73811957-sonochemistry-1617286524

CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

SONOCHEMISTRY: THEORY,

REACTIONS, SYNTHESES,

AND APPLICATIONS

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CHEMICAL ENGINEERING METHODS

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CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

SONOCHEMISTRY: THEORY,

REACTIONS, SYNTHESES,

AND APPLICATIONS

FILIP M. NOWAK

EDITOR

Nova Science Publishers, Inc.

New York

Copyright © 2010 by Nova Science Publishers, Inc.

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LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA

Sonochemistry : theory, reactions, syntheses, and applications / [edited by]

Filip M. Nowak.

p. cm.

Includes index.

ISBN 978-1-62100-147-8 (eBook)1. Sonochemistry. I. Nowak, Filip M.

QD801.S665 2009

660'.2842--dc22

2010025362

Published by Nova Science Publishers, Inc. New York

CONTENTS

Preface vii

Chapter 1 Sonochemistry: A Suitable Method for Synthesis of Nano-

Structured Materials 1 M. F. Mousavi and S. Ghasemi

Chapter 2 Industrial-Scale Processing of Liquids by High-Intensity Acoustic

Cavitation: The Underlying Theory and Ultrasonic Equipment

Design Principles 63 Alexey S. Peshkovsky and Sergei L. Peshkovsky

Chapter 3 Some Applications of Ultrasound Irradiation in Pinacol Coupling of

Carbonyl Compounds 105 Zhi-Ping Lin

and Ji-Tai Li

Chapter 4 Ultrasound and Hydrophobic Interactions in Solutions 129 Ants Tuulmets, Siim Salmar and Jaak Järv

Chapter 5 Synthetic Methodologies Using Sonincation Techniques 157 Ziyauddin S. Qureshi, Krishna M. Deshmukh and Bhalchandra M. Bhanage

Chapter 6 Sonochemotherapy Against Cancers 189 Tinghe Yu

and Yi Zhang

Chapter 7 Application of Ultrasound for Water Disinfection Processes 201 Vincenzo Naddeo, Milena Landi and Vincenzo Belgiorno

Chapter 8 Use Of Ultrasonication in the Production and Reaction of C60 and

C70 Fullerenes 213 Anne C. Gaquere-Parker and Cass D. Parker

Chapter 9 Application of Ultrasounds to Carbon Nanotubes 231 Anne C. Gaquere-Parker and Cass D. Parker

Index 265

PREFACE

The study of sonochemistry is concerned with understanding the effect of sonic waves

and wave properties on chemical systems. This book reviews research data in the study of

sonochemistry including the application of sonochemistry for the synthesis of various nano-

structured materials, ultrasound irradiation in pinacol coupling of carbonyl compounds,

ultrasound and hydrophobic interactions in solutions, as well as the use of ultrasound to

enhance anticancer agents in sonochemotherapy and the ultrasound-enhanced synthesis and

chemical modification of fullerenes.

Chapter 1 - Recently, sonochemistry has been employed extensively in the synthesis

of nano-structured materials. Rapid reaction rate, controllable reaction conditions, simplicity

and safety of the technique as well as the uniform shape, narrow size distribution, and high

purity of prepared nano-sized materials are some of the main advantage of sonochemistry.

Sonochemistry uses the ultrasonic irradiation to induce the formation of particles with smaller

size and high surface area.

Because of its importance, sonochemistry has experienced a large promotion in various fields

concerned with production of new nano-structured materials and improvement of their

properties during the recent years. However, it has encountered limitations in the case of

production of some nano-materials with specific morphology, size and properties, but the

growth of the number of researches and published articles in the field of sonochemistry

during the recent years shows a large interest and attempt to apply sonochemistry in

nanotechnology. The improvement of shape, size, purity and some other chemical and

physical properties of such produced materials has been the scope of the researchers recently.

Sonochemistry uses the powerful ultrasound irradiation (20 kHz to 10 MHz) to induce

chemical reaction of molecules. During the ultrasonic irradiation, the acoustic cavitations will

occur which consist of the formation, growth and implosive collapse of bubbles in a liquid.

The implosive collapse of the bubbles generates a localized hotspot or shock wave formation

within the gas phase of the collapsing bubbles (The hot-spot theory).

This chapter is planned to deal with the application of sonochemistry for the synthesis of

various nano-structured materials such as metals, metal carbides, metal oxides, chalcogenides

and nanocomposites with unique properties. The effect of different ultrasonic parameters on

the prepared structures including their size, morphology and properties are investigated. Also,

some applications of prepared nano-materials are introduced, e.g. electrochemical energy

storage, catalysis, biosensor and electrooxidation.

Chapter 2 - A multitude of useful physical and chemical processes promoted by

ultrasonic cavitation have been described in laboratory studies. Industrial-scale

Filip M. Nowak viii

implementation of high-intensity ultrasound has, however, been hindered by several

technological limitations, making it difficult to directly scale up ultrasonic systems in order to

transfer the results of the laboratory studies to the plant floor. High-capacity flow-through

ultrasonic reactor systems required for commercial-scale processing of liquids can only be

properly designed if all energy parameters of the cavitation region are correctly evaluated.

Conditions which must be fulfilled to ensure effective and continuous operation of an

ultrasonic reactor system are provided in this chapter, followed by a detailed description of

"shockwave model of acoustic cavitation", which shows how ultrasonic energy is absorbed in

the cavitation region, owing to the formation of a spherical micro-shock wave inside each

vapor-gas bubble, and makes it possible to explain some newly discovered properties of

acoustic cavitation that occur at extremely high intensities of ultrasound. After the theoretical

background is laid out, fundamental practical aspects of industrial-scale ultrasonic equipment

design are provided, specifically focusing on:

electromechanical transducer selection principles;

operation principles and calculation methodology of high-amplitude acoustic horns used

for the generation of high-intensity acoustic cavitation in liquids;

detailed theory of matching acoustic impedances of transducers and cavitating liquids in

order to maximize the ultrasonic power transfer efficiency;

calculation methodology of ―barbell horns‖, which provide the impedance matching and

can help achieving the transference of all available acoustic energy from transducers into the

liquids. These horns are key to industrial implementation of high-power ultrasound because

they permit producing extremely high ultrasonic amplitudes, while the output horn diameters

and the resulting liquid processing capacity remain very large;

optimization of the reactor chamber geometry.

Chapter 3 - Carbon-carbon bond formation is one of the most important topics in

organic synthesis. One of the most powerful methods for constructing a carbon-carbon bond

is the reductive coupling of carbonyl compounds giving 1,2-diols. Of these methods, the

pinacol coupling, which was described in 1859, is still a useful tool for the synthesis of

vicinal diols. 1, 2-Diols obtained in the reaction were very useful synthons for a variety of

organic synthesis, and were also used as intermediates for the construction of biologically

important natural product skeletons and asymmetric ligands for catalytic asymmetric reaction.

In particular, pinacol coupling has been employed as a key step in the construction of HIV-

protease inhibitors.

Generally, the reaction is effected by treatment of carbonyl compounds with an appropriate

metal reagent and/or metal complex to give rise to the corresponding alcohols and coupled

products, The coupling products can have two newly chiral centers formed. Threo, erythro

mixtures of diols are usually obtained from reactions. As a consequence, efficient reaction

conditions have been required to control the stereochemistry of the 1,2-diols. Recent efforts

have focused on the development of new reagents and reaction systems to improve the

reactivity of the reagents and diastereoselectivity of the products.

In some of the described methods, anhydrous conditions and long reaction time are required

to get satisfactory yields of the reaction products, some of the used reductants are expensive

or toxic; excess amounts of metal are needed. Sonication can cause metal in the form of a

powder particle rupture, with a consequent decrease in particle size, expose new surface and

increase the effective area available for reaction. It was effective in enhancing the reactivity

Preface ix

of metal and favorable for single electron transfer reaction of the aldehydes or ketones with

metal to form diols. Some recent applications of ultrasound in pinacol coupling reactions are

reviewed. The results are mostly from the author research group.

Chapter 4 - Sonochemistry and solution chemistry have been explicitly brought

together by analyzing the effect of ultrasound on kinetics of ester hydrolysis and benzoin

condensation, measured by the authors, and similar kinetic data for the solvolysis of tert-butyl

chloride, compiled from literature. For the first time the power ultrasound, reaction kinetics

and linear free-energy relationships were simultaneously exploited to study ionic reactions in

water and aqueous-organic binary solvents and the importance of hydrophobic ground-state

stabilization of reagents in aqueous solutions was discussed. This approach has opened novel

perspectives for wider understanding of the effect of sonication on chemical reactions in

solution, as well as on solvation phenomena in general.

Chapter 5 - Ultrasound generates cavitation, which is "the formation, growth, and

implosive collapse of bubbles in a liquid. Cavitation collapse produces intense local heating

(~5000 K), high pressures (~1000 atm), and enormous heating and cooling rates (>109

K/sec)" and liquid jet streams (~400 km/h), which can be used as a source of energy for a

wide range of chemical processes. This review will concentrate on theory, reactions and

synthetic applications of ultrasound in both homogeneous liquids and in liquid-solid systems.

Some recent applications of ultrasound in organic synthesis, such as, Suzuki reaction,

Sonogashira reaction, Biginelli reaction, Ullmann coupling reaction, Knoevenagel

condensation, Claisen-Schmidt condensation, Reformatsky reaction, Bouveault reaction,

Baylis-Hillman reaction, Michael addition, Curtius rearrangement, Diels-Alder reaction,

Friedal-Craft acylation, Heck reaction, Mannich type reaction, Pechmann condensation and

effect of ultrasound on phase transfer catalysis, oxidation-reduction reactions, ionic liquids

and photochemistry are reviewed. Ultrasound found to provide an alternative to traditional

techniques by means of enhancing the rate, yield and selectivity to the reactions.

Chapter 6 - Sonochemotherpy is the use of ultrasound to enhance anticancer agents.

Preclinical trials have manifested this modality is effective against cancers including

chemoresistant lesions. Sonochemotherapy is a target therapy, in which cavitation plays the

leading role. Making the occurrence and level of cavitation under control improves the safety

and therapeutic efficacy. Sonosensitizers and microbubbles enhance cavitation, being a

measure to adjust the level of cavitation. Free radicals due to cavitation have the potentials of

restructuring a molecule and changing the conformation; thus the molecular structure and

anticancer potency of a cytotoxic agent must be investigated, especially when sonosensitizer

and microbubble are employed. A potential clinical model for investigating

sonochemotherapy is the residual cancer tissues when performing palliative high intensity

focused ultrasound treatment.

Chapter 7 - Ultrasound (US) is a sound wave of a frequency greater than the superior

audibility threshold of the human hearing. Sonochemistry is the application of ultrasound in

chemistry. It became an exciting new field of research over the past decade. Some

applications date back to the 1920s. The 1950s and 1960s subsequently represented the first

extensive sonochemical research years and significant progresses were made throughout

them. Then it was realized that ultrasound power has a great potential for uses in a wide

variety of processes in the chemical and allied industries. In these early years, experiments

were often performed without any real knowledge of the fundamental physical background

about the US action. The situation changed in the 1980s when a new surge of activity started

Filip M. Nowak x

and the use of US as a real tool in chemistry began. It was in 1986 that the first ever

international symposium on Sonochemistry was held at Warwick University U.K.

Chapter 8 - In this chapter, the use of ultrasounds on fullerenes (C60 and C70) and

fullerene derivatives is described. The focus is on the articles reporting the ultrasound-

promoted treatment of these nanoparticles written in English. The ultrasound-enhanced

synthesis and chemical modification of fullerenes are detailed. The improvement obtained by

sonicating the reaction mixtures while carrying out traditional organic reactions is discussed.

This includes many types of reactions, such as oxidation, cycloaddition, reduction and

amination. Also the ultrasound-enhanced crystallization of fullerenes, producing fullerites,

and the formation of colloids when the fullerenes are sonicated in various solvent mixtures

are detailed, providing the role of ultrasound in these processes.

Chapter 9 - In this chapter, the use of ultrasounds on carbon based nanotubes is

reviewed with a focus on the English written articles. The synthesis of carbon nanotubes and

their surface modification such as oxidation and covalent functionalization under ultrasounds

are reported. The synthesis of hybrid nanocomposite materials where carbon nanotubes are

added as a reinforcement agent via ultrasound-induced assembly is not described in this

chapter. A detailed survey of the literature concerning the purification and separation of

carbon nanotubes under ultrasounds is provided. The effect of sonication on carbon nanotubes

suspensions which covers aqueous and organic solutions in the presence of surfactants is

discussed with an emphasis being placed on the effect that ultrasounds have on non-covalent

interactions between the carbon nanotubes and the components of the suspensions. The effect

of ultrasounds on the physical properties of the carbon nanotubes, especially the introduction

of wall defects is analyzed. Finally the advantages and shortcomings of sonochemistry

described in this chapter are summarized, showing a possible trend in the direction of future

research in this field.

In: Sonochemistry: Theory, Reactions, Syntheses … ISBN: 978-1-61728-652-0

Editor: Filip M. Nowak © 2010 Nova Science Publishers, Inc.

Chapter 1

SONOCHEMISTRY: A SUITABLE METHOD FOR

SYNTHESIS OF NANO-STRUCTURED MATERIALS

M. F. Mousavi1 and S. Ghasemi 1 Department of Chemistry, Tarbiat Modares

University, Tehran, Iran

2 Department of Chemistry, The University of Qom,

Qom, Iran

ABSTRACT

Recently, sonochemistry has been employed extensively in the synthesis of nano-

structured materials. Rapid reaction rate, controllable reaction conditions, simplicity and

safety of the technique as well as the uniform shape, narrow size distribution, and high

purity of prepared nano-sized materials are some of the main advantage of

sonochemistry. Sonochemistry uses the ultrasonic irradiation to induce the formation of

particles with smaller size and high surface area [1].

Because of its importance, sonochemistry has experienced a large promotion in

various fields concerned with production of new nano-structured materials and

improvement of their properties during the recent years. However, it has encountered

limitations in the case of production of some nano-materials with specific morphology,

size and properties, but the growth of the number of researches and published articles in

the field of sonochemistry during the recent years shows a large interest and attempt to

apply sonochemistry in nanotechnology. The improvement of shape, size, purity and

some other chemical and physical properties of such produced materials has been the

scope of the researchers recently [2].

Sonochemistry uses the powerful ultrasound irradiation (20 kHz to 10 MHz) to

induce chemical reaction of molecules. During the ultrasonic irradiation, the acoustic

cavitations will occur which consist of the formation, growth and implosive collapse of

bubbles in a liquid. The implosive collapse of the bubbles generates a localized hotspot or

1 Corresponding author. M.F. Mousavi, Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-

175, Tehran, Iran Tel.: +98 21 82883474/9; fax: +98 21 82883455. E-mail addresses:

[email protected], [email protected] (M.F. Mousavi).

M. F. Mousavi and S. Ghasemi 2

shock wave formation within the gas phase of the collapsing bubbles (The hot-spot

theory) [3].

This chapter is planned to deal with the application of sonochemistry for the

synthesis of various nano-structured materials such as metals, metal carbides, metal

oxides, chalcogenides and nanocomposites with unique properties. The effect of different

ultrasonic parameters on the prepared structures including their size, morphology and

properties are investigated. Also, some applications of prepared nano-materials are

introduced, e.g. electrochemical energy storage, catalysis, biosensor and electrooxidation.

1. INTRODUCTION

When ultrasound radiations interact with molecules, chemical reactions can be initiated.

Sonochemistry is an interesting research area deal with the processes occurs during the

application of powerful ultrasound (20 KHz–10 MHz). Sonochemistry arises from acoustic

cavitations. Bubbles undergo the formation, growth, and implosive collapse in a liquid under

ultrasonic irradiation. Bubble growth occurs through the diffusion of solute vapor into the

bubble. A bubble can be included evaporated water molecules and dissolved gas molecules.

When the bubble size reaches to a radius down to several µm, the bubbles collapse provides

extreme conditions of transient high temperature(as high as 5000K) and high pressure (up to

~1800 atm) within the collapsing bubbles, shock wave generation, and radical formation. The

collapsing bubbles provide reaction sites, named hot spots. At this sites, sonolysis of water

molecules to hydrogen radicals (H•) and hydroxyl radicals (OH•) is occurred which is

responsible to sonochemical reaction. Also, organic molecules in solution can form organic

radicals with a reducing ability. The size of a bubble depends on ultrasonic frequency and

intensity. Bubbles collapse occurs in very short time (nanosecond) and cooling rate of 1011

K/s is obtained. The fast kinetics of such process can hinders the growth of nuclei produced

during the collapse of bubbles. This may be the reason of formation of nanostructured

materials.

Sonochemical synthesis of different types of nanostructured materials consisted of metals

and their oxides, alloy, semiconductors, carbon carbonic and polymeric materials and their

nanocomposite have received much attention in recent years.

A number of factors can influence on cavitation efficiency and the properties of the

products. The dissolved gas, ultrasonic power and frequency, temperature of the bulk

solution, and type of solvent are all important factors that control the yield and properties of

the synthesized materials.

In the field of sonochemistry, a number of book chapter and reviews have been published

4. Y. Mastai and A. Gedanken reviewed articles in the field of sonochemistry published

before 2004 in a chapter of book entitled ―Sonochemistry and Other Novel Methods

Developed for the Synthesis of Nanoparticles‖ [2]. Also a review articles was published by

Gedanken in 2004 entitled ―Using sonochemistry for the fabrication of nanomaterials‖

focused on the typical shape of products obtained in sonochemistry [1]. Another review

articles also published dealt with insertion of nanoparticles into mesoporous materials [5] and

the sonochemical doping of various nanoparticles into ceramics and polymers [6].

In this chapter, we will present a literature survey on the various inorganic,

organic/inorganic and inorganic/inorganic systems more recently have been synthesized by

using ultrasonic method from January 2004 to January 2010s.

Sonochemistry: A Suitable Method for Synthesis of Nano-Structured Materials 3

2. SYNTHESIS OF NANOMETALS

Intensive works on metal nanostructures such as noble metals (Au, Pt, Pd) with various

size and morphology have been achieved due to their potential applications in the fabrication

of electronic, optical, optoelectronic, and magnetic devices. They can be obtained form

sonication of solution containing related metal ion in the absence and presence of capping

agents. With controlling size, shape, and crystallinity of nanometals, it can be possible to tune

the intrinsic properties of a metal nanostructure.

2.1. Gold

Gold and other noble metal nanoparticles have been extensively considered in recent

years because of their potential applications in optics, electronics, and catalysis, etc. Okitsu et

al reported the synthesis of Au nanoparticles and investigate the dependence of sonochemical

reduction rate of Au(III) to Au nanoparticles in aqueous solutions containing 1-propanol as

accelerator and their particle size to the ultrasound frequency so that the highest reduction rate

was at 213 kHz in the range of 20 to 1062 kHz [7]. The average size of Au particles was 15.5

nm in 20 mM 1-propanol.

This group also synthesized Gold nanorods by using sonochemical reduction (frequency,

200 kHz; power, 200 W) of gold ions in aqueous solution (60 mL) containing of HAuCl4 and

CTAB including 1.2 mL of AgNO3 (4.0 mM) and 240 μL of ascorbic acid (0.050 M) with pH

3.5 [8]. During the reaction, Au (III) is immediately reduced to Au (I) by reaction with the

ascorbic acid. CTAB and AgNO3 act as effective capping agents for the shape controlled

growth of gold seeds. The solution was purged with argon for 15 min and then sonicated in a

water bath (at 27 ºC) by a water circulation system. In the presence of ultrasonic, the

following reactions are proposed:

OHHOH

)))

2 (1)

)()( 22 HOHHOHCTAB + reducing species (2)

OHCTAB 2 pyrolysis radicals and unstable products (3)

MHAuMAu 0 (4)

nAunAu )( 00 (5)

1

000 )()( nn AuAuAu (6)

Where M corresponds to various reducing species, pyrolysis radicals and unstable

products. In reaction 3, pyrolysis radicals and unstable products are formed via pyrolysis of


CTAB and water. The size of the sonochemically formed gold nanorods was less than 50 nm,

and their average aspect ratio decreased with increasing pH of the solution.

At pH 7.7, irregular shaped gold nanoparticles were formed. At pH 9.8, most of the

particles formed had a spherical shape with a smaller particle size than those formed in the

lower pH solutions. Based on the obtained results, it was clear that the size and shape of the

sonochemically formed gold nanoparticles are dramatically dependent on the pH value of the

solution (Figure 1).

From the obtained results, it was demonstrated that longer gold nanorods would be

obtained if the synthesis was performed in solution with acidic pH.

Li et al. reported the synthesis of single-crystal Au nanoprisms with triangular or

hexagonal shape, 30-40 nm planar dimensions, and 6-10 nm thickness from solution of

HAuCl4 and PVP in ethylene glycol solution [9]. Ethylene glycol, the surfactant

poly(vinylpyrrolidone), and ultrasonic irradiation play important roles in the formation of Au

nanoprisms.

Single-crystalline gold nanobelts have been prepared sonochemically from aqueous

solution of HAuCl4 in the presence of α-D-glucose, a biological directing agent, under

ambient conditions (Figure 2).

Figure 1. TEM images of gold nanorods and nanoparticles formed in different pH solutions of (a) pH

3.5, (b) pH 5.0, (c) pH 6.5, (d) pH 7.7, and (e) pH 9.8 after 180 min irradiation under argon. (f) TEM

image of gold nanoparticles formed in pH 9.8 without ultrasonic irradiation.


Figure 2. a,b) SEM images and c,d) high-magnification SEM images of as-synthesized gold nanobelts;

[HAuCl4]=50 mgmL-1

, [α-D-glucose]= 0.2 m, ultrasound time=1 h.

The formation of gold nanobelts depends on the concentration of α-D-glucose. When its

concentration was as low as 0.05 M, only gold particles with a size of approximately 40 nm

were obtained [10]. In the dilute solution, the glucose can not provide effective coverage or

passivation of gold facets. The gold nanobelts have a width of 30–50 nm and a length of

several micrometers with highly flexibility. Nanobelts have thickness of approximately 10

nm. Authors also showed that only spherical particles with a diameter of approximately 30

nm were obtained in the presence of β-cyclodextrin. It was mentioned that ultrasound

irradiation can enhance the entanglement and rearrangement of the α-D-glucose molecules on

gold crystals.

Park et al. showed the effects of concentration of stabilizer (sodium dodecylsulfate: SDS)

and ultrasonic irradiation power on the formation of gold nanoparticles (Au-NPs) [11]. The

multiple shapes and size distribution of Au-NPs are observed by different ratio of Au (III)

ion/SDS and ultrasonic irradiation power.

A sonochemical method in preparation of gold nanoparticles capped by thiol-

functionalized ionic liquid (TFIL) in the presence of hydrogen peroxide as a reducing agent

reported by Jin et al. [12]. It was demonstrated that the molar ratio of gold atom in

chloroauric acid to thiol group in TFIL (Au/S) has great effects on the particles size and

distribution of gold nanoparticles. Small gold nanoparticles size of 2.7±0.3 nm can be

synthesized when ultrasound irradiation applied to a solution with the molar ratio of Au/S =

1:2 for 12 h.


2.2. Palladium

Nemamcha et al reported the sonochemical synthesis of stable palladium nanoparticles by

ultrasonic irradiation of palladium (II) nitrate solution in ethylene glycol and in the presence

of poly(vinylpyrrolidone) (PVP) for 180 min [13]. During the ultrasonic irradiation of the

palladium (II) nitrate mixture, the color of the solutions turned from the initial pale yellow to

a dark brown. The following mechanism was proposed:

HOHOH ))))

2 (7)

)()( 22222 HOHHOHCHOCHOHOHOHCHHOCH (8)

nHCHOnHOCHnPdHOHCnHOCHIInPd 22 2)0(2)( (9)

The coordination of the PVP carbonyl group to the palladium atoms causes to the

stabilization of the Pd nanoparticles in ethylene glycol. It has been shown by TEM that the

increase of the Pd (II)/PVP molar ratio from 0.13 ×10-3 to 0.53 ×10-3 decreases the number

of palladium nanoparticles with a slight increase in particle size. For the highest Pd (II)/PVP

value, 0.53 × 10-3, the reduction reaction leads to the unexpected smallest aggregated

nanoparticles.

2. 3. Tellurium

Crystalline tellurium nanorods and nanorod branched structures are successfully prepared

at room temperature via an ultrasonic-induced process in alkaline aqueous solution containing

tellurium nitrate, D-glucose and polyethylene glycol (PEG-400,CP) for 2 h treatment in an

ultrasonic bath [14]. A yellow sol was produced and was kept in darkness for 24 h to allow

the growth of Te nanocrystals. The as-obtained nanorods are single crystalline with [0 0 1]

growth orientation, and have 30–60 nm in diameter with 200–300 nm in length. Some

branched architectures, consisting of several nanorods, are also found in the products. The

formation of the branched structures is suggested to be the result of multi-nuclei growth in

monomer colloid.

2.4. Tin

Metallic tin nanorods were synthesized by a sonochemical method employing the polyol

process [15]. In the reaction a solution of SnCl2 in ethylene glycol was exposed to high-

intense ultrasound irradiation. The crystallized metallic tin nanorods have diameters of 50–

100 nm and lengths of up to 3 µm were synthesized. In the absence of the high-intensity

ultrasonic irradiation, no reduction of tin ions occurs even at temperatures as high as 500 ºC

in a closed cell.


2.5. Ruthenium

Ruthenium nanoparticles have been prepared by sonochemical reduction of a ruthenium

chloride solution in 0.1 M perchloric acid containing propanol and SDS for almost 13 h [16].

The effects of different ultrasound frequencies in the range 20–1056 kHz were investigated.

The Ru particles have diameters between 10 and 20 nm. The rate of Ru (III) reduction by the

sonochemical method is very slow. The sonochemical reduction rate has been found to

influence by ultrasound frequency. An optimum reduction rate was determined in the

frequency range 213–355 kHz.

2.6. Germanium

Wu et al. reported a method based on ultrasonic solution reduction of GeCl4 by metal

hydride (LiAlH4 and NaBH4) or alkaline (N2H4·H2O) in tetrahydrofuran (THF) and in

ambient condition [17]. The germanium nanocrystals have narrow size distribution with

average grain sizes ranging from 3 to 10 nm. Octanol was used as capping agent. To prevent

the formation of GeO2 formed in the presence of water, the anhydrous salt is added to form a

transparent ionic solution in THF.

2.7. Selenium

Single crystalline trigonal selenium (t-Se) nanotubes with diameters of less than 200 nm

and nanowires with diameters of 20-50 nm have been synthesized by the reduction of

H2SeO3 in different solvents with a sonochemical method [18]. The morphology of the

products depends on the reaction conditions including ultrasonic parameters (e.g., frequency,

power, and time), aging time, and solvent. Hydrazine hydrate was dissolved in ethylene

glycol, water, etc. to form solutions. The solution was added dropwise to the corresponding

selenious acid solution. At the same time, ultrasound was preceded to the solution, and the

ultrasonic time is 30-60 min. Selenium nanotube and nanowire formation involved several

stage:

)(

)()(

)))

))))))

4232

SetNanowires

SetlikeSphericalSeSphericalHNSeOH

(10)

2.8. Silver

Dendritic silver nanostructures were formed by means of ultrasonic irradiation[19] of an

aqueous solution of silver nitrate with isopropanol as reducing agent and PEG400 as disperser

for 2 h.


Figure3. TEM image of a silver dendritic nanostructures obtained with ultrasonic irradiation of the

aqueous solutions of 0.04 M AgNO3, 4.0 M isopropanol and 0.01 M PEG400 for 2 h .

The side branches of the dendritic silver are constructed of well crystallized small

nanorods (Figure 3). The selected area electron diffraction (SAED) image of dendritic silver

nanostructures has single crystal nature with cubic phase and the side branch direction

assembles along <011> direction.

The irradiation time, the concentration of Ag+ and the molar ratio of PEG to AgNO3 are

parameters can influence the morphology of silver nanostructured. The low molar ratio of

PEG400 to AgNO3 (1:4 ~ 1:1) result in the formation of silver dendritic nanostructures but

the molar ratio of 10:1 will cause to formation of silver nanoparticles (in the range of 40–100

nm ) instead of dendritic nanostructures. Only silver spheroidal nanoparticles were obtained at

the beginning of the reaction but silver dendrites were observed with 1 h sonication. These

dendritic nanostructures transform to hexagonal compact crystals after 6 h later.

In another work, highly monodispersed Ag nanoparticles (NPs) were prepared by a

sonochemical reduction in which Ag+ in an ethanol solution of AgNO3 was reduced by

ultrasound irradiation in the presence of benzyl mercaptan without the additional step of

introducing other reducing reagents or protective reagents [20].

3. SYNTHESIS OF METALLIC NANOALLOYS

The nanoalloys are formed when two or more kinds of metals are melted together.

Nanoalloy materials can exhibit many novel properties, including electronic, catalytic,

magnetic and corrosion-resistant properties. The sonochemical method has been used as a

new technique for preparing alloy nanoparticles. Bimetallic nanoalloys show different


properties such as high catalytic activity and catalytic selectivity in comparison with the

corresponding monometallic counterparts so that they can be used as catalysts and gas

sensors.

3.1. Sn–Bi

Sn–Bi alloy nanoparticles were prepared by sonicating bulk Sn–Bi alloy directly in

paraffin oil under ambient pressure and room temperature [21]. Twenty grams Sn and 30 g Bi

were melted together in a vessel to obtain the bulk Sn–Bi alloy. Then 0.5 g bulk Sn–Bi alloy

was added to 30 ml paraffin oil in a horniness test tube and the system was irradiated for two

hours at 1000Wcm−2 with a high intensity ultrasonic probe. The product was centrifuged

after cooled to room temperature and washed with chloroform and dried to get some gray-

black powder. They show that when the ultrasonic power was increased from 700 to 1000

Wcm-2, the size distribution reduced from 60-80 nm to 10-25 nm. They also show that the

sonication time had little impact on the size of the nanoparticles.

3.2. Pd–Sn

Kim et al. prepared Pd–Sn nanoparticles from aqueous ethanol solution of Pd(NH4)2Cl4

and SnCl2 in the presence of citric acid by applying ultrasonic irradiation and investigate the

Pd–Sn nanoparticles for the oxygen reduction reaction (ORR) in alkaline media [22]. The

average size of Pd–Sn nanoparticles thus prepared was about 3–5 nm. The initial

concentrations of Pd and Sn and their molar ratio, the concentration of ethanol and the

concentration of citric acid affect the size distribution of the Pd–Sn nanoparticles. The Pd in

Pd–Sn nanoparticles is mostly in the metallic form.

3.3. Pt-Ru

Bimetallic catalysts comprised of Pt and Ru (Pt-Ru) are important in the development of

low temperature (<~120 ºC) H2-air and direct methanol fuel cells. Korzeniewski et al.

prepared Pt-Ru nanoparticles with diameters in the range of 2–6 nm as catalyst materials to

investigate the electrochemical oxidation of CH3OH and CO [23]. In Pt-Ru catalyst, Pt

provides sites for C-H bond cleavage and CO adsorption, and Ru activates water to produce

reactive oxides that enable conversion of carbon containing fragments to CO2.

Pt-Ru Nanoparticle bimetallic electrocatalysts with XRu ≈0.1 and XRu ≈ 0.5 were

synthesized and its response toward the electrochemical oxidation of CO and CH3OH in 0.1

M H2SO4 was investigate [24]. Syntheses were carried out in tetrahydrofuran (THF)

containing Ru3+ and Pt4+ in a fixed mole ratio of either 1:10 or 1:1 using high-intensity

sonochemistry.


3.4. Co-B

Uniform spherical Co-B amorphous alloy nanoparticles were prepared by ultrasound-

assisted reduction of Co(NH3)2+6 with BH−4 in aqueous solution which the particle size

distribution was controlled by changing the ultrasound power and the ultrasonication time

[25]. During liquid-phase cinnamaldehyde (CMA) hydrogenation, the as-prepared Co-B

catalyst exhibited much higher activity and better selectivity to cinnamyl alcohol (CMO) than

the regularCo-B in the absence of ultrasonic waves.

3.5. Au-Ag

Au-Ag nanoalloys were prepared sonochemically form solution containing gold

nanoparticles and silver nitrate in the presence of different surfactant (sodium borohydride in

water; poly(vinyl pyrrolidone) in ethylene glycol; poly(ethylene glycol); sodium dodecyl

sulfate in water or propanol) [26]. It was suggested that the degradation of the surfactants

occurred during the ultrasonic treatment and allowed modification of the shape of gold

nanoparticles in their interaction with silver ions. Monodisperse gold-silver nanocomposite of

triangular or polygonal structure was obtained with reduction of the silver by NaBH4 on the

gold surface in the presence of ultrasonic irridation. Uniformly distributed gold-silver with

round shapes was resulted after sonication in poly (ethylene glycol). Multiangular Au-Ag

nanocomposites of larger size appeared after ultrasonic irradiation of the gold-silver mixture

in the presence of poly (vinyl pyrrolidone) in ethylene glycol due to the capping effect and the

relatively low rate of degradation of PVP. With SDS, worms or netlike gold-silver

nanostructures obtained after 1 h of ultrasonic irradiation of AgNO3 in propanol and water,

respectively.

3.6. Bimetallic Nanoparticles with Core-Shell Morphology

Sonochemically assisted synthesis of bimetallic nanoparticles with core-shell morphology

have been reported for materials such as Co/Cu [27], Au/Pd 28 and Pt-Ru [29].

A sequential sonolysis method was used to synthesis of Pt-Ru core shell (Pt@Ru)

structure [29]. Pt-Ru has been used as a methanol oxidation catalyst in direct methanol fuel

cells (DMFC). A potassium tetrachloroplatinate (K2PtCl4) solution containing 8 mM SDS,

200 mM propanol, and 0.1 M HClO4 were sonicated to reduce the Pt (II) to colloidal Pt (0)

during 3h at 20 °C. When all of the Pt (II) has been reduced, the RuCl3 solution was added to

the Pt colloidal solution and sonication continued. TEM image of the nanoparticles showed

that the ruthenium formed a layer around the platinum particles and Pt-Ru core-shell particles

in the range of 5-10 nm were formed (Figure 4). The platinum particle sizes are ~7 nm, while

the thickness of the ruthenium shell was estimated to be between 2 and 3 nm.

When 1mg/mL of polyvinyl-2-pyrrolidone, PVP (MW =55000) is used as the stabilizer,

the formation of colloidal platinum is very rapid and become complete within 1 h of

sonication. At the end of 1 h, when all of the Pt (II) was reduced, the RuCl3 solution was

added to the Pt colloidal solution and sonication continued.


Figure 4: (a) TEM of Pt-Ru nanoparticles synthesized by sonocation of a solution containing 1 mM

PtCl4 2-

in 200 mM propanol, 0.1 M HClO4, and 8 mM SDS followed by the reduction of 1 mM RuCl3

under argon atmosphere. The RuCl3 solution was added after the PtCl4 2-

solution was sonicated for 4 h.

The total time of sonication was 7h at 213 kHz. (b) Absorption spectra Pt-Ru nanoparticles.

The TEM images showed ultrasmall 2 nm sized particles without core-shell morphology and

only the presence of bimetallic ruthenium and platinum was confirmed by energy-dispersive

X-ray analysis of the TEM.

Figure 4b shows the change in absorption spectra of the colloidal solutions with time.

Curve a shows the absorption spectrum of PtCl4 2- solution at time t = 0 and continuing

through the addition of the RuCl3 and its reduction. Curve e shows the absorption spectrum

immediately upon addition of the ruthenium chloride. Only one prominent peak at 400 nm

appears in the curve indicating an instantaneous partial reduction of Ru (III) upon addition to

the solution.

As mentioned above, Vinodgopal et al used a sequential reduction method to prepare Pt-

Ru core-shell nanoparticles but Anandan and his coworker prepared Au-Ag bimetallic

nanoparticles by the sonochemical co-reduction of Au(III) and Ag(I) ions in aqueous

solutions containing polyethylene glycol (0.1 wt %) and ethylene glycol (0.1 M) [30]. The

average diameter of the bimetallic clusters prepared by the simultaneous reduction is about 20

nm. The stabilizing polymers can coordinate to metal ions before the reduction. This

interaction between the polymer and the metal ions lead to the formation of smaller size core-


shell nanoparticles with a narrow size distribution. They also suggested that the formation of

core-shell morphology is most likely due to the difference in the reduction rates of the

individual metal ions and the involvement of a polymer-Ag ion complex. Gold ions are firstly

reduced under the sonochemical conditions followed by the reduction of Ag+ ions on the

surface of the gold particles.

4. METAL OXIDE

During the last years, the ultrasonically assisted synthesis of metallic oxides and

hydroxides has been considered by some of researchers. Due to their importance in various

area of science, some of them are investigated in the following paragraph.

4.1. ZnO

ZnO is one of the most important multifunctional semiconductors with wide direct energy

band gap of 3.37 eV and large exciton binding energy (about 60 meV). Sonochemical

synthesis of ZnO nanostructures with different shapes such as nanowires, nanotubes,

nanoparticles have been considers by some of authors. The effects of various parameters on

the morphology of ZnO nanostructures were investigated. ZnO nanostructure with

morphologies such as flower-like clusters [31], cauliflower-like [32], nanorods [33], needle-

shape [34], trigonal-shaped [35], nanosheet [36] and Hollow ZnO microspheres [37].

Jung et al fabricated ZnO nanorods, nanocups, nanodisks, nanoflowers, and nanospheres

in a horn-type reaction vessel using an ultrasonic technique at a power of 50 W (intensity of

39.5 W/cm2) and frequency of 20 kHz (Figure 5) [38]. The kind of hydroxide anion-

generating agents, concentration of reactants, sonication time and additives are dominant

factor affect on preparation of different morphology of ZnO. For the production of ZnO

nanorods and ZnO nanocups, different concentration of Zn(NO3)2 and

hexamethylenetetramine (HMT, (CH2)6N4 as well as different sonication time (30 min for

nanorods in comparison with 2h for nanocups) were used. An increase in ultrasonication time

provides such energy indicates to the reaction System that hinders the ZnO nanorod growth.

Triethyl citrate was used as an additional chemical additive to synthesize ZnO nanodisks.

ZnO nanocrystals grow preferentially along the [0001] direction to form nanorods. The

growth rate of the ZnO crystal along the [0001] direction decreases dramatically due to the

addition of triethyl citrate.

For the synthesis of ZnO nanoflowers and nanospheres, ammonia–water (28–30 wt %)

solution were used as hydroxide anion precursors. In the case of ZnO nanospheres, triethyl

citrate was added to the mixture of zinc acetate dihydrate solution (90 mL) and ammonia–

water (10 mL). The sonochemical growth mechanism of ZnO nanostructures was suggested

by authors as follows:

HCHONHOHNCH 646)( 32462 (11)

OHNHOHNH 423 (12)


2

4

2 )(4 OHZnOHZn (13)

OHOHZnOOHZn 2)( 2

)))2

4 (14)

2

)))

2

2

2

32 OZnOOZn

(15)

The sonolysis of water produces

2O radicals in solution.

Figure 5. SEM (left) and TEM (right) images of ZnO nanostructures. (a,b) Nanorods. (c,d) Nanocups.

(e,f) Nanodisks. (g, h) Nanoflowers. (i, j) Nanospheres. (Insets: HRTEM image) .

The same authors presented in another paper a sonochemical method for fabricating

vertically aligned ZnO nanorods arrays on various substrates such as a large-area Zn sheet, Si


wafer, transparent glass, and flexible polymeric materials, in an aqueous solution under

ambient conditions [39].

A template-free, sonochemical route to prepare porous hexagonal ZnO nano-disks has

been developed by Bhattacharyya and Gedanken. The nano-disks are 260–400 nm across the

edges and 290–430 nm across the vertices[40]. Some of authors used ultrasonic irradiation to

fabricate well-defined dentritic ZnO nanostructures in a room-temperature ionic liquid [41].

The ZnO nanostructures have been used to sensing etanol [42], high performance NO2

gas sensor [43], gas sensitivity to NO [44].

4.2. CuO

The synthesis of one-dimensional (1D) Cu(OH)2 nanowires [45] in a aqueous solution of

CuCl2 and NaOH was done under ultrasound irradiation with 40 kHz ultrasonic waves at the

output power of 100% at 70 ºC for 5-60 min. The morphology of products is highly depends

on time of ultraonication. Under continuous ultrasonic irradiation, Cu(OH)2 nanowires

integrated into nanoribbons, then parts of nanoribbons crosswise grew to form 3D Cu(OH)2

nanostructures; finally, 3D nanostructures disrupted and transformed into 3D CuO

microstructures. The effect of ultrasonic irradiation time on conversion process of Cu(OH)2

to CuO was investigated. A color change of the product from the pale-blue to the black was

observed in the range of 15 to 45 min of irradiation implied the gradual conversion of

Cu(OH)2 to CuO. The XRD analyses of the products confirmed the conversion process. It

was demonstrated that the ultrasound plays two roles besides dispersion: shortening the

conversion time from Cu(OH)2 to CuO and inducing the formation of 3D CuO

microstructures. The CuO microstructures showed better electrochemical property than

Cu(OH)2.

4.3. V2O5

A sonochemical method has been developed to preparation self-assemble V2O5

nanowires with spindle-like morphology (Figure 6). Vanadium oxide (V2O5, 0.46 g, 2.5

mmol) and sodium fluoride (NaF, 0.21 g, 5 mmol) were dissolved in 50 mL of distilled water

in a 100-mL round-bottom flask and exposed to high-intensity ultrasound irradiation (20 kHz,

100 W/cm2) under ambient air for 2 h. The organization of 1D V2O5 nanostructured subunits

into spindle -like V2O5 bundles was occurred

Each bundles composed of several tens of homogeneous nanowires with diameters of 30-

50 nm and lengths of 3-7 µm. Also, a sensitive resonance light scattering (RLS) method was

developed to detect bovine serum albumin (BSA) based on the ultrasonically V2O5 bundles

[46]. An increase in the scattered light signals of V2O5 bundles were observed by the

addition of BSA. The enhanced RLS intensity at 468 nm of V2O5 bundles-BSA varies

linearly with the concentration of BSA in the range from 0.5 to 20 µg mL-1.

Synthesis of self-assembled nanorod vanadium oxide bundles by sonochemical technique

were reported by a Malaysian group [47].


Figure6. FE-SEM images of the V2O5 bundles with spindle-like morphology (a) low-magnification

SEM image of V2O5 bundles; (b) low-magnification TEM image.

The morphologies of the nanorod vanadium oxides are depended on the time of sonication so

that a uniform, well defined shapes and smaller size nanorod vanadium oxide bundles were

obtained with higher ultrasound irradiation times. Vanadium oxide bundles showed higher

activity to anaerobic oxidation of n-butane than the bulk material.

4.4. Iron oxide

Suslick and Bang used carbon nanoparticles as a spontaneously removable template for

synthesis of crystalline hollow hematite (α-Fe2O3) [48]. A mixture of Spherical carbon

nanoparticles (0.1 g) (4- 12 nm diameter) and Fe(CO)5 (0.5 mL) in 40 mL of hexadecane was

irradiated by a high-intensity ultrasound horn (operated at 20 kHz and 50 W/cm2 at 20 °C for

3 h under argon flow). The decomposition of Fe(CO)5 form high-surface-area iron shells

around the core carbon nanoparticles. The high-surface-area iron shells rapidly oxidize in

contact with air and release such heat that ignites the carbon particles. The combustion of the

nanosized carbon particles generates enough heat to crystallize the iron oxide shells to hollow

cores α-Fe2O3 (Figure 7). Mössbauer spectra confirm the presence of hematite as the only

iron species. Under the same condition, the sonication of precursor solution in the absence of

carbon nanoparticles produce agglomerated nanoparticles of ~ 6 nm.


Figure 7. (a) Bright-field and (b) dark-field TEM image of nanosized hollow hematit.

Any organic residues were completely removed by annealing the as-produced hollow

hematite at 450 °C for 2 h under air. TEM image and EDS (energy dispersve X-ray

spectroscopy) reveal that the morphology and composition of the hollow hematite remained

unchanged after the heat-treatment. The hollow hematite nanoparticles shows hysteresis loops

for at 5 and 298 K. Also, the hollow hematite nanoparticles are weakly ferromagnetic down to

5 K.

Another work has been reported to sonochemically synthesis of monodispersed magnetit

nanoparticles [49]. Dang et al. used an FeCl2 ethanol–water mixed solvent and a 2 N NaOH

aqueous solution to from a Fe(OH)2 precipitate. The Fe(OH)2 precipitate was irradiated by an

ultrasonic horn in air at 50 ºC to synthesize magnetite nanoparticles. It was demonstrated that

the formation of magnetite was accelerated in ethanol–water solution in the presence of

ultrasonic irradiation.

Monodisperse iron oxide nanoparticles with 5–20 nm can be synthesized by an

inexpensive and simple ultrasonic-assisted method at low temperature [50]. This is based on

the decomposition of iron pentacarbonyl in cis–trans decalin. They found that ultrasonic

irradiation could greatly enhance the crystallization of iron oxide nucleus at 190 °C, after the

react solution was refluxed at this temperature; monodisperse γ-Fe2O3 nanocrystals were

obtained.

4.5. Manganese Oxide

An ultrasonic technique was used to prepare MnO2 nanoparticles inside the pore

channels of ordered mesoporous CMK-3 [51]. MnO2 nanoparticles were anchored in pores of

carbon CMK-3. The size of MnO2 incorporated in CMK-3 is between 0.5 nm and 3.0 nm. In

the pores, KMNO4 reduce to MnO2. Ultrasonic technique controls the amount of loading

MnO2 inside CMK-3. CMK-3 with 20 wt. % loading of MnO2 inside CMK-3 produced an

improved discharge capacity of 223 mAhg-1 at 1 Ag-1.


Colloidal Mn3O4 nanoparticles with diameters of about 5-10 nm has been pepared by an

ultrasonic-assisted method in the absence of any additional nucleation and surfactant at

normal temperature and pressure [52]. It was shown that reveal the size and the crystallinity

of Mn3O4 nanoparticles depends on the growth temperature so that the smaller the average

diameter and the poorer the crystallinity of Mn3O4 nanoparticles was observed at lower

reaction temperature. The magnetic properties of the samples showed that Mn3O4

nanoparticles exhibited ferromagnetic behavior at low temperature (40 K).

Kumar et al. prepared sonochemically a highly dispersed and non-agglomerated α-MnO2

nano-needles of dimensions 20–30 nm from aqueous solution consisting of manganese

Mn(acetate)3 and LiOH with a pH value of 7.2 [53].

The sonochemical preparation of MnO2 was reported through the reduction of MnO4- in

water under Ar atmosphere. The effect of H2O2 formed in the sonolysis of water on the rates

of reduction of MnO4- was investigated [54]. It was shown that the rate of the sonochemical

reduction of MnO4- depend on the amount of sonochemically formed H2O2 molecules.

Mn3O4 nanoparticles were prepared by reacting MnCl2 and NaOH in water at room

temperature through a sonochemical method, operated at 20 kHz and 220 W for 20 min [55].

Also, the LiMn2O4 nanoparticles were also prepared. A thin film of the LiOH with the

thickness of about 4.5–5.5 nm was coated onto the surface of Mn3O4 under the same

sonochemical conditions and the LiOH-coated Mn3O4 particles sample was heated at the

relatively low temperature of 300–500 °C. The thickness of coated LiOH on Mn3O4 obtained

from the reaction ratio of 3:1 between LiOH and Mn3O4 was about 4.5–5.5 nm range. Then,

by heating LiOH-coated Mn3O4 particles at the relatively low temperature of 300–500 °C for

1 h, they were transformed into phase-pure LiMn2O4 nanoparticles of about 50 to 70 nm size

in diameter.

4.6. In2O3

The synthesis of monodispersed In2O3 nanoparticles and doped with rare earth ions

((Eu3+ and Dy3+)) is another work considered to investigate their photoluminescence

properties [56]. To a solution of indium ethoxide, In(OEt)3, in 20 ml ethanol containing 0.36

g cetyltrimethyl ammonium bromide (CTAB), 60 ml water was added and pH of solution was

adjusted to 10 by adding NH4OH. The irradiation of solution with a high-intensity (100

W/cm2) ultrasonic radiation operating at 20 kHz, under air at room temperature resulted in

In(OH)3 nanoparticles. After 1 h sonication, In(OH)3 nanocubes are obtained in the range of

30-35 nm. The In2O3 nanoparticles were formed by heating the In(OH)3 nanoparticles in

furnace under air at 350 °C for 1 h. On exciting at 235 nm, emission peaks around 460 nm

(blue) and two relatively less intense peaks centered around 548 nm (yellow) and 618 nm

(orange) were observed possibly due to the presence of shallow defect levels in the annealed

samples. With Eu3+/Dy3+ incorporation, the In2O3 diffraction peaks broads with respect to

diffraction peaks of undoped In2O3. With Eu3+/Dy3+ incorporation, Eu3+/Dy3+ has gone

into the In2O3 lattice and lattice undergoes distortion. In2O3 doped Dy3+ nanoparticles did

not show any luminescence due to the highly strained and distorted environment around the

dysprosium ions in the In2O3 lattice. A similar strain causes to the low emission intensity in

the In2O3 doped Eu3+ particles.


4.7. TiO2

TiO2 is an important semiconductor for a broad range of applications, such as hydrogen

production, solar cells, biological coatings and photocatalysis [57]

Mesoporous titanium dioxide nanocrystalline powders were synthesized by ultrasonic-

induced hydrolysis reaction of tetrabutyl titanate (Ti(OC4H9)4 , TBOT). TBOT was added

dropwise to 40 ml pure water in a 100 ml beaker. Pulse irradiation was done with a high

intensity ultrasonic horn (6.3 mm diameter Ti-horn, 20 kHz, and 1200 W/cm2 at 50%

efficiency) for 45 min. In pulse technique, ultrasonic wave is on for 2 s followed by 2 s off

during the whole reaction. The sonication promoted the hydrolysis of TBOT, crystallization

of TiO2 and formation of mesopore TiO2. The as-prepared products by the ultrasonic method

were composed of anatase and brooktie phases Photocatalytic decomposition was investigated

for formaldehyde and acetone. Mesopore TiO2 prepared by ultrasonic method showed better

photocatalytic activities than the samples prepared by conventional hydrolysis method.

Wang et al. prepared mixed-crystal TiO2 powder with high sonocatalytic activity under

ultrasonic irradiation in hydrogen peroxide solution [58]. The nano-sized rutile phase TiO2

powder (10.0 g) and 30% hydrogen peroxide solution (30 mL) were mixed into a glass reactor

and suspension was treated under ultrasonic irradiation for 4.0 h. A white powder was

obtained after washing and drying. This powder was heat-treated at 400 °C for 40 min. The

XRD and the FT-IR spectra of treated mixed-crystal showed both nano-sized rutile phase and

anatase phase TiO2 powders. The sonocatalytic degradation of methylene blue in aqueous

solution was investigated under ultrasonic irradiation in the presence of treated mixed-crystal

TiO2 powder. Effect of different parameters such as heat-treated temperature and heat-treated

time on degredation of methylen blue was studied. It was shown by the UV–vis spectra that

the methylene blue in aqueous solution can be obviously degraded under ultrasonic irradiation

in the presence of treated mixed-crystal TiO2 powder.

Guo et al. prepared the mesoporous TiO2 nanorods using industrial bulk Ti powder [59].

The as prepared materials contained numerous irregular olive-like nanorods aggregates. The

nanorods were 8–12 nm in width and 15–100 nm in length. The products showed higher

photocatalytic activity to toluene than Degussa P25 TiO2.

Crystalline anatase TiO2 nanoparticles was synthesized from titanium tetraisopropoxide

in the ionic liquid 1-(3-hydroxypropyl)-3-methylimidazolium-bis(trifluoromethan esul-

fonyl)amide by ultrasound assisted synthesis [60]. The spherical shaped TiO2 particles have a

small size (~5 nm) with narrow size distribution. TiO2 nanoparticles have high surface area

of 177m2 g−1 with bandgap of 3.3 eV. The spherical TiO2 nanocrystals have good

photocatalytic activity in the degradation of methylorange.

Zhou reported the preparation of nanocrystalline mesoporous Fe-doped TiO2 powders

red by the ultrasonic-induced hydrolysis reaction of tetrabutyl titanate (Ti(OC4H9)4) in a

ferric nitrate aqueous solution [61]. The photocatalytic activities of Fe-doped TiO2 powders

were investigated by the photocatalytic oxidation of acetone in air. The high activities of the

Fe-doped TiO2 powders was observed due to synergetic effects of Fe-doping and large

specific surface area of catalyst.


4.8. PbO2

Our research group synthesized β-PbO2 nano-powder by the ultrasonic irradiation of an

aqueous suspension of dispersed β-PbO (Pure yellow orthogonal phase), as precursor, in the

presence of ammonium peroxydisulfate as an oxidant [62]. The effect of pararmeters such as

oxidant concentration, temperature and ultrasonic wave amplitude on the morphology,

reaction rate and composition of products were investigated. The reaction rate increased with

an increase in temperature and ammonium peroxydisulfate concentration. It was found that

the applied ultrasonic wave determines the particle size. PbO2 samples prepared under

optimized experimental conditions have lead dioxide particles in the range of 50–100 nm.

It was observed that the use of Pb(NO3)2, instead of the lead precursor β-PbO, resulted in

the formation of PbSO4, which precipitated out at the end of the reaction. Thus, the oxidation

process should be initiated with β-PbO. When ultrasonic waves were applied to β-PbO

particles, only mechanical milling occurred and the particles were cracked. β-PbO was not

oxidized under these conditions, even with an increase in the duration of ultrasonication. In

fact, a proper oxidant is necessary to convert β-PbO to PbO2. In the presence of ammonium

peroxydisulfate, the increased concentration of hydroxyl radical facilitated the oxidation of β-

PbO to PbO2 under ultrasonic irradiation. The XRD results reveal that only β-PbO2 is formed

under optimum conditions. When the reaction mixture was stirred instead of ultrasonically

irradiated, only a fraction of the lead oxide was converted to lead dioxide, and lead sulfate

was the main reaction product.

4.9. Other Metallic Oxide

Other metallic oxides such as bismuth oxide [63], lead oxide [64 65], magnesium

oxide66, molybdenum oxide [67], mercury oxide [68], Tungsten oxide [69] and tin oxide[70]

have been synthesized by sonochemistry methods in the past several years.

Figure 8. SEM of lead dioxide (β-PbO2) samples prepared from a solution containing 0.2 g β-PbO and 5

g (NH4)2S2O8 at 60 ºC and ultrasonic amplitude of 84µm (Diffrent magnification).


4.10. Rare-Earth Oxide

Wang et al. in 2002 used a sonochemical method to prepare CeO2 nanoparticles [71].

After that, Miao et al. succeed to synthesis of CeO2 nanotubes by a sonochemical method

under ambient air in alkali aqueous solution without any template [72]. The CeO2 nanotubes

had diameters of 10–15nm and length of 150–200 nm. They also showed that when 3 M KOH

aqueous solutions is used, only rod-like assemblies composed of CeO2 nanoparticles with the

size of 5 nm were appeared. The CeO2 nanotubes were obtained only at concentration higher

than 5 M KOH.

In 2007, Zhang et al proposed a sonochemical method to synthesis of Polycrystalline

CeO2 nanorods with 5-10 nm in diameter and 50-150 nm in length at room temperature [73].

Polyethylene glycol (PEG) was used as a structure-directing agent. In the absence of PEG, the

agglomerated nanoparticles was formed instead nanorods. To a solution of Ce(NO3)3

containing 1 g of PEG600, NaOH solution (0.005 g/mL) was added gradually (5 mL/min)

under ultrasonication for ~1 h at room temperature until the pH value was 10. TEM images of

the CeO2 showed nanorods with the clear (111) and (220) lattice fringes with the interplanar

spacing of 0.31 and 0.19 nm, respectively. The UV-vis absorption spectrum of the CeO2

nanorods exhibits a strong absorption band at the UV region due to the charge transfer

transitions from O 2p to Ce 4f bonds. The BET specific surface area of CeO2 nanorods was

calculated 154.5 m2 g-1 in comparison with 5.7 m2 g-1 of commercial CeO2.

Nanoparticles of the single (Eu3+, Dy3+, Tb3+), double(Eu3+/Dy3+, Eu3+/Tb3+,

Dy3+/Tb3+), and triple (Eu3+/Dy3+/Tb3+) doped Gd2O3 (gadolinium oxide) nanoparticles

were prepared via a sonochemical technique [74]. The particles sizes were in the range of 15

to 30 nm. The triple doped samples showed multicolor emission on single wavelength

excitation.

5. THE SONOCHEMICAL SYNTHESIS OF MIXED OXIDES

Mixed Oxides such as aluminates, molybdates, manganates and etc. have been found

many applications in sensors, electrooptic and electromagnetic devices because of their

prominent properties. The sonochemical method is one of the simple route have been used

during the last years to prepare nanostructure mixed oxides.

5.1. MVO4

Much of work on metal vanadate is focused on bismuth vanadate (BiVO4) because

BiVO4 has been recently recognized as a strong photocatalyst for water water decomposition

and organic pollutant decomposing under visible light irradiation due to its narrow band gap

75.

A facile sonochemical approach has been developed for the synthesis of BiVO4

photocatalyst by Zhou and his coworkers. In a typical preparation, aqueous solutions of

Bi(NO3)3 and NH4VO3 were mixed together in 1:1 molar ratio and exposed to high-intensity

ultrasound irradiation for 60 min. The average crystal size of as-prepared BiVO4 particles is

ca. 50 nm and samples exhibited surface areas of ca. 4.16 m2/g. The as-prepared BiVO4


nanocrystals had strong absorption in visible light region with an obvious blue-shift compared

with that of the bulk sample. The band gap was estimated to be ca. 2.45 eV. BiVO4

nanocrystals showed high photocatalytic activities to decolorization of methyl orange under

visible light (λ > 400 nm).

Shang et al. used polyethylene glycol (PEG 20000) as surfactant[76]. An aqueous

solutions of Bi(NO3)3 and NH4VO3 in 1:1 molar ratio as well as polyethylene glycol (1 g)

was exposed to high-intensity ultrasonic irradiation (6 mm diameter Ti-horn, 600W, 20 kHz)

for 30 min in ambient condition. The pH value was adjusted to about 7 by NH3. Nanosized

BiVO4 consisted of small nanoparticles with the size of ca. 60 nm. The nanosized BiVO4

exhibited excellent visible-light-driven photocatalytic efficiency for degrading Rhodamine B

(RhB) with good stability. When the RhB solution was irradiated with visible-light (λ > 420

nm) in the presence of calcinated well-crystallized BiVO4 sample, about 95% of RhB was

degraded after being irradiated for 30min and the spectral maximum shifted from 552 to 500

nm.

The lanthanide orthovanadate LnVO4 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,

Tm, Yb, Lu) nanoparticles had been prepared from an aqueous solution of Ln(NO3)3 and

NH4VO3 without any surfactant under ultrasonic irradiation[77]. It was observed that the

morphology of the LnVO4 nanoparticles was affected strongly by ultrasonic irradiation. The

as-formed LnVO4 particles have a spindle-like shape with an equatorial diameter of 30-70

nm and a length of 100-200 nm. Each particles (as aggregates) are composed of smaller

nanoparticles of 10-20 nm.

The sonochemical synthesis of Lanthanide orthovanadates RVO4 ( R = La, Ce, Nd, Sm,

Eu and Gd) was reported in the presence of Polyethylene glycol (PEG-900) and amphiphilic

triblock copolymer Pluronic P123 as structure-directing agents at room temperature [78].

When the P123 surfactant was used, the Lanthanide orthovanadates with nanorod shape was

observed. With the surfactant PEG, nanorods of NdVO4, nanospindles of GdVO4 and

nanoparticles of other orthovanadates were obtained.

5.2. MTiO3

The metal tiatanates, BaTiO3, PbTiO3, and PbTiO3 have been reported to be synthesized

by sonochemical methods [79]. Wang and his coworker prepared PbTiO3 fine powders with

narrow size distribution (40–60 nm) by a sol-gel method with lead acetate Pb(OCOCH3)2,

tetrabutyl titanate Ti(OBu)4 as precursors via ultrasound irradiation.

The formation of BaTiO3 particles was reported by a Japanese group [80]. They used

ultrasonic irradiation to form narrow size distribution of aggregated particles. This method

caused to formation of the aggregation of the original 5–10 nm BaTiO3 particles. It is thought

that under ultrasonic irradiation, Ti-based sol forms by the hydrolysis of TiCl4 in Ba2+

aqueous solution. Ti ions dissolve form the Ti-based sol to form Ti(OH)62- octahedron and

the nucleation of BaTiO3 occurs around the Ti-based sol. Ultrasound influences the synthesis

of BaTiO3 particles mainly through acceleration the dissolution of Ti ion from Ti-based sol

and the nucleation of BaTiO3 particles.

Xu et al. developed a sonochemical method for the synthesis of spherical BaTiO3

nanoparticles by sonicating a strong alkaline solution including BaCl2 and TiCl4 [81]. They


showed that the reactant concentration influence the particle size. This group also synthesizes

the SrTiO3 nanoparticles with the same sonochemical method.

5.3. MAl2O4

Metal aluminate, MAl2O4 (M = Cu, Zn and Mg), has been also prepared by

sonochemical methods [82]. Spinel copper aluminate (CuAl2O4) nanoparticles were prepared

by sonicating an aqueous solution of copper nitrate, aluminium nitrate, and urea. Upon

heating at 900 ◦C for 6 h, the precursor formed nanosized CuAl2O4 particles with an average

size of 17nm. The BET surface area of CuAl2O4 anoparticles was about 110 m2 g−1. The

photochemical catalysis degeneration of methyl orange on CuAl2O4 noparticles as

photocatalyst was investigated under the irradiation of 125W Hg lamp (λ > 400 nm). The rate

of the methyl orange degradation was measured to be as high as 98% in 2 h.

They reported another work that investigate the effect of processing conditions on

preparation of nanosized copper aluminate (CuAl2O4) spinel using Cu(NO3)2 and Al(NO3)3

as starting materials and urea as a precipitation agent at a concentration of 9 M [83]. The

reaction was carried out under ultrasound irradiation at 80 ºC for 4 h and a calcination

temperature of 900 ºC for 6 h.

High surface area MgAl2O4 has been synthesised by a sonochemical method. Two kinds

of precursors were used, alkoxides and aluminium nitrates/magnesium acetates in the

presence and absence of cetyl trimethyl ammonium bromide (CTAB). In the case of alkoxides

precursors the as-made product is a mixture of hydroxides of aluminium and magnesium, and

after heating at 500 ºC pure MgAl2O4 phase was not obtained [84]. While with

nitrates/acetates a gel is obtained after sonication, containing the metal hydroxides and

ammonium nitrate. Heating at 500 ºC transforms the as-made products into MgAl2O4 spinel

phase with the surface area of 267 m2/g. In the case of nitrates/acetates precursors, the CTAB

reduces the formation of large stable aggregates.

Zinc Aluminate ZnAl2O4 and Zinc Gallate ZnGa2O4 doped with Mn2+ and some of

lanthanide ions such as Dy3+, Tb3+, Eu3+ were synthesized through a sonochemical process

85. Photoluminescence studies were done on prepared samples. The doped samples showed

multicolor emission on single wavelength excitation.

5.4. MWO4

PbWO4 nanostructures with different morphologies, such as polyhedral, spindle-like, and

dot-shaped, have been synthesized via a mild sonochemical route from an aqueous solution of

lead acetate and sodium tungstate (Na2WO4) in the presence of complexing reagent

nitrilotriacetate acid (NTA) [86]. H3NTA is a precursor of a multidentate organic ligand

(NTA3-), incorporating carboxylic acid groups and one N-donor atom, capable of

coordinating to several metal centers. The mechanism of the formation of PbWO4

nanocrystals is probably related to the coordination of Pb2+ and NTA3- to form Pb-NTA

complex. To explain what has been occurred in the reaction vessel, it was suggested that Pb-

NTA complex is formed due to coordination of Pb2+ and NTA3-. In the presence of


ultrasound irradiation (20 kHz, 60 W/cm2), the dissociation of the complex was occurred and

PbWO4 was formed. The mechanism is summerised as follow:

32 NTAPbPbNTA (16)

4

2

4

2 PbWOWOPb (17)

Different shapes of PbWO4 nanocrystals, i.e. polyhedral, spindle-like, and dot-shaped

morphologies were obtained with controlling the pH value and the amount of complexing

reagent. A pH range of 5-9 is optimal. Figure 9a shows that the products prepared at pH 9.0

are polyhedrons with dimension of (400-500) nm × (600-700) nm. At the pH value was

decreased of 7.0, the homogeneous spindle-like nanorods with diameters of 50-60 nm at the

center and lengths of about 200-250 nm (Figure 9b) was obtained. Figure 9c shows the dot-

shaped product prepared with a pH value of 5.0. The average size of these polycrystalline

particles is about 10 nm.

Under pH 7-9, nitrilotriacetic acid exists as NTA3- and the predominant species in

solution remains a 1:1 complex of PbNTA-. With pH decreasing, NTA3- would partly

combine H+ in the solution. When pH was lower than 5, NTA3- would exist as HxNTAx-3

and its complexing ability with metal ions would therefore decrease. At the pH > 11, another

complex, Pb(OH)x2-x, was formed instead of PbWO4 due to the strong complexing ability

between Pb2+ and OH-. Room-temperature photoluminescence of PbWO4 nanocrystals

showed green emissions at 480-500 nm with different luminescence intensity. The optical

properties of these PbWO4 nanocrystals differ from those of the bulk crystals.

In another work, Geng et al. prepared nanosized lead tungstate (PbWO4) hollow spindles

via a sonochemical process by using triblock copolymer Pluronic P123- (EO20PO70EO20) as

a structure directing agent [87]. The concentration of polymer had vital role in preparation of

PbWO4. Hollow PbWO4 nanospindles were obtained in the polymer concentration of 4 gL-1.

PbWO4 hollow spindles can be formed by templating the P123 micellar aggregates induced

by the ultrasonic irradiation. Pb2+ ions in the solution are easily attracted on the micellar

surfaces by forming Pb-(PEO-PPO-PEO) units and provide nucleation domains for the

subsequent reaction between Pb2+ and WO42- to form PbWO4 nanoparticles.

Figure 9. TEM images of samples prepared at pH values of (a) 9.0, (b) 7.0, and (c) 5.0. The initial

concentrations of Pb2+

, WO42-

, and H3NTA were 20, 20, and 40 mM, respectively.


The same procedure was used to preparation of ZnWO4 nanorods [88]. ZnWO4 nanorods

were successfully synthesized via powerful ultrasonic irradiation. An aqueous solution of

sodium tungstate (Na2WO4) was slowly added to a solution of zinc acetate, 3 g P123, 20mL

ethanol and 200mL deionized and Ultrasound irradiation was applied to solution by a high-

intensity ultrasonic probe, 20 kHz, 250 W/cm2. The photocatalytic activity of ZnWO4 in

degradation of rhodamine-B (RhB) under 365nm UV light illumination was investigated.

Also, Metal tungstates (MWO4, M = Ba, Sr and Ca) were synthesized using the

corresponding M(NO3)2 and Na2WO4 in ethylene glycol by ultrasonic irradiation [89]. Their

average sizes of round shaped nanoparticles of metal tungstates were 42.0 ± 10.4, 18.5 ± 5.1

and 13.1 ± 3.3 nm for M = Ba, Sr and Ca, respectively.

5.5. MoO4

Lead molybdate (PbMoO4) and lead tungstate nanoparticles were synthesized from

solution of Pb(NO3)2 and Na2MO4 dissolved in 50 ml ethylene glycol by applying

ultrasound waves for 1 h [90]. The particle sizes were 29.09 ± 5.22 nm and 21.05 ± 2.68 nm

for PbMoO4 and PbWO4, respectively.

Bismuth molybdate (α-Bi2Mo3O12 phase) nanorods were synthesized by pyridine

intercalative sonochemical method [91]. Spherically α-Bi2Mo3O12 powder was dissolved in

pyridine and sonicated at 30–40°C under nitrogen atmosphere, for varying time periods (2, 4,

6, 8, and 10 h). The diameter of the α-Bi2Mo3O12 nanorods were about 10 nm and length in

few hundreds of nanometer to μm after sonicating in pyridine for 6 h. The controlled heating

of pyridine-intercalated nanorods to 450 °C was resulted in a-Bi2Mo3O12 phase nanorods

free of pyridine.

5.6. Ferrites

Ferrites are widely used in ferrofluid technology, magnetic resonance imaging, drug

delivery and data storage. The synthesis of spinel ferrites MFe2O4 ((M = Mn, Co, Ni, Cu and

Zn)) such as copper ferrite (CuFe2O4) [92] and zinc ferrite (ZnFe2O4) [93a] were reported.

Sivakumar et al. [93b] used a ultrasound assisted emulsion (consisting of rapeseed oil and

aqueous solution of Zn2+ and Fe2+ acetates) and evaporation protocol to synthesize zinc

ferrite (ZnFe2O4) nanoparticles (Figure 10). The as-synthesized sample consisted of

crystalline zinc ferrite particles with an average diameter of ~4 nm and the heat-treated ferrite

particles (350 °C for 3 h) with ~12 nm.

The small amount of oil present on the surface of the as-synthesized ferrite sample was

removed by heat treatment at 350 °C for 3 h.

Ferrites with formula MFeO3 also were reported. Das et al. reported the preparation of

nanosized BiFeO3 powders by sonochemical technique [94].

Nanocrystalline rare earth orthoferrites MFeO3 (M=Gd, Er, Tb and Eu) were prepared by

Sivakumar et al. using Fe(CO)5 and rare earth carbonates precursor through sonochemical

method [95]. A distinct advantage of the sonochemical method is the preparation of

nanocrystalline orthoferrites at a remarkably reduced calcination temperature. The magnetic

properties of different orthoferrites were reported.


Figure 10. TEM of as-prepared and heat-treated ZnFe2O4 nanocrystals (scale bar is 20 nm).

Same authors reported a sonochemical method for preparation of strontium hexaferrite by

a sonochemical method employing Fe(CO)5 and SrCO3 [96]. A SrCO3 hexagonal rod was

synthesized using strontium nitrate and urea in the presence of ultrasonic irradiation.

Stoichiometric amounts of SrCO3 and Fe(CO)5 was dissolved in decalin and irradiated with

ultrasound (using the titanium horn tip with power of 29.7W/cm2) in an air atmosphere at

0°C for 4 h to get the strontium hexaferrite powder. The resultant precursor was then calcined

at 900°C for 14 h in air atmosphere, which is lower than the conventional solid-state reaction

(1300°C). It was suggested that the application of ultrasound on the Fe(CO)5 generates

amorphous Fe2O3. The amorphous Fe2O3 was then dispersed or coated on SrCO3 during the

ultrasound irradiation. SrFe12O19 exhibited an intrinsic coercivity field (Hc) of ~4600 Oe

and a saturation magnetization (Ms) of ~60 emu/g at 20 K and ~32 emu/g at 300 K. The Hc

value remains more or less temperature independent over the 20–300K range. The

magnetization vs. temperature pattern exhibits strong temperature dependence over a range of

300–800 K probably due to the presence of single-domain nanoparticles and consequent

superparamagnetism.

6. NANOCOMPOSITES

Sonochemistery is one of the techniques have been used to synthesis different categories

of nanocomposites such as inorganic/inorganic and inorganic/organic materials [97].

6.1. Metal Oxide-Metal (Oxide) Nanocomposite

Perkas and et al. used sonochemical irradiation of iron (II) acetate aqueous solution in

presence of silver nanopowder to deposite magnetite nanoparticles on silver nanocrystals

[97]. The crystalline size of silver nanoparticles was calculated as 50 nm and magnetite

nanoparticles ~ as 10 nm. Ag-Fe3O4 composite was well arranged in the series of chains

(Figure 11 a and b).


Figure 11. (a) initial nanosilver powder (b) TEM images of Ag–Fe3O4 composite obtained by

sonochemical method and (C) HRTEM image of Ag–Fe3O4 composite.

The characterization of the product reveals the presence of two phases of the silver and

the magnetite without any chemical interaction between them. It was suggested that local

melting of silver occur when the magnetite nucleus is thrown at the silver surface by high

speed sonochemical microjets and this is probably the phenomenon causes the anchoring of

magnetite to the nanosilver surface. The total saturation magnetization of the composite is

rather low – 1.8 Emu/g. However, it most considered that only 5.2 wt% of the nanocomposite

is corresponded to Fe3O4 and its magnetization would be about 35 Emu/g Fe3O4. The Ag-

Fe3O4 nanocomposite showed superparamagnetic behavior in a magnetic field.

Pradhan et al. also synthesized gold-magnetite nanocomposite materials via sonochemical

methods (Figure 12) [98]. Magnetite nanoparticles (1 mg suspended in 100 μL of methanol)

with diameter of ca. 30 nm were added to a 50 mL solution of 0.1 mM HAuCl4 (aq)

containing methanol (100 μL), diethylene glycol (100 μL), or oleic acid (100 μL) as solvent

modifiers sparged with argon during the experiment [99]. The solution was then sonicated in

a jacketed, water cooled (20 C) reaction vessel under an argon atmosphere for 10 min at 50%

amplitude using an ultrasonic processor. The resulting solution was then transferred into a test

tube and kept in front of a magnet. The gold–magnetite nanocomposite material was pulled

against the wall of the test tube by the magnet.

The coercivity of the treated magnetite was 75 Oe, while the gold–magnetite

nanocomposite material exhibited a coercivity of 200 Oe. The changes in magnetic properties

are likely due to changes in the surface characteristics of the magnetite. Gold could contribute

to changes in the surface states and magnetic properties.


Figure 12. TEM images of gold–magnetite nanocomposite material formed by sonication of magnetite

in aqueous HAuCl4 with added (a) methanol (b) oleic acid additives. Dark particles are gold, grey

particles are magnetite.

Also, during sonication, the capping ligands can be removed and cause a change in the

surface charge or magnetic domains. The additives were also found to change the gold

particle loading and the Fe/Au ratio in the composite materials. With oleic acid added,

substantially smaller gold particles were observed, and the Fe/Au ratio was intermediate

between that of the materials prepared with methanol and diethylene glycol.

Mizukoshi et al. reported immobilization of noble metal nanoparticles (Au, Ptand Pd) on

the surface of maghemite with irradiation of aqueous solutions containing noble metal ions

(HAuCl4, Na2PdCl4, H2PtCl6), polyethyleneglycol monostearate (PEG-MS), and magnetic

maghemite nanoparticles [100]. The noble metal ions were reduced by the effects of

ultrasound, and uniformly immobilized on the surface of the maghemite. XRD patterns of

prepared nanocomposites showed peaks originated from 111 planes of noble metals with

peaks of maghemite. TEM images showed that the diameters noble metal particles depended

upon the concentration of PEG-MS, pH of the solution and the concentration of noble metal

ions, but not upon the maghemite concentration. The average diameter of immobilized Au

was 7–13 nm, and the diameters of Pd and Pt were several nm. It was suggested that the

nucleation of noble metal occurred in the homogeneous bulk solution and then the nuclei

were immobilized on the surface of the Maghemite. Then, the growth of noble metal nuclei

were continuing on the surface of the maghemite.

Another work was reported by Mizukoshi et al. which prepared the magnetically

retrievable palladium/Maghemite nanocomposite catalysts by sonochemical reduction method

[101]. Such a catatalyst show high catalytic activities for the reduction of nitrobenzene and

could be readily retrieved by magnets and verified the durability of the catalytic performance.

Mizukoshi et al. also introduced Au/γ-Fe2O3 composite nanoparticles which could

selectively adsorb sulfur-containing amino acids [102]. Adsorbed amino acids were

successfully manipulated by applying all external magnetic fields.

Nanocomposites of Ag nanoparticles/mesoporous γ-Al2O3 were synthesized by

sonochemical method [103]. The as prepared product consisted of Ag nanoparticles dispersed

in the bayerite [Al(OH)3]/boehmite [AlO(OH)] matrix. The Ag nanoparticles were

incorporated in a mesoporous structure of γ-Al2O3 upon calcination of product under Ar

atmosphere at 700 °C for 4 h. For a nanocomposite containing 3.7 wt % Ag nanoparticles, the

BET surface area is more than that of γ-Al2O3 because the Ag nanoparticles remained on the


surface of mesoporous alumina whereas for 10.5 wt % Ag nanoparticles, the BET surface

area decreased. In this case, the Ag nanoparticles blocked the pores, and also increased the

diameter of the pores of mesoporous alumina.

Insertion of Pt nanoparticles into Mesoporous (MSP) CeO2 reported by an ultrasound-

assisted reduction procedure [104]. With incorporation of highly dispersed Pt into the CeO2

(MSP) by the sonochemical method, the specific surface area, pore volume and size of the

CeO2 support decreased. The observed changes could be attributed to incorporation of the

metallic Pt particles in the CeO2 interparticle volume. Pt/CeO2 nanoparticles have excellent

properties in EA combustion at low temperature. The catalytic activity of these catalysts was

higher than that of the Pt catalysts on the CeO2 support prepared by the classic incipient

wetness- impregnation method. It was demonstrated that the higher the dispersion of the

CeO2 support and the Pt phase, the better the catalyst properties. Ultrasonic technique causes

to the homogeneity and better dispersion of the Pt in CeO2 support.

The best results with 100% selectivity to CO2 at the lowest temperature were achieved

with the Pt catalysts sonochemically incorporated into the mesoporous CeO2 support

previously synthesized by the ultrasound method.

ZnO nanorod/Ag nanoparticle composites was synthesized by ultrasonic irradiation of a

mixture of ZnO nanorods, Ag(NH3)2+, and formaldehyde in a aqueous solution 105. TEM

images of ZnO/Ag composites reveal that the ZnO nanorods are coated with spherical Ag

nanoparticles with a mean size of several tens nanometer and fcc structure.

The sonochemically synthesized Pt (Pd) nano-particles (~2 nm) were impregnated into

zirconia (3 mol% yttria-stabilized zirconia, 3Y-TZP) nano-aggregates (20–45 nm) (Figure 13)

106.

Figure 13. TEM of 3Y-TZP porous nano-aggregates impregnated with 1.5 wt.% of platinum.

As shown in Figure 13, the primary crystallites with an average size of ~5nm are

aggregated and nanoaggregates with a mean aggregate size of 20–40 nm are formed. With

low temperature sintering (1150 °C for 30 h), it can be possible to produce the Pt–3Y-TZP


and Pd–3Y-TZP (0.5–2 wt. % of platinum) nano-composites with uniform distribution of the

Pt (Pd) grains (in the range of 20–60 nm) and with a zirconia average grain size of 120 nm.

Bhattacharyya and Gedanken reported [107] the preparation of γ-Al2O3-doped porous

ZnO nanocomposite by sonochemistry. The nanoparticles of γ-Al2O3 partially or fully block

the pores of porous ZnO.

6.2. Organic-Inorganic Nanocomposite

The sonochemical assisted syntheses of organic-inorganic nanocomposites have been

prepared by some researchers[108].

6.2.1. Natural Fibers

Perelshtein et al prepared CuO-cotton nanocomposite and investigate its antibacterial

activity [108]. Copper oxide nanoparticles (~ 10–15 nm) were synthesized and subsequently

deposited on the surface of cotton fabrics using ultrasound irradiation. The antibacterial

activities of the CuO-fabric composite were tested against Escherichia coli (Gram negative)

and Staphylococcus aureus; (Gram positive) cultures. The antibacterial effect is due to the

copper oxide nanoparticles. CuO nanoparticles can generate some active species that are

responsible for damaging the bacteria's cells.

Figure 14. (a) HR-SEM images of pristine cotton fabric coated with CuO nanoparticles (magnification

×20,000). (Inset: magnified image (×100,000) of the nanoparticles coated the fiber).

In a similar work, silver nanoparticles were deposited on the surface of natural wool

fibers under ultrasonic irradiation [109]. The sonochemical irradiation of slurry containing

wool fibers, silver nitrate, and ammonia in an aqueous medium for 120 min under an argon


atmosphere yielded a silver-wool nanocomposite. The average silver particle size was 5-10

nm, but larger aggregates of 50-100 nm were also observed. Silver adhere strong to the wool

through the interaction of silver with sulfur moieties related to the cysteine group.

6.2.2. Polymeric Based Nanocomposites

6.2.2.1 Poly(Methylacrylate) and Poly(Methylmethacrylate)

Preparation of ceria nanoparticles embedded in polymethylmethacrylate (PMMA) has

been reported by means of sonochemistry [110]. An average size of the ceria is found to be

similar to 5 nm by XRD and TEM measurements. In Ceria–PMMA composite, the band gap

found is 3.55 eV.

Parra et al. studied the preparation of composite materials based on PMMA with

nanometric hydroxyapatite (Ca10(PO4) 6(OH)2) under ultrasonic radiation for different times

[111]. In the synthesis of Hydroxyapatite (HA), ammonium phosphate [(NH4)2HPO4] and

calcium hydroxide [Ca(OH)2] were used as precursors. The precursors of the HA and

commercial PMMA in 2-butanone were placed simultaneously in a reactor under ultrasonic

radiation at 20 kHz, for periods of 15, 25 and 35 min. Composite materials were obtained

from the in situ synthesis of hydroxyapatite (HA) in dissolved PMMA.

Figure 15. TEM bright field image of PMMA/ HA composites.

The FTIR spectra showed the interactions between the ester group of PMMA and the

phosphates groups of HA. Hydroxyapatite particles encapsulated in a thin film of PMMA,

forming ‗‗pockets‘‘ of the composite material (Figure 15). XRD results show the formation of


HA in the amorphous PMMA matrix. The appearance of broad peak in XRD implies

nanometric crystal size of HA.

In another reported work, wang et al. sonochemically prepared polyacrylamide and

gamma-zirconium phosphate (Zr-P) nanocomposites by intercalative polymerization 112

.

6.2.2.2. Polystyrene

Polystyrene (PS)/Fe3O4 nanocomposite were prepared with miniemulsion

polymerization of styrene in the presence of Fe3O4 nanoparticles under ultrasonic irradiation

[113]. Each (PS)/Fe3O4 nanoparticles and PS latex with no encapsulated Fe3O4

nanoparticles were found in PS/Fe3O4 magnetic emulsion. The nanoparticles are spherical

and their size was in the range of 20 to 80 nm. PS/Fe3O4 emulsion and nanocomposite

exhibit magnetic properties and can be separated from the magnetic emulsion by an external

magnetic field.

In another paper, Kai et al. proposed a method based on sonochemistry to prepare

silver/PS nanocomposite [114]. The preparation of Ag/PS nanocomposite was achieved by

dispersion polymerization of styrene in a water–ethanol (1/6 wt/wt) solution, with poly(N-

vinyl pyrrolidone) (PVP) as stabilizer and 2,2'-azobisizobutyronitrile (AIBN) as initiator in

the presence of nano-silver particles under ultrasonic irradiation with a power output of 300

W at 20 kHz for 3.0 h. The monomer conversion and polymerization increased when nano-

silver particles was added to reaction vessel. The conversion of monomer can reach about

70% in 3.0 h. When bare nano-silver particles without pretreatment are introduced into the

polymerization medium, polystyrene particles are covered with some small silver beads

(because of their high hydrophilicity) and complete encapsulation does not occur. It was

observed that in the presence of the surfactant, SDS, no silver bead is detected on the whole

surface of the samples and the nano-silver particles are encapsulated in the polystyrene

particles.

6.2.2.3. Polypropylene

Deposition of silver nanoparticles on porous Polypropylene (PP) polymer was

investigated by an ultrasound-assisted reduction of AgNO3 in the presence of poly(vinyl

pyrrolidone) (PVP) as stabilizing agent [115]. PVP prevent the agglomeration of the reduced

silver nanoparticles. With PVP, a homogeneous distribution of silver nanocrystals with 50 nm

in size and a relatively high silver content (0.5–0.6 wt % Ag) was formed on the PP beads

surface. It was suggested that microjets formed during the bubble collapsing can throw the

silver nucleus to the polymer‘s surface and cause to local melting of the PP at the collision

sites. At collision sites, the thermal degradation of polymer chains cause to the formation of a

small amount of pure carbon.

The appearance of the high-intensity bands characteristic of pristine carbon at ~1344 and

1580 cm-1 in the Raman spectra of silver coated PP for after coating PP with nanosilver was

caused by the localized melting of the polymer at their points of contact with silver

nanoparticles.

XPS studies also showed the presence of Ag (0) in silver-coated polymer PP. The peaks

observed in the energy region of the Ag 3d transition are symmetric and centered at 367.9 and

373.9 eV. Antimicrobial test show that the beads of the silver PP composite have high

antibacterial activity against microorganisms.


6.2.2.4. Conducting Polymer

Among conductive polymer, polypyrrole and polyaniline have been used to prepare

nanocomposite with different materials such as nobel metal and metal oxide [116]. Colloidal

dispersions of hybrid nanocomposite composed of gold and platinum nanoparticles (Au- and

Pt-NPs) and polypyrrole (PPy) were prepared by a sonochemical method, in which metal ion

and pyrrole monomer in an aqueous solution were reduced and oxidized, respectively, by

ultrasonic irradiation in the presence of sodium dodecyl sulfate (SDS). TEM of Au-NPs show

small Au-NPs dispersed in PPy matrix and the average diameter of Au-NPs/PPy are 15 nm

(Figure 16).

Figure 16. TEM image of (a) Au-NPs/PPy (b) Pt-NPs/PPy nanocomposite prepared by ultrasonic

irradiation for 4 h.

The authors also investigated the effect of poly(N-vinyl-2-pyrrolidone) as a stabilizer in

preparation of Au-NPs/PPy (Figure 16 b) 117

.

Same authors also preapared polypyrrole-encapsulated platinum nanoparticles (PPy/Pt-

NPs) by a sonochemical synthesis and used it as catalysts for the liquid phase hydrogenation

of substituted alkenes in methanol or methanol/water mixtures 118

.

The nanocomposites of polyaniline/silver 119

, polyaniline/Au 120

, polyaniline/Y2O3 121

and

polyaniline/Fe3O4 122

have been synthesis with the aid of ultrasonic irradiation.

The ultrasonically synthesis of PANI/Fe3O4 nanocomposite was reported with in situ

polymerization of aniline in the initially neutral medium and in the presence of Fe3O4

nanoparticles and oxidant. Fe3O4 nanoparticles were dispersed on the nanoscale by ultrasonic

irradiation and the polymerization of aniline was begun by the addition of an oxidant,

ammonium persulfate (APS). PANI deposited on the surfaces of the Fe3O4 particles and all

Fe3O4 nanoparticles was encapsulated with PANI. The maximaum conductivity of PANI was

obtained with 2:1 molar ratios of oxidant to aniline. The conductivity and magnetic properties

of the PANI/Fe3O4 composite can be controlled by the Fe3O4 content so that with increasing

the Fe3O4 content, a decrease in the conductivity and an increase in the magnetic properties of

the PANI/Fe3O4 composite were observed. The decrease of the conductivity is raised from the

addition of nonconducting Fe3O4 nanoparticles.


Polyaniline (PANI) nanotubes containing Fe3O4 nanoparticles were also synthesized

under ultrasonic irradiation of the aqueous solutions of aniline, ammonium peroxydisulfate

(APS), phosphoric acid (H3PO4), and the quantitative amount of Fe3O4 123

.

6.3. Carbonaceous Nanocomposite

Kawaoka et al. used sonochemical method to synthesized amorphous manganese oxide

and acetylene black (HSMO/AB) [124]. A solution containing 1.20 g of NaMnO4 was

dissolved in 750 ml of deionized water and 0.65 g of acetylene black (AB) was irradiated

with ultrasound (600W of total power and 100 kHz in frequency) was for 6 h in air

atmosphere. The acetylene black particles were homogeneously coated with amorphous

manganese oxide with ca. 35 nm in diameter (Figure 17). The thickness of the amorphous

substance varied from 1 to 10 nm. Energy dispersive spectrometer (EDS) analysis showed

that the amorphous substance was composed of Carbon, oxygen, sodium, and manganese. It

was suggested that AB particles were coated with the amorphous phase of hydrated sodium

manganese Oxide (HSMO).

Figure 17. Low-resolution TEM of HSMO/AB nanocomposite.

The capacity of the HSMO/AB nanocomposite tested under large current density, 10 A g-

1, is 185 mAh g-1. At operating voltage of 2.5 V, the power and energy density are 20 kW

kg-1 and 90 Wh kg-1, respectively.


This Japanese group also published another article in which the preparation of manganese

oxide/carbon nanocomposite was described [125]. They optimized the synthesis conditions,

such as the reaction temperature and pH and specific surface area of the carbon. The

manganese oxide/carbon nanocomposite was used as the cathode material of a high-power

lithium-ion battery. They showed that the use of a carbon with a higher specific surface area

caused to the higher specific capacity and lower capacity drop.

In other work, Cao et al used the sonochemical method to prepare mesoporous carbon-

tin oxide (SnO2) nanocomposite [126]. The resulting nanocomposite is consist of SnO2 with

3 nm in size dispersed on ordered mesoporous carbon.

Jang et al prepared nanocomposites between β-WC (also known as WC1−x ) and Pd

nanoparticles supported on carbon [127]. When a Pd-loaded GKB (Graphitic Ketjen Black),

Pd/C, obtained by reducing PdCl2 with NaBH4 in the presence of GKB, was used as the

support, a nanocomposite composed of Pd and β-WC nanoparticles was obtained by a

sonochemical decomposition of W(CO)6 followed by heat-treatment. By varying the amount

of W(CO)6 in the sonochemical reaction, two samples with different W-contents denoted as

β-WC(12)/Pd/C and β-WC(39)/Pd/C were synthesized. β-WC is a high temperature phase,

stable above 2785 ◦C. The conventional synthesis method such as carburization of tungsten

oxide precursors cannot generate such high temperatures and most of studies on tungsten

carbides have been restricted to W2C or α-WC whereas nanoparticles of β-WC can be

synthesized by a sonochemical reaction method due to extreme conditions generated by this

method. This is an evidence of preference of sonochemistry to other conventional methods.

The prepared tungsten carbide–palladium nanocomposites with different amount of W were

examined as hydrogen oxidation reaction (HOR) catalysts. It was discussed that when too

excessive W was deposited, a part of the deposit reacts with the Pd nanoparticles forming a

Pd–W alloy which showed much lower HOR activity than non-alloy Pd nanoparticles.

Nanocomposite based on carbon nanotube has been considered by some researchers

during last years[128]. After preparation β-WC/Pd/C nanoparticles, Jang et al also used the

similar ultrasonic method to prepare composite of Pt and WC1−x nanoparticles supported on

multiwalled carbon nanotube (MWNT). They investigated the electrochemical properties of

WC1−x/Pt/MWNT nanocomposite especially for the hydrogen oxidation reaction (HOR).

The synthesis of carbon nanotube (CNT)-supported Rh nanoparticles was reported by a

sonochemical method [129]. For this purpose, 20 mg of carboxylate-functionalized MWNTs

were dissolved in 20 mL ethanol, and the solution was sonicated for 1 h. The well-dispersed

CNT solution was added to a solution with 400 µL of 0.1 M RhCl3 aqueous solution and 40

mg of borane morpholine complex (C4H12BNO) and sonication was continued for another

20 min. Borane morpholine complex has a milder reducing ability compared with NaBH4 and

produce Rh nanoparticles with a narrow size distribution. TEM image of sonochemical

synthesized CNT/Rh nanocomposite is shown in Figure 18.

As shown in Figure 18, rhodium (Rh) nanoparticles with an average diameter of 2.5 (0.7

nm is deposited uniformly on multiwalled carbon nanotubes (MWNTs). Also, it was shown

that without sonication and functionalized MWNTs, well-dispersed small metallic

nanoparticles cannot be formed and deposited on CNTs. The carboxylate groups of

functionalized MWNTs (acid washed) provide sites for anchoring the metallic nanoparticles.


Figure 18. TEM images of CNT-supported Rh nanoparticles prepared with sonochemical method.

The XPS spectrum of the Rh/MWNTs shows binding energies (BEs) at 307.2 and 312.1

eV related to BEs of Rh(3d5/2) and Rh(3d3/2), respectively, of Rh(0). The EDX analysis also

showed that the mass of Rh in the composite was 11.2 ±0.1 wt %. The catalytic activities of

the Rh/MWNT catalyst were investigated for hydrogenation of neat benzene and benzene

derivatives in comparison with commercially available Rh nanocatalysts at low-temperature.

Results show that complete ring saturation of polycyclic aromatic hydrocarbons (PAHs) can

be achieved under mild hydrogenation conditions using the Rh/MWNT. The catalytic activity

of the Rh/MWNT catalyst is much higher compared with a commercially available Rh

nanocatalyst.

Sonochemistry can be used to synthesize the metal oxide and carbon nanotube

nanocomposite. Zhang et al. used a simple sonochemical route to prepare CNT/CeO2. Firstly,

CNT was dispersed in a 0.05 g/mL Ce(NO)3 alcohol aqueous solution (Valcohol:Vwater =

1:1) with high-intensity ultrasonic radiation at room temperature [130]. Then, CeO2 was

deposited on CNT with slowly addition of NaOH aqueous solution to above solution. The

final pH value was 10. TEM of CNT/CeO2 composite shows many tiny interconnected grains

with average grain size of 4 nm. The selected area electron diffraction (SAED) pattern shows

a ring pattern corresponding to the face-centered cubic polycrystalline structure of CeO2.

Furthermore, some of articles were focused on preparation of Polymer/CNT

nanocomposites with sonochemistry [131]. Polyaniline/CNT and poly (methyl methacrylate-

co-n-butyl acrylate) (P(MMA-BA))/ carbon nanotubes (CNTs) are example of such

nanocomposites. It was shown that the MWNT/PANI nanocomposites causes enhanced

electric conductivity and thermal stability in comparison with pure PANI. Also, the smooth,

uniform, and flexible P(MMA-BA)/CNTs composite films were prepared from the composite


emulsion [132]. Tensile tests of film suggest that with the modulus and the yield strength of

composite film increased with increasing in the CNTs content.

6.4. Other Nanocomposite

Ghule et al. reported the synthesis of Ag/Bi2Mo3O12 nanocomposite by ultrasonic

method [133]. Silver nanoparticles with an average size of ca. 10 nm were uniformly

deposited on the surface of α-Bi2Mo3O12 nanorods. α-Bi2Mo3O12 nanorods (ca. 100 nm

diameter) was prepared by ultrasonication of preformed α- Bi2Mo3O12 spherical

nanoparticles (ca. 200 nm) in pyridine. To prepare Ag/Bi2Mo3O12 nanocomposite, a mixture

of alpha-Bi2Mo3O12 nanorods and Ag2O in pyridine was irradiated by ultrasonic wave.

Calcination of sample at 450 °C produces pyridine-free α-Bi2Mo3O12 nanorods with

deposited Ag nanoparticles.

7. NANOMATERIALS WITH CORE-SHELL MORPHOLOGY

Nanostructured materials with core-shell morphology can find many applications in areas

such as photonic crystals, catalysts, and biotechnology [134.] In the following paragraph

some of nanoparticls with core shell morphology are reviewed.

7.1. Nanoparticle with Metal Core

Nikitenko et al prepared the air-stable Fe/Fe3C nanocrystalline particles have by

sonicating of Fe(CO)5 in diphenylmethane solutions under argon and subsequently annealing

the as prepared amorphous products in an inert atmosphere for 2 h. Nanocrystalline particles

have a core-shell structure where a coating of Fe3C and carbon protects the body-centered

cubic Fe in the core from oxidation. The iron nanoparticles are coated by a crystalline shell

with a thickness of about 5 nm. The size of the particles, their composition and magnetic

properties could be controlled by changing the sonication conditions and annealing

temperature. Particles heated at 300 and 400 °C have narrow range distribution ca. 20-40 nm

and 20-100 nm and with round shape morphology. Particles annealed at 700 °C are composed

mainly of tetragonal and round particles. An increase in the annealing temperature to 800 °C

causes the formation of hexagonal and tetragonal particles. Material obtained under

appropriate conditions possesses a high saturation magnetization close to that of bulk iron

(Ms/M0 = 0.97-1.06) and good, soft magnetic properties (coercive field HC = 0.50-0.05 A m-

1).

Aluminum-oleic acid core-shell nanoparticles have been synthesized using the titanium-

catalyzed thermal decomposition of Alane N,N-dimethylethylamine in a 0.4 M toluene

solution in which the thermal energy was supplied via acoustic cavitation [135]. Titanium

(IV) isopropoxide was the catalyst of reaction. The aluminum-oleic acid core-shell

nanoparticles prepared with 3.8 mM oleic acid have spherical nanoparticles with an average

size of ~30 nm and a size distribution estimated at 20-70 nm. TEM of the second sample

prepared with the concentration of 11.4 mM oleic acid showed spherical nanoparticles of a


much smaller diameter with an average size of ~5 nm with a size distribution of 2-15 nm.

Oleic acid may act to cap the surface of the growing nanoparticles, thus limiting particle size.

7.2. Nanoparticles with Metal Oxide Core

Hu et al. reported the sonochemically synthesis of Fe3O4–FeP core–shell nanoparticles

with Fe3O4 core of 5–10 nm and FeP shell of 2–3 nm [136]. They also reported the

preparation of FeP hollow nanoparticles with outer-diameter of 5–10 nm and inner-diameter

of 3–8 nm (Figure 19). Trioctylphosphine (TOP) (as P source) is used to react with iron

pentacarbonyl for the formation of iron phosphide and trioctylphosphine oxide (TOPO) is

used to control the size and growth morphology of resulting materials. The TOPO/TOP

mixture was sonicated under aerobic condition in a sealed bottle at 65–70 ºC in water bath.

Figure 19. Characterization of core–shell Fe3O4–FeP particles, which have been sonochemically

synthesized for 4 h: (a) SEM image, (b) TEM image.


The core–shell Fe3O4– FeP particles and FeP hollow nanoparticles showed the M–H loop

at room temperature and low temperature under magnetic field up to 7 T. The M–H curve

shows a soft ferromagnetic behavior. For the core–shell sample, the low temperature

measurement shows that the coercivity reaches 500 Oe, which is smaller than 760 Oe of

sample FeP. Fe3O4 is magnetically a much soft material, at 10 K with coercivity ranging from

200 Oe for 4 nm to 450 Oe for 16 nm nanoparticles, the increasing of Fe3O4 results in a

decreasing of coercivity.

7.3. Nanoparticle with Sio2 Core

Morel et al. report a rapid sonochemical synthesis of monodisperse nonaggregated Core -

shell Fe3O4@SiO2 magnetic nanoparticles (NPs) 137

. The Fe3O4 NPs were prepared by

coprecipitation of Fe(III) and Fe(II) in alkaline solutions in the presence of ultrasonic

irridation. A freshly prepared mixture of 1.5 mmol FeCl3 and 0.75 mmol FeCl2 in 5 mL of

0.05 M HCl was rapidly injected via a fine plastic tube to 40 mL of 2 M ammonia solution

containing 0.01 M of hydrazine under power ultrasound at 30-32 °C in an argon flow.

OHOFeOHIIFeIIIFe 243 88)()(2 (18)

Fe3O4 NPs have smaller size and a narrow size distribution (4-8 nm) than the silent

reaction. Sonication of Fe3O4 NPs suspension in alkaline ethanol-water solutions of tetraethyl

orthosilicate (TEOS) cause to hydrolysis of TEOS and Fe3O4 NPs coated with silica are

prepared. The reaction is accelerated many-fold in the presence of a 20 kHz ultrasonic field.

Silica shell thickness of Fe3O4@SiO2 magnetic nanoparticles increase with sonication time.

TEM images shows an increase in the silica shell thickness is from 1.0 -1.5 nm after 1 h of

sonication to 3.0-3.5 nm after 3 h of ultrasonic treatment. Fe3O4@SiO2 NPs prepared with

sonochemistry exhibit a higher magnetization value than that for NPs obtained under silent

conditions. High speed of sonochemical coating prevents the magnetite from oxidizing.

The sonochemical preparation of FePt/SiO2 and FePt/ZnS/SiO2 core-shell was

demonstrated by Wang et al (Figure 20) 138. Silica microspheres were modified with a two

layer polyelectrolyte of aqueous poly (ethyleneimine) and poly (acrylic acid). In the case of

FePt/SiO2, 0.010 g of modified SiO2 in 12 mL of ethylene glycol was sonicated for about 10

min and platinum (II) acetylacetonate (Pt(acac)2) (0.060 g) and iron acetylacetonate (0.078 g

of Fe(acac)2 or 0.108 g of Fe(acac)3 were then added into the mixture and sonication was

continued for 2-4 h under an Ar gas flow. It was seen that, magnetic FePt nanoparticles with

size of 3-5 nm, forms a densely packed shell with uniform coating. It was demonstrated that

the amine and carboxylic functional groups in the polyelectrolyte layer provided nucleation

sites for FePt nanoparticles. To prepare FePt/ZnS/SiO2 core-shell, the unmodified silica was

precoated with ZnS, and then FePt nanoparticles nucleate on the ZnS shell. The thickness of

ZnS shell and FePt shell on FePt/ZnS/SiO2 is ~30 nm and ~15 nm, respectively.

FePt/SiO2 core-shell particles exhibit coercivity of 3.5 and 12.5 kOe at room temperature

when annealed under a high vacuum at 400 °C for 20 min and 600 °C for 10 min. the

coercivity of the FePt/ZnS/SiO2 core-shell sample annealed at 530 °C is 12.0 kOe.


Figure 20. TEM images of fcc phase FePt core-shell spheres: (a) average size ~240 nm FePt/SiO2 using

Fe(III)(acac)3 as iron precursor, (b) HRTEM image of dense 3-5 nm FePt clusters on the FePt/SiO2

shell (inset: ~120 nm FePt/SiO2 using Fe(II)(acac)2 as iron precursor, and SAED pattern of FePt shows

fcc phase), (c) FePt/ZnS/SiO2 (inset, the thickness of ZnS shell and FePt shell is ~30 nm and ~15 nm,

respectively), (d) HRTEM of FePt and ZnS nanoparticles on the shell .

Figure 21. (a) TEM of sonochemically prepared hollow MoO3 nanospheres after HF etching of

MoO3/SiO2 (before thermal annealing) (b) After thermal annealing at 350 °C.


Dhas and Suslick reported the synthesis of MoS2/SiO2 and MoO3/SiO2 core shell

materials uniform coating using sonochemical method 139

. Hollow shells of the MoS2 and

MoO3 were obtained with washing the MoS2- or MoO3-coated silica with 10% HF in aqueous

ethanol. Thermal annealing of MoO3/SiO2 at 450 °C before HF etching was caused to

conversion of hollow MoO3 spheres to truncated cubic hollow crystals (Figure 21).

7.4. Chalcogenide Core-Shell

Ultrasonic irradiation was employed to aqueous synthesis of CdTe/CdS core-shell

nanocrystals by using preformed TG-capped CdTe nanocrystals as template cores and

thiourea as the sulfur source [140]. It was found that ultrasound facilitated the decomposition

of thiourea, leading to the formation of gradient CdS shell on CdTe cores. The resultant core-

shell nanocrystals presented dramatically improved photoluminescence (PL) quantum yields

(QYs), 10 times higher than the original nanocrystals. In comparison with the original CdTe

nanocrystals, CdTe/CdS nanocrystals show bright emission with an obvious red shift of

spectra.

8. OTHER NANOMATERIAL

8.1. Metal Phosphate

Monetite (anhydrous calcium hydrogen phosphate, CaHPO4) with orderly layered

structure assembled by nanosheets was synthesized from solution containing Ca(NO3)2 and

NaH2PO4 in the presence of cetyltrimethylammonium bromide (CTAB) by a sonochemical

method [141]. The thicknesses of the nanosheets are 100-200 nm with the lateral sizes of

about 2 μm.

Hydroxyapatite (Ca10(PO4)6(OH)2, HAP), the prime constituent of tooth and bone

mineral was synthesized sonochemically from aqueous solution of Ca(H2PO4)2 and

glycosaminoglycans (GAGs) with adding saturated Ca(OH)2 aqueous solution [142]. The

mixture was irradiated for different time (0.5 h, 3 h and 5 h) by an ultrasonic cleaner at 40

kHz and 250 W. TEM images showed that nanoparticles with short rod-like shape with 20–50

nm length and 12–25 nm width or spherical shape with 10–25 nm were obtained.

Nanosized, platelike hydroxyapatite (HAp) was synthesized using a homogeneous

precipitation method under ultrasound irradiation[143]. The internal structure of these

platelike formations consists of specifically oriented and laterally connected HAp nanorods

with a length of about 500 nm and a diameter of about 100 nm (Fig . 22)

The SAED pattern indicates that the nanorods are single crystals. The FTIR spectrum

show the characteristic bands for PO43- appear at 472, 583, 601, 961, 1032, and 1108 cm-1.

XRD results also proved the appearance of HAp phases.

Yu et al. reported the preparation of lanthanide orthophosphate LnPO4 (Ln = La, Ce, Pr,

Nd, Sm, Eu, Gd, Tb, Dy, Ho) nanoparticles via ultrasonic irradiation of inorganic salt aqueous

solution under ambient conditions [144].


Figure 22. (a) TEM and (b) SAED pattern of a single nanorod of Hap.

TEM images show that the hexagonal structured lanthanide orthophosphate LnPO4 (Ln =

La, Ce, Pr, Nd, Sm, Eu, Gd) products have nanorod bundles morphology, while the tetragonal

LnPO4 (Ln = Tb, Dy, Ho) samples prepared under the same experimental conditions are

composed of nanoparticles. HRTEM micrographs and SAED results show that LnPO4

nanostructures are polycrystalline in nature. They also prepared Eu3+-doped LaPO4 samples

and investigated their photoluminescent properties. Eu3+-doped LaPO4 exhibit an orange–

red emission.

BiPO4 nanorods were successfully synthesized via a sonochemical method without any

surfactant under ambient air [145]. Nanorods have diameters of 40-60 nm and lengths of 2-

5µm, which shows a large aspect ratio of 50-80 (Figure 23). The BiPO4 nanorod have single-

crystalline nature with a preferential growth oriented along the (001) crystalline plane.

Figure 23. (a) SEM images of BiPO4 nanorods, (b) TEM image and SAED pattern recorded on a single

BiPO4 nanorod.

It was demonstrated that the synthetic parameters such as pH of solution and sonication

time effect on morphology of BiPO4. Uniform BiPO4 nanorods with a large aspect ratio could


only be obtained when the pH of reaction system was adjusted at range of 0.5-1. The

morphologies of the products is the result of the sensitive influence of the pH on the solute

concentrations ([Bi3+

] and [HnPO4(3-n)-

]).

8.2. Metal Carbonate

Sonochemistry has been used to prepared various type of nanostructure metal carbonate

such as BaCO3[146], CdCO3 147 and CeCO3OH [148]. Yang et al.[149] reported the

sonochemical preparation of MnCO3 submicrocubes and highly oriented MnCO3 nanocrystal

assemblies with an ellipsoidal morphology.

The aqueous solution containing MnCl2 and urea was sonicated with an ultrasonic probe

immersed directly in solution (operate at 20 kHz, ~ 80 W/cm2) at 80 °C for 90 min [150]. The

reaction was repeated in the presence of aerosol OT (AOT) and SDS (sodium dodecyl

sulfate).

In the presence of AOT as a surfactant, MnCO3 submicrocubes with sizes of about 500

nm were observed which is significantly smaller than those prepared without using AOT. In

the presence of SDS, highly oriented ellipsoidal assemblies were observed. The assemblies

were porous and constructed of ca. 5 nm nanocrystals. A thermal treatment of MnCO3 at 600

°C in air produces nanoporous Mn2O3. Thermal treatment of MnCO3 samples prepared

without surfactant and in the presence of AOT produced a nanoporous cubic Morphology.

The ellipsoidal morphology of sample prepared in the presence of SDS was retained after

decomposition of MnCO3 to Mn2O3.

8.3. Metal Fluoride

BaF2 nanocrystals doped with 5.0 mol% Eu3+ (BaF2:Eu3+) nanospheres [151] EuF3

nanoflower [152] and Dumbbell-like YF3 nanostructures [153] are metal fluorides family

were prepared by ultrasonic-assisted method. Rare-earth fluorides with controllable shapes

and sizes have attracted intense research interest due to their particular photoluminescence

properties and potential applications in optics, optoelectronics, biological labeling and

catalysis. EuF3 nanoflower prepared through the reaction of Eu(NO3)3 and KBF4 under mild

ultrasonic irradiation.

To prepare Dumbbell-like YF3 nanostructures, Y2O3 was dissolved in 10% dilute

HNO3. Then, 15.0 mL N,N-dimethylformamide (DMF) was added to the above solution

(water/DMF ratio of 5/15). NH4F (3 mmol) was added under stirring. Subsequently, the

mixed system was transferred into a 50 mL plastic flask and irradiated by ultrasonic wave

with 40 kHz and ultrasonic power of 100% (200 W) at 65 °C for 2 h. TEM images of sample

show a series of dumbbells each comprised of abundant nanorods. The morphology of the

product could be affected by the volume ratios of water/DMF in initial solutions. Table 1

summarizes the observed morphology depended on water/DMF ratio.


Table 1. morphology of YF3 as function of Water/DMF ratio

Water/DMF ratio Morphology of YF3 nanostructures

20/0 (pure water) spindle-like (particles with about 350 nm in length)

15/5 spindle-like

10/10 rod-like

5/15 dumbbell-like

0/20 (pure DMF) near-spherical

When the volume ratio of water/DMF was decreased to 15/5, the morphology does not

change markedly. After the volume ratio was decreased to 10/10, the spindle-like particles

with the thin in two ends and the thick in middle had converted into morphology. Further

decreasing the volume ratio of water/DMF to 5/15 produces particles. When reaction was

completed in pure DMF (0/20), the as-prepared product was composed of a great number of

nanoparticles with a mean diameter of ~30 nm.

8.4. Single-Walled Carbon Nanotube (SWCNT)

Jeong et al. reported a sonochemically method to prepare SWCNT. A solution of

ferrocene and p-xylene was mixture with silica powder (diameters of 2-5 mm) and irradiated

with ultrasonic waves by a 1/2-in. titanium tip 200-W probe pulsed 65% under ambient

conditions for 20 min [154]. Ferrocene is decomposed during sonication and provide Fe

nanoparticles as catalyst for nanotube growth. Also, p-xylene and ferrocene provide carbon

source for SWCNT growth and silica powder acted as a nucleation site for SWCNT growth.

Silica powder was broken into small pieces during the sonication. HF solution was used to

remove silica particles at end of reaction. It was found that high-purity SWCNTs were

obtained at relatively low concentration of ferrocene (0.01 mol %).

Li et al. reported the synthesis of new hydrocarbons (hydrocarbon nanotubes and nano-

onions) and carbon nanostructures (carbon nanotubes and nano-onions) via the sonochemical

reactions of organic solvents such as CHCl3, CH2Cl2, and CH3I on hydrogen-passivated

silicon nanowires (SiNWs) as templates under ambient conditions [155].

8.5. Polyaniline

PANI is one of the most important conducting polymers. PANI nanotubes and nanofibers

doped with different mineral (HNO3, H3PO4, HClO4) and organic acid (camphorsulfonic

acid) were synthesized under ultrasonic irradiation [156]. Ammonium peroxydisulfate was

used to oxidize aniline monomer. It was demonstrated that different dopant acids produced

PANI nanotubes (Figure 24) and nanofibers with similar morphology.

At low concentration of dopant acid, nanotubes could be observed but with high

concentration of dopant acid, nanofibers are formed. Also, the concentration ratio of [dopant

acid]/[aniline] effect the morphology of polyaniline nanostructures.


Figure 24. TEM of PANI nanotubes synthesized under ultrasonic irradiation doped by perchloric acid.

Li et al. reported the chemically synthesized polyaniline nanofiber via ultrasonic

irradiation [157]. The effect of various parameters such as ultrasonic power, frequency, and

reaction temperature was investigated on morphology of polymer. It was found that increase

of ultrasonic power (up to 250 W) or the reaction temperature (up to 75°C) produce PANI

nanofibers with more uniform diameters. The length or aspect ratio of PANI nanofibers

decreased with increasing ultrasonic power, whereas longer nanofibers with larger aspect

ratios were obtained at a higher polymerization temperature. Also, the polymers prepared at

higher frequencies showed higher purity; for example the polymer prepared at 50 kHz

showed the highest uniformity and smoothest surfaces.

8.6. Metal Chalcogenides

Sonochemistry provide a facile synthetic method to prepare nanostructures of metal

chalcogenides. Chalcogenides (S-2

, Se-2

, and Te-2

) of metal have semiconductive properties

and found extensive applications in various fields such as non-linear optic detectors,

photovoltaic solar cells and optical storage media. In next section, various types of

nanochalcogenides i.e. metal sulfides, selenides and tellurides are considered.

8.6.1. Metal Sulfides

Various types of metal sulfides have been synthesized based on ultrasonic techniques

such as ZnS [158], HgS 159, MoS2 [160],Ag2S [161], In2S3 [162]and CuS [163].

Among various types of metal sulfide CdS and PbS have been explored extensively

because of its unique properties 164. Lead sulfide (PbS) is an important π–π semiconductor

with small bulk band gap (0.41 eV at 300 K) and a larger excitation Bohr radius of 18 nm.

Also, CdS is one of the most important II–IV group semiconductors with narrow band gap of

2.4 eV and has received considerable attention in solar cells, catalysis, quantum size effect

semiconductor, optoelectronic devices, photo-electrochemistry and biological labeling.


Wang et al. reported the preparation of PbS hollow nanospheres with diameters of 80-250

nm through a surfactant-assisted sonochemical route (Figure 25). The shells of the hollow

spheres estimated to be around 20 nm and are composed of small PbS nanoparticles with

diameters of about 12 nm. An aqueous solution containing Pb(CH3COO)2, thioacetamide

(TAA), and sodium dodecylbenzenesulfonate (DBS) were transferred to a ultrasonic cleaning

bath (49 Hz, 50W) and sonicated for 4 h. It was suggested that DBS has the tendency to self-

aggregation and to form vesicles with different sizes under the ultrasound wave, which

directly determine the diameters of the spheres. As DBS is an anionic surfactant, the surface

of spheres has negative charge and Pb2+ ions are easily attracted on the vesicle surfaces. The

adsorbed ions provide nucleation domains for the subsequent reaction between Pb2+ and H2S

to form PbS nanoparticles. The formation process of PbS nanoparticles is suggested as

follows:

OHHOH

)))

2 (19)

)(2 232 CSNHCHRSRSHRSH (20)

PbSPbS 22 (21)

nPbSnPbS )( (22)

The sonochemical process produces H2S gradually, which avoids the rapid reaction and

causes that PbS nanoparticles grow on the surface of spheres.

In the absence of surfactant only irregular PbS rods were observed. Moreover, when

cetyltrimethyl ammonium bromide (CTAB) was used as surfactant, well-crystalline PbS rods

with of 0.3-0.4 µm in width and 3.5-7 µm in length were observed. Table 2 and 3 summarize

some of synthetic parameters and morphological structure of PbS and CdS, respectively.

Figure 25. (a) TEM and (b) HRTEM images of the PbS hollow spheres.


Table 2. The conditions of preparation of PbS nanostructures

Table 3. The conditions of preparation of CdS nanostructures

Metal

sources

solvent Sulfur

sources

template

or

structure

directing

agent

Ultrasonic parameters Morphology

(Size)

Ref. no.

CdSO4 H2O

and

(CH3)2

CHOH

Na2S2O3 Hydroxyethyl

cellulose

(Mw,

123,000)

40-kHz ultrasonic wave at 100-

W output power at room

temperature

Nanoparticles

, nanowires

and

dendritic-like

shape

[170]

CdCl2 H2O Na2S Polyvinylpyrr

olidone K30

(PVP)

Pulse sonication

(ton=6s, toff=14s),

100 W, 20 kHz, for 30 min to 2 h

Nanoparticles

(3-5 nm)

[171]

CdCl2 H2O Na2S Polyacrylica

mide

CdCl2 solution was added

into the Na2S solution within 5

min under ultrasonic condition

Hollow

nanoparticle

chains

[172]

Cadmium

acetate

H2O Na2S Amino-acid

histidine

(as chelating

agent)

Sonochemical bath (33 kHz, 350

W) at room temperature at

different

ultrasonic irradiation time

Nanoparticles [173]

Metal sources solvent Sulfur sources template

or

structure

directing agent

Ultrasonic

parameters

Morphology

(Size)

Ref.

no.

Pb(CH3COO)2

ethanol,

distilled water,

ethylene glycol

and

polyethylene

glycol-200

Thiourea -

high-intensity

ultrasound

irradiation

under ambient

air for 30 min

Different

morphologic

al shapes

depend on

solvent type

[165]

PbCl2 Water, pH=7 Na2S2O3

EDTA as

complexing

agent

titanium horn,

20 KHz,

40Wcm-1 for

4 h at room

temperature

Nanobelts [166]

Pb(CH3COO)2 Water Thioacetamide CTAB

ultrasonic

cleaning bath

(49 Hz, 50 W)

for 2 and 4 h

nanocubes,

nanorods

and

nanotubes

[167]

Pb(CH3COO)2 Water Thioacetamide

Nitrilotriacetic

acid as capping

agent

ultrasonic

cleaning bath (

40kHz 250 W)

for 40 min at

40 and 70 °C

Dentritic

and star like

Nanostructur

es

[168]

Pb(NO3)2 Water Thioacetamide

Polyethylene

glycol-6000 as a

kind of capping

polymer

High-intensity

ultrasound

irradiation (50

kHz, 100 W)

under ambient

air

Nanorods [169]


8.6.2. Metal Telluride

Metal tellurides are another category of metal chalcogenides that sonochemistry have

been used extensively to prepare them [174].

Mercury telluride (HgTe) nanorods (with diameters of ~15 nm and lengths of up to 200

nm) and nanoparticles were synthesized via sonochemical method from mercury perchlorate

hydrate and tellurium powder in an ethylenediamine solvent system in the presence of 1-

thioglycerol as a complexing agent.

Zheng et al. prepared Nanocrystalline Bi2Te3 by sonochemical methods at 70 °C using

Te and BiCl3 as the reactants and NaBH4 as the reductant [175]. The prepared powders

consist of granular and flake nanoparticles. The size of the particles are about 10~20 nm. It

was observed that the addition of EDTA suppressed the formation of pure Bi2Te3 phase.

EDTA form complex compounds with Bi3+ ions, decreasing the activity of Bi3+ ions in the

solution and hindering the formation of Bi2Te3.

Bi2Te3 hexagonal nanoflakes with controllable edge length ranging from ~150 nm to as

small as ~10 nm were synthesized via an ultrasonic-assisted disproportionation route, using

Te powder and Bi(NO3)3 in the mixed solvent of glycerol and water or ethylene glycol (EG),

or EG containing certain amount of polyvinyl pyrrolidone (PVP, K-30) [176]. The reaction

mechanism may be as followed:

OHTeOTeOHTe 2

2

3

2 3263 (23)

32

23 32 TeBiTeBi (24)

Te2-

is produced in the disproportionation of Te in alkaline solution react with Bi(III) to

give out Bi2Te3. The ultrasonic irradiation accelerates the reaction rate due to the

mechanochemical effects of ultrasound waves and cause to formation of relatively small and

uniform nanocrystals. The size of the Bi2Te3 nanoflakes changed when using different

solvents. Also, When PVP was added into EG, the size of the nanoflakes decreased. PVP

could adsorb onto the faces of the nanoflakes and hinder their growth, resulting in the

formation of nanoflakes with smaller size.

8.6.3. Metal Selenide

Some of metal selenide have been recently synthesized are Ag2Se, HgSe, CdSe, Bi2Se3.

Table 4 present some of works focused on metal selenides and summarized their synthetic

parameters under ultrasonic irradiation.

8.7. Coordination Polymers

The ultrasonic method is also expected to be useful, but few instances have been reported

for nano structures coordination polymers. Some papers have been reported by Morsali and

co-workers. Hedge balls nano-structure of a new Pb(II) two-dimensional coordination

polymer, [Pb(3-pyc)(N3)(H2O)]n (1), {3-Hpyc =3-pyridinecarboxylic acid}, have been

synthesized using a thermal gradient approach and by sonochemical irradiation [182].


Table 4. The conditions of preparation of MeS nanostructures

Metal

sources solvent

Selenium

sources

template

or

structure

directing agent

Ultrasonic

parameters

Morphology

(Size)

Ref.

no.

AgNO3 Water Se powder

NH3, citric

acid or KSCN

as complexing

agents

Pulse sonication with

Ti-horn at 500 W.

(time of irradiation in

every reaction was 50

working cycles ton=

60 s and toff=10 s)

Ag2Se

Nanoparticles [177]

Hg(Ac)2

polyol

solvent such

as ethylene

glycol,

diethylene

glycol and

polyethylene

glycol 200

Se

powder -

Pulse sonication with

a

high-intensity

ultrasonic probe Ti-

horn, 20 kHz, 80

W/cm2)( ton= 27 s

and toff=3 s)

Taper shaped

HgSe nanorods [178]

CdCl2 Water,

pH=10 Na2SeSO3 β-cyclodextrin

High-intensity

ultrasonic titanium

horn (20 KHz, 75

W/cm2) under

ambient conditions

for 20 min.

Hollow

spherical CdSe

quantum

dot assemblies

[179]

BiCl3 Water H2SeO3 -

High-intensity

ultrasound

(59 kHz, 45 W) for

15 h at 25 °C

Bi2Se3

nanobelts (8–

10nm in

thickness, 20–80

nm in width, and

several

micrometers in

length)

[180]

Bismuth

nitrate

Water,

pH=11 Na2SeSO3

EDTA as

complexing

agent

High-intensity

ultrasonic probe 20

kHz, 60 W/cm2) for

1 h

Bi2Se3

nanoparticles [181]

Single-crystal X-ray diffraction of compound 1 shows a two-dimensional polymer with

the coordination number seven for Pb(II) ions. Calcination under air produces nano-sized

particles of PbO. Reduction of the particle size of the supramolecular compound to a few

dozen nanometers results in a lower thermal stability when compared to single crystalline

samples.

A new nanostructured Bi(III) supramolecular compound, {Bi2(4,4΄-Hbipy)1.678(4,4΄-

Hbipy)0.322(μ-I)2I5.678]•(4,4΄-bipy)} (1), 4,4΄-bipy = 4,4΄-bipyridine } was synthesized by a

sonochemical method [183]. Calcinations of compound 1 under two different atmospheres,

air and nitrogen, results in nano-structures Bi2O3 and BiI3.

Some others coordination polymers from La (II) [184], Mn(II) [185], Bi(III) [186], Pb(II)

187 with different ligands and morphologies have been prepared.


CONCLUSION

The sonochemistry is a new area of research has been considered during last years

because of its simplicity and possibility of operating under ambient conditions.

Sonochemistry have been proven to be a useful route for the preparation of novel materials

with unusual structures and properties which found their application in various technological

applications such as sensors, optoelectronic device, photocatalyst, fuel cells and energy

storage device and etc. The advantages of this method include a rapid reaction rate,

controllable reaction conditions, and the ability to form materials with uniform shapes,

narrow size distributions, and high purities. Also, sonochemistry provide easy conditions for

synthesize of some materials that other methods can not able to operate at these conditions.

The chemical effect of ultrasonic irradiation arises from the acoustic cavitation which is the

formation, growth, and implosive collapse of bubbles in the liquid medium. The implosive

collapse of the bubbles generates local hot spots or shock wave formation within the gas

phase of the collapsing bubble. These local hot spots produce high temperature (~ 5000 K)

and pressure (~1800 K) which provide a unique environment for the growth of materials with

novel structures.

ACKNOWLEDGMENTS

The authors gratefully acknowledge M. Yousef Elahi for her assistance in preparation of

this chapter. Also, the cooperation of M. A. Kiani, A. Abbasi and Z. Bagheryan is

acknolewdeged.

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Chapter 2

INDUSTRIAL-SCALE PROCESSING OF LIQUIDS BY

HIGH-INTENSITY ACOUSTIC CAVITATION: THE

UNDERLYING THEORY AND ULTRASONIC

EQUIPMENT DESIGN PRINCIPLES

Alexey S. Peshkovsky1 and Sergei L. Peshkovsky Industrial Sonomechanics, LLC, New York, NY 10040, USA

ABSTRACT

A multitude of useful physical and chemical processes promoted by ultrasonic

cavitation have been described in laboratory studies. Industrial-scale implementation of

high-intensity ultrasound has, however, been hindered by several technological

limitations, making it difficult to directly scale up ultrasonic systems in order to transfer

the results of the laboratory studies to the plant floor. High-capacity flow-through

ultrasonic reactor systems required for commercial-scale processing of liquids can only

be properly designed if all energy parameters of the cavitation region are correctly

evaluated. Conditions which must be fulfilled to ensure effective and continuous

operation of an ultrasonic reactor system are provided in this chapter, followed by a

detailed description of "shockwave model of acoustic cavitation", which shows how

ultrasonic energy is absorbed in the cavitation region, owing to the formation of a

spherical micro-shock wave inside each vapor-gas bubble, and makes it possible to

explain some newly discovered properties of acoustic cavitation that occur at extremely

high intensities of ultrasound. After the theoretical background is laid out, fundamental

practical aspects of industrial-scale ultrasonic equipment design are provided, specifically

focusing on:

electromechanical transducer selection principles;

operation principles and calculation methodology of high-amplitude acoustic horns

used for the generation of high-intensity acoustic cavitation in liquids;

1 Correspondence to: Alexey S. Peshkovsky, Ph.D., Industrial Sonomechanics, LLC, 150 Bennett Avenue, Suite

5K, New York, NY 10040, e-mail: [email protected].

Alexey S. Peshkovsky and Sergei L. Peshkovsky 64

detailed theory of matching acoustic impedances of transducers and cavitating

liquids in order to maximize the ultrasonic power transfer efficiency;

calculation methodology of ―barbell horns‖, which provide the impedance matching

and can help achieving the transference of all available acoustic energy from

transducers into the liquids. These horns are key to industrial implementation of

high-power ultrasound because they permit producing extremely high ultrasonic

amplitudes, while the output horn diameters and the resulting liquid processing

capacity remain very large;

optimization of the reactor chamber geometry.

1. INTRODUCTION

A multitude of important physical and chemical processes promoted by ultrasonic

cavitation can be implemented on industrial scale by utilizing high-capacity flow-through

ultrasonic reactor systems. These systems permit processing large volumes of liquids and

commonly comprise an ultrasonic-frequency electrical signal generator, an electromechanical

transducer, which converts the electrical signal into an ultrasonic vibration, an ultrasonic

horn, which amplifies and transmits the vibration into the liquid, and a flow-through reactor

chamber (flow cell), which contains the flowing liquid. A general schematic of such system is

presented in Figure 1 [1, 2]. Several conditions must be fulfilled in order to ensure effective

and continuous operation of an ultrasonic reactor system:

a) technologically necessary intensity of ultrasonic cavitation must be achieved in the

liquid;

b) size and homogeneity of the cavitation region formed in the liquid must be

maximized (well developed cavitation region);

c) reactor chamber design should be such that all of the liquid is directed through the

cavitation region (no liquid bypass);

d) utilized electromechanical transducer must be electrically save, capable of

continuous operation for extended periods of time, and able to provide high radiation

power levels;

e) ultrasonic horn must be capable of amplifying the vibration amplitudes (high gain),

while maintaining maximum possible size of the resulting cavitation region (large

output diameter);

f) mechanical stresses present in the electromechanical transducer and the ultrasonic

horn must not approach the limiting values for the fatigue strength of the

corresponding materials;

g) entire system as well as each of its components must not be in danger of becoming

overheated during continuous operation at full power.

High-quality engineering calculations of the ultrasonic reactor system components can

only be properly performed if all energy parameters of the cavitation region itself are

correctly evaluated, since this region represents the active acoustic load of the

electromechanical transducer (through the ultrasonic horn) and is the target "consumer" of all

produced ultrasonic energy.

Industrial-Scale Processing of Liquids by High-Intensity Acoustic Cavitation … 65

Figure 1. Schematic of the Ultrasonic Reactor System is presented. 1 – ultrasonic electrical generator, 2

– electromechanical transducer, 3 – ultrasonic horn (in this case, a barbell horn), 4 – mounting flange, 5

– reactor chamber, 6 – working liquid inlet, 7 - working liquid outlet.

We will, therefore, start by providing a detailed model of acoustic cavitation, explaining

the mechanism by which the ultrasonic energy is absorbed in the cavitation region. A

discussion of the design principles of the main ultrasonic reactor system components will

follow.

2. SHOCK-WAVE MODEL OF ACOUSTIC CAVITATION

In the design and calculation of powerful ultrasonic sources for ultrasonic reactors, it is

necessary to know the exact value of the intensity of acoustic energy radiated into the

working liquid. This information is usually obtained experimentally because no adequate

physical model of acoustic cavitation that would allow one to obtain such data through

calculation exists. The development of an adequate model of acoustic cavitation, although of

great importance, has in the past been severely restricted by considerable mathematical

difficulties connected with the necessity of finding numerical solutions of nonlinear equations

describing the cavitation region (the visible region of large cavitation bubble population) [3].

The utilized direct analytical solutions of these equations in different approximations do not

give practical results suitable for the design of ultrasonic equipment [4, 5].


The literature on acoustic cavitation mainly tends to involve numerical models of spatio-

temporal characteristics of the cavitation region [6-8]. Large number of theoretical acoustic

cavitation models has been developed along with the corresponding methods of numerical

analysis of such models. Computer simulation-based investigations of acoustic cavitation

have also been proposed, involving complex non-linear physicomathematical models and

including many aspects of spatial movement of cavitation bubbles in an acoustic field, spatial

distribution of the characteristics of these fields in a liquid, interaction between the bubbles

themselves, properties of acoustical flow, etc [9-12]. Water is most frequently used for the

experimental verification of such theoretical models.

No adequate explanation of the mechanism by which dissipation of the primary acoustic

energy of a radiator occurs in a liquid at cavitation is, however, available from the literature.

Additionally, no theoretical method permitting to calculate this energy in a manner adequate

to the available experimental data currently exists. Meanwhile, the exact knowledge of the

mechanisms by which the heating of a liquid in the presence of a cavitation-inducing acoustic

wave occurs is quite important not only for the understanding of the related sonochemical

processes, but also for the practical design parameter calculations that would permit

constructing improved high-capacity ultrasonic radiators and reactors.

2.1. Visual Observations of Acoustic Cavitation

Several authors provided common [13], high-speed [14] and stereoscopic high-speed [15]

photographs of the cavitation region, obtained in the presence of relatively low-intensity

acoustic fields. At these conditions, the cavitation region is located some distance away from

the radiating surface and has a typical pattern similar to that of an electrical discharge.

Photographs of the cavitation region formed by powerful ultrasonic radiators have also

been provided [16, 17]. The diameters of the radiating surfaces of the radiators were greater

than the sound wavelengths in the given liquid at the working frequencies. In these cases,

plane acoustic waves are radiated into the liquid. The photographs show that at relatively low

acoustic radiation intensity, the cavitation region is also located some distance away from the

radiating surface, has an irregular pattern and is composed of thread-like collection of

cavitation bubbles. As the radiation intensity goes up, however, the cavitation region

approaches the radiating surface and grows in size. When the intensity reaches the value of,

approximately, 1.5 W/cm2, the cavitation region ―sits‖ on the radiating surface and its shape

starts to resemble an upside-down circular cone. The so-called ―cone bubble structure‖ begins

to form. Further radiation intensity increases have little effect on the shape and position of the

cone bubble structure. The photographs in the abovementioned studies show that at high

radiation intensity the cone bubble structure is in contact with the radiating surface. Reference

[18] provides photographs of the radiating surface of a metal radiator which was utilized for a

period of time to create high-intensity cavitation in a liquid. The surface of the radiator

contains clear traces of metal degradation due to cavitation.

Therefore, it can be concluded with certainty that at high radiation intensities, acoustic

cavitation starts at the surface of the acoustic radiator. This location in the liquid is known,

according to theory, to have the lowest value of tensile strength due to the constant presence

of adsorbed gas inclusions at the metal surface [4].


However, at low radiation intensities just above the cavitation threshold, the cavitation

region is always formed at a significant distance away from the radiating surface, which

contradicts the abovementioned theory. Clearly, the tensile strength of the liquid at any

location away from the metal surface should be higher than near it, since the concentration of

the preexisting bubbles (inceptions) that ―weaken‖ the liquid at that location should diminish

with time.

2.2. Justification for the Shock-Wave Approach

At low radiation intensity, harmonic acoustic wave is not capable of inducing cavitation

even at the weakest location in the liquid near the radiating surface. Formation of cavitation

away from the radiating surface in this case can be explained by the effect of the increase of

the planar acoustic wave-front steepness during its propagation through a liquid. As a result

of such an increase, at some location in the liquid a discontinuity in the wave profile is

formed. Since such discontinuity is physically not possible in a continuous media, a shock-

wave with a steep front is formed as a result. This effect has to do with the acoustic radiation-

induced nonlinearity of the compressible media properties and is very well known and

documented [19].

This explanation, however, seems contradictory to the common shock-wave theory, since

the attainable amplitude of vibration velocity of the radiating surface is always much lower

than the speed of sound in the pure liquid and, therefore, the necessary conditions for the

creation of such a discontinuity in the wave profile are not fulfilled. The explanation may,

nevertheless, still be valid due to the following two considerations. It is well known that

during propagation of an acoustic wave of slightly lower intensity than the cavitation

threshold, an ensemble of tiny bubbles is formed in the liquid. This occurs due to the so-

called ―rectified diffusion‖ [4]. It is also well known that the speed of sound in a liquid

containing gas bubbles is significantly lower than that in a pure liquid [20, 21], and, under

certain conditions, it may become similar to the amplitude of vibration velocity of the

radiating surface.

It may, therefore, be considered that the bubbles formed in an acoustic wave due to

rectified diffusion help forming a discontinuity in the profile of the acoustic wave at a

location away from the radiating surface by significantly lowering the sound speed in the

liquid. Further, at the location of the discontinuity in the acoustic wave, these tiny bubbles

begin to undergo such rapid nonlinear movements that they loose dynamic stability and,

consequentially, rapidly multiply forming the cavitation region.

The abovementioned observations and analysis formed the basis of the shock-wave

model of acoustic cavitation described in this section. The model shows how the primary

energy of an acoustic radiator causing the cavitation of liquid is absorbed in the cavitation

region owing to the formation of spherical shock waves inside each cavitation bubble.

Calculation of the total energy absorbed in the cavitation region using the concept of a

hypothetical spatial wave moving through the cavitation region is possible with this model

using the classical system of the Rankine-Hugoniot equations. Additionally, the proposed

model makes it possible to explain some newly discovered properties of acoustic cavitation of

water that occur at extremely high oscillatory velocities of the radiating surfaces.


2.3. Theory

Let us assume that an acoustic radiator emitting a plane-wave is used to generate

cavitation in a liquid. The diameter of the radiator‘s output surface is comparable with the

length of the acoustic wave in the liquid at the given frequency of vibrations. The frequency

of the acoustic radiator vibrations should be considered to be much lower than the resonance

frequency of the cavitation bubbles. We assume that the liquid always contains an equilibrium

concentration of dissolved gas as well as some cavitation nuclei (tiny spherical bubbles filled

with the gas) and, consequentially, the liquid possesses no tensile strength during rarefaction

caused by the acoustic waves. As, for example, indicated in the reference [4], water that has

not been purified of gas inclusions ruptures at the negative acoustic pressure of,

approximately, 1 bar. The density of the liquid with the tiny cavitation nuclei is taken to be

equal to the density of the pure liquid, ρf. Surface tension of the liquid and the presence of

stable (non-cavitational) gas bubbles are neglected. Thus, within the framework of the model,

only the so-called low-frequency transient gas cavitation is considered. We, additionally,

assume the liquid to be non-viscous, non-compressible and non-volatile.

Let us represent acoustic cavitation in the liquid as a sequence of the following events.

When an acoustic rarefaction wave passes through a volume of the liquid, an explosive

growth of cavitation nuclei occurs, leading to the formation of gas-filled cavitation bubbles.

Possible parameters of such a rarefaction wave are described, for example, in [22]. A mixture

of the spherical bubbles and the liquid is, therefore, formed. The gas dissolved in the volume

of the liquid passes inside the free space formed by the bubbles. The density of the liquid

medium, therefore, drops. At this point, the bubbles are so small, compared to the acoustic

wavelength, that the liquid/bubble mixture can be considered a continuous medium. The

rarefaction wave phase is followed by a compression wave phase, whose passage results in a

collapse of all gas bubbles, restoring the density of the liquid to ρf. The reverse diffusion of

the gas back into the liquid during compression is insignificant and should be ignored. This

particular stage of acoustic cavitation completes the total cavitation cycle and is further

considered here in great detail, since it is this stage that is mainly responsible for the

sonochemical effects of acoustic cavitation.

2.3.1. Oscillations of a Single Gas Bubble

The problem of the liquid motion during compression of an empty spherical bubble in

liquid was solved by Rayleigh (see reviews [4, 5]). On the basis of this solution and Ref. [19],

the instantaneous pressure distribution in the liquid can be written as:

4

22

2

2

UUrUpp ff

(1)

Here, p∞ is the pressure in the liquid at infinity, U is the velocity of the bubble boundary

(wall), ξ = R/r, r is the current bubble radius, and R is the current radial coordinate. For the

boundary of a gas-filled bubble at ξ = 1, the following equality must be met:


)2

3( 2UrUpp fg

(2)

Here, pg is the gas pressure in the bubble. This expression is the well-known Noltingk-

Neppiras equation (see reviews [4, 5]).

For an empty bubble, taking pg = 0 and p∞ = p0, integration of equation (2) gives

Rayleigh‘s equations for the velocity of the bubble wall movement and the time of the bubble

collapse:

)1r

r(

3

p2U

3

3

in02

f

(3)

5.0

0

f

inp

r915.0

Here, p0 is the static pressure, and rin is the initial bubble radius.

From equations (1) and (2), an expression for the instantaneous pressure distribution in

the liquid during the compression of a gas-filled bubble can be obtained:

)11

(2

Up)

11(pp

4

2

fg

(4)

Let us single out a spherical liquid volume that includes a gas bubble. The gas

bubble/surrounding liquid system has a certain acoustic compressibility, which determines the

velocity of the propagation of small perturbations or the velocity of sound in this volume.

Using the linearized form of the Noltingk-Neppiras equation, one can obtain an expression for

the velocity of sound in such a system, as it was done, for example, in the work [21]. The

velocity of sound, with the abovementioned assumptions taken into account, is determined

using the following expression:

5.0

f

g)

)1(

p(c

(5)

Here, α is the volumetric gas concentration in the singled-out liquid volume that includes

the gas bubble. From equation (5) it can be seen that the velocity of sound at a given gas

pressure in the bubble has a minimum at α = 0.5. For example, at pg = 1 bar the minimum

velocity of sound cmin = 20 m/s. It should also be noted that the velocity of sound in the range

0.4 < α < 0.6 changes little.

A gas bubble is formed during the half-period of the liquid rarefaction in the acoustic

wave. Under the abovementioned assumptions, this occurs at the moment when the pressure


in the liquid near the wall of a cavitation nucleus decreases to zero, i.e. the negative acoustic

pressure is equal to p0. At that point, the gas pressure in the formed bubble is also very small.

Further, during the subsequent period of increase in the acoustic pressure, the bubble is

compressed, and the gas pressure in it also increases. During the subsequent compression

half-period, in the singled-out liquid volume near the gas bubble wall a spherical flow in the

direction of the bubble center is formed, which is described by equation (4). From equation

(5) it is seen that the velocity of sound for the singled-out system gas bubble/surrounding

liquid depends on the gas pressure in the bubble pg and the value of coordinate ξ, along which

the boundary of the singled-out volume passes. If we start reducing the singled-out volume,

while the radius of the bubble and the gas pressure in it are constant, the velocity of sound in

this system will fall to a certain limit and then will grow again. This means that in the

considered spherical volume near the moving wall of the bubble, there is a critical spherical

region, where the sound velocity, cmin, is at the minimum at a given gas pressure in the

bubble, pg. The position of this region is determined from the condition 0.4 < α < 0.6. It is

located close to the bubble wall in the coordinate range 1.18 < ξ < 1.35. For the simplicity of

further analysis of equation (4), it is taken that the velocity of the flow of the liquid particles

in the critical region is equal to the velocity of the bubble wall movement, U.

In the model being considered, it is assumed that when the gas bubble/surrounding liquid

system is compressed by the external pressure, p∞, the velocity of the flow of the liquid

particles in the critical region near the bubble wall increases to such a degree that at a certain

gas pressure in the bubble, pg, it reaches the minimum velocity of sound in the system under

consideration, i.e. U = cmin.

At a ratio of the initial radius of an empty bubble to its current radius, rin/r = 2, and static

pressure, p0 = 1 bar, the value of U ≈ 21 m/s reached according to equation (3) is indeed close

to cmin = 20 m/s.

Let us represent the pressure at infinity as a sum of the static and the acoustic (excessive)

pressures, p∞ = p0 +p′∞ and transform equation (4) taking into account that U = cmin:

)11

(p2p

)1

1)(pp(p4g

g

0

(6)

This expression describes the extreme condition of equilibrium of the system. Equation

(6) shows that during compression of the flowing liquid, in the vicinity of the gas bubble a

pressure impulse is formed, which is stationary with respect to the bubble wall. The amplitude

of the excess pressure in this impulse is p - p0 = 1.4pg + 0.5 δp′∞, where δp′∞ = (p′∞ - p0). This

value is reached at the coordinate ξ ≈ 2 located upstream from the critical region. As we show

below, the quantity, δp′∞, does not need to be considered for small oscillation velocities of

acoustic radiators.

When the velocity of the bubble wall motion exceeds the minimum velocity of sound, U

> cmin, the equilibrium state described by equation (6) becomes destroyed, and the pressure in

the liquid at the bubble wall downstream from the critical region decreases to p0. The velocity

of the bubble wall movement also reduces because the driving pressure difference decreases.


Figure 2. Instantaneous distribution of the excessive pressure in liquid near the cavitation bubble wall at

U > cmin is shown. The quantity δp′∞ is not taken into account.

At the same moment, the excessive pressure amplitude in the impulse increases stepwise

up to the value p - p0 = 1.4p0 + 0.5δp′∞, since the boundary condition in equation (2) is

changed and the pressure near the bubble wall becomes pg = p0. This occurs because the

bubble pressure signal does not penetrate upstream from the bubble wall when U > cmin.

Due to destruction of the dynamic equilibrium (retardation of a part of the flow), the

pressure impulse located in the liquid upstream from the critical section disintegrates and

begins to move relative to the bubble boundary in the form of a converging spherical wave.

The supposed instantaneous distribution of excessive pressure in the impulse near the gas

bubble wall at U = cmin is shown in Figure 2.

Phenomena similar in essence are observed during the breakup of arbitrary pressure

discontinuity in a gas, during hydraulic impact, and during the flow of gases and gas-liquid

mixtures through nozzles. See, for example, the works [6, 8], as well as the studies on Laval

nozzles and water hammers.

In accordance with the assumed form of pressure distribution in a converging spherical

wave shown in Figure 2, the excessive pressure at the bubble wall first increases smoothly up

to the value of p - p0 = 1.4pg + 0.5δp′∞, and, accordingly, the gas pressure inside the bubble

increases smoothly (isothermally) as well. Then, when an abrupt excess pressure jump (up to

the value of p - p0 = 1.4p0 + 0.5δp′∞) approaches the bubble wall, a spherical shock wave is

formed in the gas inside the bubble. The pressure jump itself, evidently, is equal to 1.4(p0 -

pg). After focusing in the center of the gas bubble, the spherical shock wave is reflected, and

the bubble ―explodes‖ from the inside, breaking up into small fragments. The collapse of the

gas bubble or, more precisely, its shock destruction occurs. Gas pressure and temperature

inside the bubble during the focusing and the subsequent reflection of the shock wave reach

very large, albeit theoretically restricted, values [19]. When the collapse of the gas bubble is


completed, its small fragments are left in the singled-out liquid volume, which are equal in

size to the original cavitation nuclei, and the density of the singled-out liquid volume

becomes close to the initial liquid density, ρf. As we show below, when the oscillation

velocities of the ultrasonic radiators reach very high values, cavitation may follow a different

mechanism, which does not involve breaking the gas bubbles up into small fragments, but

rather exhibits bubble behavior approaching that of an empty Rayleigh cavity.

This approach permits easily eliminating a seemingly clear contradiction that follows

from the Noltingk-Neppiras equation: how can a gas-filled bubble implode with a very high

rate if the gas pressure inside the bubble during compression rapidly increases, while the rate

of the gas diffusion from the bubble, according to [4, 5], is negligible. In the proposed model,

the gas bubble does not implode in the literal sense of the word, but is destroyed by a

spherical shock wave reflected after focusing in its center. The presence of a well-known

phenomena accompanying acoustic cavitation, such as sonoluminescence, erosion and

dispersion of solids, emulsification of liquids, etcetera, can be well explained from this point

of view. Additionally, the mechanism of the dissipation of the primary acoustic energy during

the liquid cavitation becomes clear. This is the mechanism of the heating of a compressible

medium in a shock wave, which is well described in the literature (see, for example, [19]).

2.3.2. Cavitation Region

During the rarefaction of a liquid in an acoustic wave, a mixture of a great number of

spherical gas bubbles with the liquid (cavitation region) is formed. Let us call this gas-liquid

mixture present in the cavitation region, the ―continuum‖. In the previous section, the course

of events during the collapse of a single bubble in some small volume of liquid was

described. To extend these events over the entire continuum, a transition to spatial description

is necessary. At that, the results of this transition must depend neither on the dimensions and

the form of the continuum itself nor on the sizes and the spatial distribution of the bubbles in

it.

During the compression stage, an acoustic radiator creates a pressure impulse in the

liquid beyond the continuum in the form of a plane acoustic wave. Since the velocity of sound

in the continuum is finite, the collapse of a multitude of gas bubbles located arbitrarily in the

continuum must also occur simultaneously only in some narrow layer, as the impulse of the

acoustic pressure approaches it, i.e. it must have a wave character. In the current model

representation, the result of the superposition of many spherical shock waves, which are

formed near each gas bubble during its collapse in a narrow layer of the continuum, is a

spatial wave (SW) moving through the continuum. Such a representation is the most exact

and visual way of extending the events occurring during a single gas bubble collapse, over the

entire continuum.

In the real situation, the cavitation region in a liquid may take very complex, branched

shapes. The spatial distribution of bubbles in the region also may be quite non-uniform and

the sizes of the bubbles may vary. When the transition to the presented spatial description of

cavitation is made, for the results to be independent of the shape of the cavitation region as

well as of the spatial distribution and the sizes of the bubbles, in our initial equations we will

further utilize hypothetical physical parameters related to the cavitation region as a whole. In

other words, instead of operating with local values of density, changes in the internal energy

and so on, we will use the values averaged over the whole cavitation region. As demonstrated


below, these values disappear when further modifications of the fundamental equations are

made.

The experimental investigations of acoustic cavitation described below conducted for the

verification of the presented model were carried out using calorimetry of the entire

environment and, therefore, provide only the spatially averaged values due to a relatively high

thermal conductivity of the liquid. Therefore, the final purpose of the calculations following

this model is the determination of a cumulative value of the changes in the internal energy of

the environment, as a result of acoustic cavitation.

The spatial wave (SW) described above has a bore wave-like character, however, the

continuum density and pressure inside the SW front change stepwise. This occurs because the

cavitation bubbles collapse inside its front, following the process outlined in section 2.3.1.

The presence of such a wave is the final stage of acoustic cavitation, within one cycle of the

continuum rarefaction - compression. In other words, according to the model, it is assumed

that the collapse of the gas bubbles occurs inside a relatively narrow front of a hypothetical

SW, being formed and moving through the continuum in each compression half-period of an

acoustic radiator.

The width of the SW front, inside which the collapse of the bubbles and the change of the

continuum density occur, can be estimated as the product of the empty bubble collapse time,

according to equation (3) and the wave front movement velocity with respect to the

continuum, h = cτ. A rough estimate for the wave front movement velocity can be made using

expression (5). Then, at α = 0.1 (taken from the literature data [22] and characteristic for the

initial stage of acoustic cavitation) we obtain h ≈ 3rin. According to the estimation performed

in the work [4], the maximum radius of a gas bubble in water does not exceed 2∙10-4

m, since

larger bubbles rapidly rise to the surface. Hence, the value is: h ≤ 6∙10-4

m, which is smaller

than the dimensions of the continuum itself by many orders of magnitude. Thus, the specified

wave has a front that is very narrow relative to the dimensions of the entire continuum.

Getting over this barrier, therefore, the physical parameters of the continuum change

stepwise.

It is necessary, further, to establish a relation between the continuum parameters ahead of

and behind the SW front, as well as the relationship between these parameters and the

oscillatory velocity of an acoustic radiator. It is important to note that the velocity of the

specified wave can be lower than the velocity of sound in the continuum.

The SW moving through the continuum is not only a physical abstraction used for the

construction of the model, but can, apparently, exists in reality. In this case, however, we are

not faced with an ordinary shock wave, which arises in a compressible continuum when the

piston movement velocity is higher than the sound velocity in the continuum. Such shock

waves in a gas-liquid suspension obtained by bubbling a gas through a liquid are described in

detail in literature [21]. Here, it is assumed that in a gas-liquid suspension formed as a result

of the liquid rarefaction in an acoustic wave, another type of bore wave-like shock waves may

exist, which is associated with the radial movement of the liquid in the vicinity of each

bubble.

It is well known that when a jump (discontinuity) of a physical quantity arises in a

compressible continuum, a solution should be sought using the general conservation laws in

the form of the Rankine-Hugoniot equations [19]. These equations reflect the ratios of the

steady-state physical parameters of the compressible continuum before and after the passage

of the shock wave front.


Figure 3. Schematic of the continuum‘s flow during compression is shown (1 – acoustic radiator, 2 –

flow region after the SW passage, 3 – flow region before the SW passage).

Figure 4. Processes occurring during acoustic cavitation are illustrated. Line 1 represents the rarefaction

of the continuum with cavitation nuclei in an acoustic wave, line 2 represents a nonlinear process of the

growth of cavitation bubbles in the rarefaction wave, line 3 represents a preliminary compression of the

continuum in an acoustic precursor wave, line 4 represents the continuum transition from one state to

the other when the SW passes.


Additionally, there appears a possibility to analytically calculate the values of important

parameters, without considering in detail the transient processes inside the SW front, which

are connected with the complex kinetics of a collapsing gas bubble.

Let us introduce the following designations: ph is the pressure in the liquid phase of the

continuum near the bubble wall after the SW passage; pl, ρl = ρf (1- αl), αl are, respectively,

the pressure in the liquid phase of the continuum near the bubble wall, the density and the

volumetric gas content of the continuum before the SW passage. A scheme of the continuum

flow is presented in Figure 3. It is assumed that a SW moves through the continuum, and that

the gas bubbles collapse inside the narrow front of this wave. Also shown in this figure is the

supposed pressure profile in the continuum.

Figure 4 shows the supposed processes occurring during one cycle of acoustic cavitation

in a liquid. The pressure in the liquid phase of the continuum near the gas bubble wall in an

arbitrary state is plotted on the ordinate, and the continuum specific volume is plotted on the

abscissa. Line 1 represents the rarefaction of the continuum with cavitation nuclei in an

acoustic wave. Line 2 represents a nonlinear process of the growth of cavitation bubbles in

the rarefaction wave. Line 3 represents a preliminary compression of the continuum in an

acoustic wave (for a single gas bubble, this corresponds to a rise in the gas pressure in the

bubble on the smooth section of a converging spherical wave, as described in section 2.3.1).

Line 4 represents the continuum‘s transition from one state to the other when the SW passes

(for a single gas bubble, this corresponds to a rise in the gas pressure in the bubble on the

steep section of a converging spherical wave, as described in section 2.3.1). In this scheme, it

is assumed in advance that the velocity of the SW movement through the continuum can be

lower than the sound velocity in the continuum itself ahead of SW. Additionally, the SW

front itself serves as a source of the acoustic wave, propagating forward in the direction of the

shock wave movement. In this connection, there is a preliminary compression of the

continuum, and line 4 begins above the abscissa axis.

This kind of an acoustic wave is called precursor. Precursor does not cause the collapse

and disintegration of the bubbles because of a small value of its amplitude. Similar

representations are used for initially loose or porous environment. In such environment,

during the compression phase, the shock-wave front is formed only due to the parameters of

the compression process itself since this environment tends to change the specific volume of

pores (cavities) abruptly (stepwise) under pressure [23-25].

Let us introduce the following additional designations: pl = p0 + p'l, ph = p0 + p'h; p'l and

p'h are the excessive pressures in the liquid phase of the continuum near the bubble wall

before and after the SW passage, respectively; ul and uh are the continuum flow velocities

relative to SW before and after its passage, respectively; el and eh are the specific internal

energy of the continuum before and after the SW passage, respectively; v is the current

oscillatory velocity of an acoustic radiator; vt is the critical oscillatory velocity of an acoustic

radiator, which corresponds to the cavitation onset (cavitation threshold). Note that a stepwise

increase in the continuum density from ρl to ρf at the SW front corresponds to a change in

pressure from pl to ph. The relative movement of the liquid and the gas bubbles is neglected.

Let us now write the system of conservation equations (Rankine-Hugoniot equations) for

the continuum parameters on both sides of the SW front:

hfll uu ,


22

hfhlll upup ,

h

2

h

f

h0l

2

l

l

l0 e2

uppe

2

upp

, (7)

hlt uuvv

The fourth equation of system (7) shows that a change in the continuum‘s movement

velocity getting over the SW front is equal to the excessive oscillatory velocity of an acoustic

radiator, which exceeds the critical value, vt.

This system of equations can be transformed to the following form:

)vv(2

)ppp2(I t

hl0

, (8)

lh

2

tl

pp

)vv(

Here, I = (eh – el)ρfuh is the flux density of the energy dissipated inside the SW as a

consequence of the dissipation processes related to the bubble collapse and ηl = αl/ρl is the

volume of all cavitation bubbles per unit mass of the liquid phase of the continuum before the

SW passage.

The average flux density of the acoustic energy (acoustic energy intensity) absorbed in

one acoustic wave period can be presented in the following way:

/

0

a /Idt)tsin(I2

I

(9)

2.4. Set-up of the Equations for the Experimental Verification

For the resulting equations (8) to be verified experimentally, it is necessary to determine

the particular values of p'h , p'l , ηl and vt.

2.4.1. Low Oscillatory Velocities of Acoustic Radiator

From equation (6) and the analysis given in section 2.3.1, it follows that the maximum

excessive pressure at the SW front is equal to p'h = 1.4p0 + δp′∞. As mentioned above, the

liquid utilized for the construction of the theoretical model, does not possesses tensile strength

during rarefaction. Consequentially, the explosive growth of the cavitation nuclei and their

conversion into gas bubbles in the rarefaction wave takes place at the negative pressure equal

to the static pressure, p′∞ = p0. It is possible to assume that for small oscillation velocities of

the acoustic radiator near the cavitation threshold a symmetry of acoustic pressure amplitudes

during the half periods of compression and rarefaction is conserved. Consequentially, in this


case, δp′∞ = 0 and p'h = 1.4p0. It will be shown below that for large radiator oscillatory

velocities it is no longer possible to ignore the quantity δp′∞. Note that the value of p'h ≈ 1.4p0

actually corresponds to the threshold of water cavitation, at least, in its initial stage. This fact

was experimentally established in [26].

Above, it was assumed that during the rarefaction of a liquid in an acoustic wave, all gas

dissolved in a unit volume of the liquid passes into the bubbles formed in this volume. The

oscillations of the gas bubbles before the onset of their collapse are isothermal, and the mass

of the gas in them does not change. From the analysis of equation (6) given in section 2.3.1, it

follows that p'l = 1.4pg, hence, the condition p0η0 = 0.71p'lηl must be met. Here, η0 is the

equilibrium volume of gas dissolved in a unit mass of the liquid at the pressure, p0.

The quantity vt is the critical oscillatory velocity of an acoustic radiator, which

corresponds to the cavitation threshold. In view of the conditions described above, one can

assume that for a plane acoustic wave, (vt)rms = 0.71p′∞ / ρf cf = 0.71p0 / ρf cf .

It should be borne in mind that the value of vt in each particular experimental case can be

different from the specified theoretical value. This is connected with the fact that the practical

value of vt depends on a large number of different parameters of liquid (physical nature, purity

degree, gas content, volatility, sample preparation history, etc.). Besides, vt also depends on

the conditions of the conducted measurements (frequency of ultrasound, degree of isolation

from external radiation, temperature, etc.)

From the second equation of system (8) we obtain:

2

rmst00

0

2

0l

)vv(42.1p

p4.1p

(10)

Now from the first equation of system (8) in view of equations (9, 10) we obtain the final

equation for the average flux density of the acoustic energy (intensity of acoustic energy)

absorbed in the cavitation region:

rmst2

rmst00

000a )vv(

)vv(42.1p

p41.01p76.0I

(11)

For the initial stage of acoustic cavitation, at a small value of (v-vt)rms, the final equation

is as follows:

rmst

0

a )vv(07.1p

I

(12)

It is important to point out that in equations (11, 12) the quantities related to the spatial

distribution of gas bubbles in the continuum and their size, as well as the form and shape of

the continuum itself are not present.


2.4.2. High Oscillatory Velocities of Acoustic Radiator

From the main system of equations (7), one can obtain the expression for the SW velocity

relative to the unperturbed continuum, 5.0

flhl )1(/)pp(u . The ratio of ul to

the sound velocity, c, in the continuum according to equation (5), using equation (10) and

taking into account that pg = 0.71p′l, can be written as:

5.0

00

2

rmst

5.0

g

lhl

p

)vv(2

p

pp

c

u

(13)

From this expression, it is seen that at (v-vt)rms ≥ 1 m/s, the SW movement must become

supersonic, making it a real shock wave in the classical sense. When the SW movement is

supersonic, a precursor is absent because it is absorbed by the faster shock wave. The density

and the pressure of the gas inside the bubbles in this case are initially small since they are not

compressed beforehand by the precursor. From the analysis of equation (10), it is seen that at

(v-vt)rms > 3 m/s the gas pressure in such bubbles becomes approximately an order of

magnitude lower than the static pressure, p0, and continues to decrease. A spherical shock

wave in rarefied gas inside such a bubble is not formed and, accordingly, the bubble does not

break up into small fragments as a result of the collapse. The behavior of the bubble becomes

close to the behavior of an empty Rayleigh cavity.

It is also important to keep in mind that the minimum width of the shock wave front in a

gas is on the order of the molecule free path [19]. At a normal density of the gas, this distance

is about 10-7

m. With a decreasing gas density, this distance increases proportionally and

becomes close to the characteristic size of the bubble itself 10–5

m. Under these conditions, a

spherical shock wave inside the bubble cannot be formed, and the bubble is compressed like a

Rayleigh cavity.

At the final stage of the collapse of the bubble, the gas pressure in it increases to such a

degree that it can hold back the liquid‘s pressure. At that, the pressure and temperature of the

compressed gas can reach very high values (theoretically unrestricted under the assumptions

of this model [19]). In this case, at the excess pressure, p'h = 1.4p0, the continuum behind the

SW is a gas-liquid suspension with some density ρh = ρf (1- αh). If the conditions identified in

the beginning of section 2.3, assumed for the construction of the model, are to be met, the

continuum behind the front of SW is additionally compressed by the acoustic radiator until

density ρf is reached. This corresponds to a pressure increase at the SW front up to the value

of p'h = 1.4p0 + δp′∞ = 1.4p0 + 0.5ch2δρ = 1.4p0 + 0.5ch

2ρfαh, where δρ = ρf – ρh = ρf αh is the

additional increase in the continuum‘s density behind the SW front, necessary to reach the

quantity ρf, and ch is the speed of sound in the gas-liquid suspension with density ρh. For high

oscillatory velocities of acoustic radiator similar to the sound speed in the continuum, p'h =

1.4p0 + ρf αh v2rms, since in this case it can be taken that c

2 = 2v

2rms.

The value vt is neglected. Since δp'∞ should be taken into account only at high v and the

second term of equation (11), which corresponds to the excessive pressure p'l, is negligible,

we leave it unchanged. Let us now write equation (11) in the final form in view of equation

(9):


rmst

0

2

rmshf

2

rmst00

000a )vv(

p

v29.0

)vv(42.1p

p41.01p76.0I

(14)

2.4.3. Interpretation of the Experimental Results of the Work [26]

A large series of experiments aimed at studying acoustic cavitation of water at low

oscillatory velocities of acoustic radiator are presented in the work [26]. Experiments were

conducted in degassed water with the concentration of the dissolved air equal to 30% of the

nominal concentration in the equilibrium state at the room temperature and the normal static

pressure.

For the interpretation of these data, let us introduce the following designations: ΣIa=

0.5(p'h)2γ = p0

2γ is the total intensity of the acoustic energy radiated into water; Ia0= 0.5(p'h)

2γf

= p02γf is the intensity of the acoustic energy propagating beyond the bounds of the cavitation

region. Here, γ is the specific acoustic radiation admittance of the continuum, γf = 1/ρfcf. The

difference of these intensities is the intensity of the acoustic energy absorbed in the cavitation

region. Thus, when compared with the theoretical results of the given model, the experimental

values of γ for each oscillatory velocity obtained in [26] were recalculated by the following

expression:

0f

0

a p)(p

I (15)

In representing the data of the work [26], the values of (vt)rms were determined directly

from the experimental plots of this work at the point of characteristic inflection.

2.5. Experimental Setup

To measure the acoustic energy absorbed in a cavitating liquid at increased static pressure

p0, an acoustic calorimeter described in section 3.2.3 of this chapter was used. Static pressure

in the calorimeter was produced with compressed nitrogen. Settled tap water at 200 C was

used. The static pressure, p0, varied in the range 1.0 – 5.0 bar; the water density, ρf = 998

kg/m3; sound velocity in the water, cf = 1500 m/s; the volume of air dissolved in unit mass of

water, η0 = 2.2∙10-5

m3/kg. Each experimental point shown on the plots was obtained as a

mean value of 10 measurements.

2.6. Experimental Results

Experimental data for small oscillatory velocities of an acoustic radiator, v, and different

static pressures, p0, are shown in Figure 5. The values of vt used in the treatment of these

experimental data were calculated from the expression (vt)rms = 0.707p0 /ρf cf for different

static pressures. Also shown in this figure are the experimental data from [26] for ultrasound

frequencies of 19 and 28 kHz, closest to the frequency 17.8 kHz used in the present work,

which are interpreted by equation (15).


Figure 5. Intensity of acoustic energy absorbed in water at cavitation is shown as a function of the

excessive oscillatory velocity of an acoustic radiator for pressures of × - 1 bar, + - 2 bar, ■ - 3 bar, □ - 4

bar, ○ - 5 bar, at frequencies of ▌- 28 kHz and ▀ - 19 kHz from the work [26]. Line 1 is plotted from

equation (12); line 2 is plotted from equation (11).

The values of the cavitation threshold obtained from the corresponding plots of [26] for

both frequencies (vt)rms = 0.08 m/s. Figure 5 also shows the theoretical lines calculated from

equations (11) and (12), which are represented by the solid and the dotted lines, respectively.

A good agreement between the theoretical lines themselves and the experimental data

with these lines at small values of v can be clearly seen. With increasing (v-vt)rms > 0.2 m/s,

the experimental points diverge from the straight line plotted from equation (12) and

approach the line plotted from equation (11).

Figure 6 shows the experimental results for all oscillatory velocities of the acoustic

radiator, v, which were used in the experiments at normal static pressure, p0 = 1 bar. Also

shown in this figure are the theoretical lines plotted from equations (11) and (14). From

Figure 6 it is seen that at intermediate values of v the experimental points are located near

practically coincident lines plotted from equations (11) and (14), which are represented by the

dotted and solid lines, respectively.

At high oscillatory velocities, (v-vt)rms > 3 m/s, the specified theoretical relationships

diverge, and the experimental points are located according to a more general relationship (14)

at αh = 0.4. It can be seen that the theoretical and the experimental data are in good agreement

up to the highest values of the oscillatory velocity, v.


Figure 6. Intensity of acoustic energy absorbed in water at cavitation is shown as a function of the

excessive oscillatory velocity of an acoustic radiator. Line 1 is plotted from equation (14); line 2 is

plotted from equation (11).

A spread of the experimental points on the curve in Figure 6 in the region 2 m/s < (v-

vt)rms < 3 m/s is also observed. Here, the beginning of the divergence of the theoretical curves

1 and 2 is observed as well. These phenomena are, apparently, associated with the

establishment of the supersonic regime of the SW movement and a considerable decrease in

the gas pressure in the bubbles. The indication of the possibility of the supersonic regime of

radiation at acoustic cavitation was first made in the work [27]. The phenomenon itself was

called the second threshold of acoustic cavitation. The region located over the second

threshold at (v-vt)rms > 3 m/s was called the region of acoustic supercavitation. The closest

related known phenomenon is called hydrodynamic supercavitation and is described, for

example, in [28].

Since, as the stated theory assumes, at supercavitation the spherical shock wave is not

formed in the gas inside the bubbles, at oscillatory velocities (v-vt)rms > 3 m/s the

characteristic changes of the secondary effects of cavitation, which are used in the

sonochemical technology, must be observed.

An experimental verification of this effect was conducted by observing the cavitation-

induced ultrasonic dispersion of solid particles. During the experimental setup, it was

assumed that the transition to the supercavitation regime should in some way be reflected in

the manner in which the dispersion occurs. The experimental study was conducted during the

ultrasonic dispersion of graphite particles with the initial size 200-250 in settled tap water

under normal conditions.


Figure 7. Dispersing effect of acoustic cavitation (dispersion of graphite powder in water) determined

by the degree of the 420 nm wavelength light absorption is illustrated as a function of the excessive

oscillatory velocity of an acoustic radiator.

To avoid any possible influence of the reactor geometry on the results of the

measurements, the acoustic calorimeter described in section 3.2.3 was used as an apparatus

for dispersing. For the analysis of the relative transparency of the obtained dispersions, the

degree of the light absorption (at the wavelength of 420 nm) in them was measured using a

photo-colorimeter. From the measurement results presented in Figure 7 in relative units, it can

be seen that the obtained curve reaches a maximum and then discontinues at 2.5 m/s < (v-

vt)rms < 3 m/s. A subsequent smooth rise of this curve in the supercavitation region is also

observed, which is most likely associated with the intense acoustic streaming, rather than with

the effect of cavitation itself.

It appears that it is in the acoustic supercavitation region where highest possible

temperatures during the compression of the rarefied gas inside the bubble oscillating as a

Rayleigh cavity can be expected. Pressure at the bubble wall at the moment of focusing

theoretically approaches infinitely high values because the gas compression is exerted by the

moving dense bubble wall acting as a spherical plunger, rather than by a spherical acoustic

wave [19]. In the same region, the highest intensities of the cavitation-induced sonochemical

processes occurring at high temperatures may be observed. At the same time, processes

connected with erosion, dispersion of solids and the like can be inhibited in the

supercavitation region.


2.7. Section Conclusion

The proposed shock-wave model of acoustic cavitation describes real events occurring in

water at cavitation since calculations based on the equations that follow from the model are in

good agreement with the results of the experiments. The presented experimental data extend

into the region of super-high oscillatory velocities of an acoustic radiator and agree well with

the theoretical model. The model makes it possible to obtain the resulting equation for the

calculation of the energy absorbed by liquids during cavitation without having to consider in

detail all the complex processes of the absorption of the acoustic energy, which are connected

with the nonlinear oscillations of the gas bubbles during their collapse.

Within the framework of this model, the existence of a transition from the subsonic

regime of acoustic cavitation to the supersonic regime is predicted. The possibility of a

change in the character of the oscillations of a cavitation bubble at high values of v is

theoretically shown. The conducted experimental studies confirm such possibility.

As will be shown below, simple algebraic expressions that follow from the proposed

model can be used in practical engineering calculations for designing powerful ultrasonic

horns for sonochemical reactors.

3. SELECTION AND DESIGN OF THE MAIN COMPONENTS OF HIGH-

CAPACITY ULTRASONIC SYSTEMS

The greatest mechanical stress areas in a sonochemical reactor system are concentrated in

the electromechanical transducer and the ultrasonic horn components. The same components

are also exposed to the highest thermal loads, related to the formation and maintenance of

acoustic waves. Selection of the appropriate electromechanical transducer type, therefore, is

of great importance, as is the ultrasonic horn design and the choice of material from which it

is constructed.

3.1. Electromechanical transducer selection considerations

Ultrasonic transducers are devices used to convert electric energy coming from a power

generator into mechanical energy in the form of ultrasonic vibrations. There are two main

types of ultrasonic transducers used in the high-power ultrasonics field: magnetostrictive and

piezoelectric (high-power piezoceramic).

For continuous flow-through liquid processing applications, magnetostrictive transducers

have multiple advantages over the piezoelectric devices. These transducers are constructed

from high-strength metallic alloys (5,000 – 7,000 MPa) and permit reaching high levels of

acoustic power intensity (up to 100 – 150 W/cm2). The main disadvantage of magnetostrictive

transducers is their relatively low efficiency (below 50%). On the other hand,

magnetostrictive transducers are electrically safe and do not overheat because they are

relatively low voltage driven and are liquid cooled. In addition, these transducers provide

high total radiation powers and relatively high output amplitudes, are very stable, reliable and

do not age. These devices are, therefore, well suited for continuous long-term industrial


operation under factory conditions and are ideal for industrial liquid processing with flow-

through ultrasonic systems.

For comparison, the advantage of piezoelectric transducers is their high efficiency (up to

95%). These devices, however, are characterized by much lower levels of acoustic power

intensity and relatively short life-spans due to low mechanical strengths of the involved

materials (only about 15 – 30 MPa). Additionally, piezoelectric transducers are high-voltage

driven and are air cooled, which for some applications may make them an explosion hazard.

They can also easily become overheated and damaged, which is why they cannot be used for

extended periods of time or in high-temperature environments. These devices, however, are

widely used in such important high-power ultrasonics fields as plastics welding, cleaning,

machining, etc., where pulsed-mode operation or lower amplitudes are appropriate. This

explains these transducers‘ high popularity and availability. When used in liquid processing

applications, however, piezoelectric transducers are frequently run at a much lower power

than available, in a pulsed mode or with short periods of ―on‖ time [29-31].

In view of the above discussion, we will only consider magnetostrictive transducers in

this chapter.

3.2. High Power Acoustic Horn Design Principles

Despite being capable of producing much higher vibration amplitudes than piezoelectric

devices, magnetostrictive transducers still cannot provide sufficient amplitudes for a correct

operation of an ultrasonic reactor system. Acoustic (ultrasonic) rod horns are, therefore, used

in conjunction with these transducers to amplify the vibration amplitude and deliver the

ultrasonic energy to the working liquid. Commonly used acoustic horns (Figure 8), in general,

consist of two cylindrical sections, input (larger diameter, in contact with transducer) and

output (smaller diameter, in contact with the liquid), which are connected to each other by

one transitional section, which may have a conical, exponential, catenoidal, or a more

complex shape, or may be omitted all together (stepped horn) [32-34]. Although widely used,

these horns suffer from an important limitation: they are incapable of providing matching

between the transducers and the liquid loads, leading to inefficient acoustic power

transmission.

For optimal operation, the maximum cross-sectional dimension of any portion of a

resonant horn or transducer may not exceed, approximately, one quarter-wavelength of the

corresponding longitudinal acoustic wave at the horn‘s resonance frequency [35].

Consequently, a common converging horn (for which the output diameter is smaller than the

input diameter) with a maximum allowed input diameter always ends up having a working

(output) tip diameter that is smaller than this limitation. The final size of the tip depends on

the gain factor of the horn, and becomes reduced as the gain factor increases. This is

problematic when the processes are carried out on industrial scale, since deposition of

substantial acoustic power is needed to create acoustic cavitation in large volumes of water.

While using converging horns permits increasing the acoustic energy intensity (or vibration

amplitude) radiated into the load, it is impossible to achieve the technologically necessary

levels of total radiated acoustic power, since the cross-sectional area of the horn tip in contact

with the load is small.


Figure 8. A typical high-gain converging horn is shown. High vibration amplitude of the output tip is

achieved at the expense of the tip area.

Therefore, it is intuitive that the use of the converging horns does not permit transferring

all available power from a transducer into a load.

To increase the total radiation area, the horns are sometimes connected to planar resonant

systems, such as large discs or planes [36]. These additional elements, however, significantly

complicate the construction of the horns, introduce additional mechanical connections and,

therefore, reduce life span and reliability.

In this section we will describe design principles that have been successfully

implemented in the development of a family of acoustic horns, whose shapes permit

achieving high gain factors and large output surfaces simultaneously. These horns can be

designed to accurately match an ultrasonic source (transducer) to a liquid load (water, in this

case) at cavitation, maximizing the transference of the available acoustic energy into the load

and creating a large cavitation zone. These devices are easy to machine and have well-

isolated axial resonances and uniform output amplitudes.

3.2.1. Criteria For Matching Magnetostrictive Transducer to Water at Cavitation

In an ideal case, without accounting for the internal losses, the highest acoustic energy

intensity that a resonant magnetostrictive transducer can transmit into a load is limited by two

main factors - the magnetostrictive stress saturation,m (the maximum mechanical stress

amplitude achievable due to the magnetostrictive effect for a given transducer material), and

the maximum allowed amplitude of oscillatory velocity, limited by the fatigue strength of the

transducer material, mV , such that [37]:

cV

Ee

mm

mm

2

1

(16)

where me is the deformation amplitude associated with

m , E is Young's modulus, 1 and 2

are the coefficients that take into account the features of the transducer construction [33, 37],


m is the stress amplitude of the material fatigue strength, is the transducer material‘s

density, and c is the thin-wire speed of sound in the material. The highest potential acoustic

energy intensity radiated under conditions of perfect matching between the transducer and the

load is represented by the quantity:

mmm VI 5.0 (17)

It should be noted that the acoustic load under consideration, water at cavitation, has a

purely active character, and, therefore, is appropriately described by the term ―acoustic

resistivity”,ar [26], such that vpr aa , where v is the amplitude of the output oscillatory

velocity of acoustic horn and ap is the acoustic pressure averaged over the entire radiating

surface of the horn. Practically, this means that virtually all of the acoustic energy deposited

into water at cavitation is converted into heat [38]. Under the term ―matching‖ we will further

mean supplying a magnetostrictive transducer with a multi-element acoustic horn having a

gain factor, 1G (G is defined as a ratio of the output to input oscillatory velocities,

mVv , which allows the transference of a maximum of the available acoustic power of the

transducer, mI , into the load.

Acoustic energy intensity, aI , generated in a purely active load by the longitudinal

vibrations of an acoustic rod horn with an output oscillatory velocity amplitude, v , is

represented by:

vpvrI aaa 5.05.0 2 (18)

Taking outainm SISI as a matching condition, we obtain:

2GNpa

m

(19)

where inout SSN , inS and

outS are, respectively, the input and the output cross-sections

of the acoustic horn, while inS is taken to be equal to the output cross-section of the

magnetostrictive transducer, tS (please see Figure 9). The left-hand side of equation (19)

reflects the degree of under-loading of an acoustic transducer, and the right-hand side

describes matching capabilities of an acoustic horn.

As shown theoretically in section 2.4 and experimentally confirmed, the connection

between the acoustic pressure,ap , and the static pressure,

0p , during the well developed

cavitation can be expressed by equations (11) and (14).


Figure 9. General schematic is shown, describing matching between an electromechanical transducer

and a load achieved by using an acoustic rod horn of an arbitrary shape. Sin and Sout are, respectively,

the input and the output cross-sections of the acoustic horn; St is the output cross-section of the

electromechanical transducer.

To demonstrate this, let us consider the case of moderate (although much greater than the

threshold value) amplitudes of ultrasonic vibration of the horn and apply equation (12).

Assuming tvv , and taking into account that vpI aa 5.0 , we obtain for the amplitude

value of ap the expression: 02 ppa .

Therefore, the following can be written:

0

1

2 p

Ee

p

m

a

m (20)

It is clear that for high vibration amplitudes, a more complex expression based on

equations (11) and (14) can be derived in a similar manner.

It is seen from equation (20) that the degree of under-loading of an acoustic transducer

depends only on the characteristics of the transducer itself and the static pressure of water.

Theoretically, for most common magnetostrictive materials, the calculated values of am p/

are between 15 and 44. In this calculation, the values of 0p = 105 N/m

2 and 1 = 0.45 were

assumed. However, for a real magnetostrictive transducer, which is an electro-acoustic

instrument, the maximum acoustic energy intensity generally does not exceed 70 -100 W/cm2.

This is due to such limitations as an insufficient ultrasonic generator power, voltage and

current rating of the electrical wire forming the transducer's coil, cooling system capacity, etc.

Consequentially, the practical values of the degree of under-loading are much lower than the

corresponding theoretical limits for the magnetostrictive materials themselves, and for most

models are between 5 and 10.

It is less evident how to use the right-hand side of equation (19), which reflects the

matching capabilities of a horn. In this case, before the resonance calculation of a matching

horn it is necessary to determine the maximum acoustic energy intensity for the utilized

magnetostrictive transducer, em WI . Then, from (17) and (19) we obtain:

m

e

Vp

WGN

0

2 2 , (21)


where eW - specific (with respect to Sin ) electrical power of the magnetostrictive transducer,

- its efficiency (commonly 5.0 ).

Figure 10. Schematic defining the parameters of a five-element matching horn is shown. The horn

having d1 / d3 > 1 is shown by a solid line, and the horn with d1 / d3 < 1 is shown by a dotted line.

Parameters L1 – L5 correspond to the lengths of each element.

The next step should be selecting an optimal, from the technological standpoint, range of the

values for the gain of the horn, G, which is commonly determined during the preceding

laboratory studies of a given process. It is then easy to derive the value for N necessary for

the resonance calculations of the matching horn and construction of the ultrasonic reactor.

In spite of a variety of types and shapes of the acoustic horns known from the literature

and used in practice, until recently none existed for which the relationship 12 GN , when

1G , would hold true. It is, however, clear that in order to be able to match magnetostrictive

transducers to water at cavitation, it is necessary to utilize acoustic horns that would meet the

matching criterion, 1GN 2 .

3.2.2. Five-Elements Matching Horns

3.2.2.1. Design Principles

The theory of acoustic horns is based on the mathematical problem of longitudinal

vibrations in multi-element rods that have cylindrical elements as well as elements of variable

cross-sections [39]. We will consider only the horns with axially symmetric shapes. Other

types of horns (for example, wedge-shaped) can be considered in an analogous way. In the

current work, we will restrict the discussion to five-element horns, although no theoretical

restriction for the number of elements exists.

We assume that during the passage of stress waves through a horn, the wave front

remains planar, while the stresses are uniformly distributed over the horn‘s cross-section. This

assumption limits us to "thin" horns, whose resonance lengths significantly exceed their

diameters. For all practical purposes, it is sufficient to require that the maximum cross-

sectional dimension of any portion of a resonant horn not exceed, approximately, one quarter-

wavelength of the corresponding thin wire acoustic wave at the horn‘s resonance frequency

[35].

Schematic and designation of parameters for a general case of a five-element rod horn are

given in Figure 10, where two possible situations are presented: a horn with 131 dd is

shown by a solid line; a horn with 131 dd is shown by a dotted line. Under the assumed


approximation, the problem is reduced to one-dimension, and only includes cone-shaped

elements with variable cross-section. For the steady-state mode, the equation of vibrations for

displacements, u , takes the following form:

.01 2 ukuSS

u (22)

where ck is the wave number, f 2 is the angular frequency of vibrations,

and f is the frequency of vibration.

The solutions of equation (22) for each of the horn‘s elements can be written as:

kzBkzAu sincos 111 01 zL

kzBkzAFu sincos 222 20 Lz

kzBkzAu sincos 333 322 LLzL

(23)

kzBkzAFu sincos 444 43232 LLLzLL

kzBkzAu sincos 555 5432432 LLLLzLLL

Then, using the boundary conditions for the horn‘s element, we obtain the system of

equations for displacements, u , and strains, u .

At 1Lz , inuu 1 , inFuES 11

, 0inF

inukLBkLA 1111 sincos ;

inFkLBkLAEkS 11111 cossin

At 0z , 12 uu , 12 uu

12 AFA ;

122 kBkFBAF ;

1231 dLdd ;

12 dF ; FF

At 2Lz , 23 uu ,

23 uu


22222323 sincossincos kLBkLAFkLBkLA ;

2222222323 cossincossin kLFkBAFkLFkABFkLkBkLkA ;

1231 dLdd ;

32 dF ; 12 LFF ; (24)

At 32 LLz ,

34 uu , 34 uu

323323324324 sincossincos LLkBLLkALLkBLLkAF

323323

32443244

LLkcoskBLLksinkA

LLkcosFkBAFLLksinFkABF

;

3453 dLdd ;

32 dF ; FF

At 432 LLLz ,

45 uu , 45 uu

43244324

43254325

sincos

sincos

LLLkBLLLkAF

LLLkBLLLkA

;

4324443244

43254325

cossin

cossin

LLLkFkBAFLLLkFkABF

LLLkkBLLLkkA

;

3453 dLdd ;

52 dF ; 14 LFF

At 5432 LLLLz ,

outuu 5, 05 u

outuLLLLkBLLLLkA 5432554325 sincos

0cossin 5432554325 LLLLkBLLLLkA

The gain factor of the horn can be expressed as:


1111

5432554325

sincos

)(sin)(cos

kLBkLA

LLLLkBLLLLkA

u

uG

in

out

(25)

where ndF 2 ,

nd is the diameter of the corresponding cylindrical element of the horn, nA

and nB are the constant coefficients for the corresponding elements of the horn,

nL is the

length of the corresponding element of the horn, n is the order number of the horn element,

is the cone index of the horn element with variable cross-section, inu and

outu are the

amplitudes of displacements at the horn input and output, respectively. The boundary

conditions for the force acting on the horn‘s input, 0inF , and for the strain at the horn

output, 05 u , in this system of equations indicate that the horn has a total resonance length

and does not have an acoustic load.

From the system of equations (24), one can obtain all necessary characteristics of a five-

element horn: lengths and diameters of the elements, gain factor, distribution of vibration

amplitudes, and distribution strains along the horn. From this system of equations, it is also

easy to obtain solutions for any horns with conical elements (for example, with fewer than

five elements). Horns with other shapes of the variable cross-section elements (for example,

exponential or catenoidal) can be considered in an analogous way, taking into account the

variation of sound velocity in such elements.

3.2.2.2. Analysis of Five-Element Horns

To solve the system of equations (24) and to present results in a convenient form, a

computer program has been written allowing all the indicated above characteristics of five-

element horns to be obtained in real time for subsequent analysis. The input parameters are:

operating frequency of the horn, acoustic properties and fatigue strength of the horn's

material, and the diameter-to-length ratios of the horn elements. For the convenience of

comparison of horn parameters, we further assume 151 dd .

From all possible solutions of the system of equations (24), only the series of five-

element acoustic horns will be considered, which will be referred to as "barbell horns". This

series of horns, in the authors' opinion, is the most useful for industrial applications, in

particular, for building industrial ultrasonic reactor systems.

Figure 11 shows a half-wave barbell horn and its design parameters. A photograph of this

horn is also presented in Figure 14 (b). The maximum value of the matching capability of this

horn is 42 GN . The resonance length of this horn corresponds to one half of the ultrasonic

wavelength in the metal from which the horn is constructed, with dispersion taken into

account. Its small resonance dimensions are convenient in terms of manufacturing and

minimizing the side surface radiation, and should be particularly noted. Some useful

parameters of this type of horn are presented in Table 1.

Figure 12 shows a spool-shaped barbell horn and its design parameters. This horn is

atypical because its main radiating surface is lateral, and it mainly radiates a cylindrical wave

into the load, as opposed to a plane wave radiated by other matching horns.


Figure 11. Half-wave barbell horn is shown with d1 = d5; d1/d3 = 3.0; kL2 = 0.5; kL3 = 0.2; kL4 = 0.3,

along with (a) the distribution of the oscillatory velocity, V, and strain, e, along the horn; (b) drawing of

the horn; (c) plot of the distribution of the horn‘s parameters.

Table 1.

kL1 G KL5

0.5 1.79 0.215

1.0 3.17 0.128

1.5 3.78 0.093

2.0 3.46 0.058

Due to the symmetric form of the spool-shaped barbell horn, its gain factor is always

equal to 1, the node of displacements is located in the middle, and lateral surfaces move in

anti-phase. When using lateral radiation, the horn‘s matching capabilities are quite high since

there are no limitations on its overall length.


Figure 12. Symmetrical spool-shaped barbell horn is shown with d1 = d5; kL1 = kL5 = 0.1; kL3 = kL4 =

0.5, along with (a) the distribution of the oscillatory velocity, V, and strain, e, along the horn; (b)

drawing of the horn; (c) plot of the distribution of the horn‘s parameters.


Table 2

d1/d3 KL3

2.0 0.877

3.0 0.384

4.0 0.179

5.0 0.085

When such horns are connected into a sequential string (radiating part of the long spool-

shaped barbell horn, shown in Figure 17 (a)), they can radiate a cylindrical wave of high total

power and produce a well-developed cavitation region of a large volume. Some useful

parameters of this type of horn are presented in Table 2.

Above, we have considered horns whose lengths are less than or close to half of the

length of the acoustic wave in the horn material, the so-called half-wave barbell horns. The

system of equations (24) also allows one to obtain solutions for full-wave barbell horns. One

of such horns intended for the radiation of a plane acoustic wave of a very high power into

water is full-wave barbell horn shown in Figures 13 and 14 (a). Its design parameters, as a

function of 31 dd , are presented in Figure 13 (c). The matching capabilities of full-wave

barbell horn can reach the values of 202 GN or more. These horns are very promising for

matching of high-power magnetostrictive transducers that have large cross-sections.

For example, the highest design power radiated into water at cavitation by this horn,

made of high-quality titanium alloy, taking into account the fatigue strength limitations and

limitations on output diameter under normal static pressure, is about 5 kW at a frequency of

20 kHz.

Due to the significant potential of full-wave barbell horn for industrial applications of

ultrasound, we also provide its exact parameters in Table 3. These parameters are convenient

for practical calculations.

3.2.3. Experimental Results

For the experimental verification of the described horn design principles we have chosen

full-wave barbell horn of the type shown in Figures 13 and 14 (a). Direct calorimetric

measurement of acoustic energy transmitted by this horn into water at cavitation was selected

as a method of this horn‘s performance evaluation, as well as for obtaining experimental

results presented in section 2.6. The measurements of the acoustic energy absorbed in the

cavitation region were conducted with the apparatus shown in Figure 15. Settled tap water at

the temperature of 20 0С was used. The apparatus was based on an acoustic radiator

consisting of a titanium horn connected to a magnetostrictive transducer, which operated at

the resonance frequency of 17.8 kHz. The working power of the ultrasonic generator coupled

to the magnetostrictive transducer was 5 kW. The oscillation amplitude of the

magnetostrictive transducer was kept constant in all experiments at 1.67 m/s (rms). It was

measured by placing a magnetic ring with an inductive coil on the transducer next to its

output surface. Voltage was created in the coil as the transducer oscillated. The amplitude of

this voltage corresponded to the oscillation amplitude and was measured by an oscilloscope.

Prior calibration of this device was performed, in which the vibration amplitude was

measured directly by a microscope.


Figure 13. Full-wave barbell horn is shown with d1 = d5; kL1 = kL3; kL2 = kL4 = 0.5, along with (a)

the distribution of the oscillatory velocity, V, and strain, e, along the horn; (b) drawing of the horn; (c)

plot of the distribution of the horn‘s parameters.

Table 3

d1/d3 G kL1 kL2 kL5

1.5 2.176 1.383 0.405 2.853

2.0 3.527 1.290 0.693 2.725

2.5 4.918 1.245 0.916 2.640

3.0 6.285 1.224 1.099 2.574

3.5 7.597 1.216 1.253 2.519

4.0 8.834 1.215 1.386 2.470

4.5 9.987 1.217 1.504 2.426

5.0 11.049 1.222 1.609 2.384


Figure 14. Full-wave (a) and half-wave (b) high-gain barbell horns are shown. High vibration amplitude

of the output tip is achieved without having to sacrifice the tip diameter. These particular barbell horns

have output tip diameters of 65 mm and provide ultrasonic amplitudes (a) up to 120 microns peak-to-

peak and (b) up to 80 microns peak-to-peak.

A set of replaceable full-wave barbell horns was constructed to provide the necessary

stepped change in the amplitude of the oscillatory velocity of the output end in contact with

water. The set consisted of nine such horns with different gain factors (greater or smaller than

unity), all of which had equal input and output diameters of 60 mm. Maximum oscillation

velocity of some of these horns reached very large values, close to maximum theoretically

possible for the best titanium alloys. Greatest achieved oscillation velocity was 12 m/s (rms).

Therefore, maximum gain factor for the set was 7.2.

Static pressure in the calorimeter was produced with compressed nitrogen. The

measurements of the resulting temperature of water were performed using a set of

thermocouples. A change in the temperature of water during ultrasonic treatment was not

more than 2 – 5 0C.

Experimentally measures acoustic energy intensity levels absorbed in the cavitation area

are presented above in Figures 5 and 6. The dispersing effect of acoustic cavitation is shown

in Figure 7.


Figure 15. Schematic of acoustic calorimeter is presented. 1-magnetostrictive transducer, 2-replaceable

full-wave barbell horn, 3-external wall of calorimeter, 4-heat insulation gasket, 5-cover with porous

sound-absorber, 6-internal wall of calorimeter, 7-sealing ring, 8-set of thermocouples, 9-gas supply, 10

– microphone, 11-point of control over amplitude of transducer vibrations.

Performance verification of the horns with different gain factors conducted during the

experiments showed that all of them possessed resonance and gain characteristics well

corresponding to the theoretically predicted values. In no case was it necessary to make any

adjustments to the horns after they were originally machined.

The region of acoustic energy intensity with the values above 105 W/m

2 is very little

studied, especially from the technological standpoint. The reason for this, in the authors‘

opinion, is that the traditional cone-shaped horns, widely used in ultrasonic technology, are

incapable of providing large total radiation power, since their oscillation amplitudes are

inversely proportional to the areas of their output surfaces. At large gain factors, the output

surface area becomes very small, which complicates the development of sonochemical

reactors capable of processing significant volumes of liquids. Thus, for example, a traditional

stepped horn having an input diameter of 60 mm and a gain factor of 7.2 has the output

diameter of, approximately, 20 mm. Therefore, at the maximum experimentally achieved

acoustic energy intensity of 106 W/m

2, this stepped horn is capable of depositing no more


than 300 W into its liquid load. Our full-wave barbell horn, used in the experiments presented

in this section, on the other hand, delivers, approximately, 2.7 kW of total power, providing a

power transfer efficiency increase by almost an order of magnitude.

3.3. Section Conclusion

Matching a magnetostrictive transducer to water is a matter of selecting the horn type that

fulfills expression (19) at a given gain factor G , and of subsequent calculation of its

resonance dimensions with the use of equations (24). The most powerful horn, among the

designs described above, is full-wave barbell horn, which was chosen for the experimental

investigations. During the experiments, evaluation of a set of such horns with different gain

factors showed that all of them had the resonance and the gain factor characteristics that

corresponded very well to those predicted theoretically. It was also experimentally verified

that matching of the acoustic horns with water at cavitation, according to the theory described

above, is truly established for all values of the output oscillation velocities of the horns.

It should be noted that matching an acoustic transducer to a load using an acoustic horn is

not the only possible method of matching. Another powerful matching factor, which results

from the specific properties of water at cavitation, is the static pressure, p0, according to the

expression (11) and the experimental results. It is clear that the best results are obtained when

these two matching techniques are used jointly.

In conclusion, we would like to add that barbell horns also perform well in non-aqueous

liquids and solutions with significant viscosity, and permit building very effective ultrasonic

reactors, suitable for treatment of such liquids, for example oils, epoxy resins, honey, polymer

melts, metal melts, etc.

Photographs presented in Figure 16 illustrate primary (a) and secondary (b) cavitation

zones formed during the operation of full-wave barbell horn having an output diameter of 65

mm providing acoustic energy intensity of 2x105 W/m

2 in the primary cavitation zone below

output tip.

In certain applications of powerful ultrasonic systems, however, it is more important to

increase the residence time of the working liquid in the reactor, than to maximize the output

amplitude. This is especially important during preliminary preparation for further high-

amplitude processing, such as during pre-dispersion, pre-emulsification, treatment of high-

viscosity liquids, etc. In these cases, it is convenient to utilize a long spool-shaped barbell

horn, incorporated into a reactor chamber. Figure 17 shows such a horn (a) as well as the

cavitation zones formed by it in a relatively viscous liquid, glycerin (b). This figure shows

that two well developed secondary cavitation zones are formed near the two "necks" of the

long spool-shaped barbell horn, constructed as two spool-shaped barbell horns connected in

series.

In semi-industrial ultrasonic reactor systems with relatively low transducer power (1 - 2

kW), it is convenient to use half-wave barbell horns, shown in Figure 14 (b). These horns are

compact and have minimal losses due to the side-surface radiation.

All photographs shown above were obtained using ultrasonic equipment produced by

Industrial Sonomechanics, LLC. Videos of the corresponding cavitation processes are

available at the company‘s website: http://www.sonomechanics.com.


Figure 16. Experimentally obtained photographs of well developed stable cavitation zones are shown.

The zones were created in an unrestricted volume of water by a barbell horn, having the following

operational parameters: output tip diameter – 65 mm, ultrasound frequency – 18 kHz, acoustic energy

intensity – 20 W/cm2. Part (a) shows the primary cavitation zone under the horn tip; part (b) shows the

secondary cavitation zone produced near the neck of the barbell horn (marked with a white line).

Figure 17. Photograph of a long spool-shaped barbell horn is shown in part (a). Photograph taken

during operation of this horn in glycerin is displayed in part (b), showing multiple secondary cavitation

zones formed near its transitional sections.


4. ULTRASONIC REACTOR CHAMBER GEOMETRY

During a flow-through ultrasonic process, it is important to make sure that all working

liquid is directed through the active cavitation zone, otherwise inhomogeneous processing

may result, leading to a lower-quality product. Eliminating the low cavitation intensity areas

in the reactor also helps increase the power density that the system can deposit into a liquid

load. Optimization of the ultrasonic reactor chamber geometry, therefore, leads to an

improvement in the technological effects obtained during the operation of the reactor.

In a common reactor chamber the treated liquid enters through the inlet at the bottom,

passes through the primary cavitation zone of a horn, Figure 16 (a), flows along the horn's

side surface and comes out through the outlet at the side of the chamber at the top. If a barbell

horn is utilized, there is also a secondary cavitation zone near the transitional sections, as

shown in Figure 16 (b), which accounts for approximately 20 % of the total radiated

ultrasonic power. An optimized reactor chamber design would efficiently direct all treated

liquid through both of these cavitation zones.

It has been explained above that the shape of a well developed cavitation zone formed at

the bottom of a barbell horn resembles an upside-down circular cone. Therefore, it is

beneficial to shape the bottom of the reactor chamber in the same manner, as shown in Figure

18. An approximately 20 % increase in the absorbed acoustic energy can be achieved due to

the presence of a cone insert at the bottom of the reactor chamber, which optimizes the

volume and the shape of the main cavitation zone at the output tip of the barbell horn [2]. To

take the full advantage of the secondary cavitations zone, a liquid deflector ring may be

inserted near the neck of the barbell horn (its second cylindrical section), as shown in Figure

18. Supplying the reactor chamber with both of these features dramatically improves the

homogeneity of ultrasonic exposure of the working liquid and increases the total power

deposition.

5. FINAL REMARKS

Industrial implementation of ultrasonic reactor systems has not reached its full potential.

This is especially true when processes require high ultrasonic amplitudes, for example in

production of nanoemulsions or nanodispersions. On the other hand, a large number of

laboratory studies exist that demonstrate high potential effectiveness of ultrasonic processing

in these and other areas [40, 41].

Since prior to the introduction of barbell horns the ultrasonic amplitude amplification was

commonly done with converging horns, high-amplitude industrial-scale ultrasonic equipment

was not available. Consequentially, transferring the results of many laboratory studies

involving high-amplitude ultrasound to the plant floor has not been possible. Low-amplitude

(below 30 microns peak-to-peak (pp)) industrial ultrasonic equipment has been around for

several decades. This equipment, however, has had limited capability to translate optimized

ultrasonic processes to commercial scale due to its inability to provide high-intensity

cavitation in large reactor volumes. Additionally, this equipment has generally relied on

piezoelectric transducer designs, which for industrial-scale liquid processing applications

suffer from several important limitations compared with magnetostrictive devices.


Figure 18. Schematic of an optimized flow-through ultrasonic reactor is presented, where 1 – electro-

acoustical transducer, 2 – barbell horn, 3 – working liquid outlet, 4 – reactor chamber, 5 – upside-down

circular cone insert, 6 – working liquid inlet, 7 – circular reflection surface.

The ultrasonic cavitation theory and main hardware design principles presented in

this chapter provide the background necessary to be able to construct high-capacity industrial

ultrasonic systems with up to 10,000 L/h processing capability, able to operate at extremely

high ultrasonic amplitudes in excess of 150 pp. Using these systems, any laboratory study

results can be directly implemented on industrial scale by simply increasing the horn tip

diameter and the corresponding reactor volume and boosting the power of the generator and

the transducer. All of the process parameters optimized during the laboratory study

(ultrasonic amplitude, reactor residence time, pressure, etc.) can be retained, while the system

productivity is increased by orders of magnitude.

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Chapter 3

SOME APPLICATIONS OF ULTRASOUND

IRRADIATION IN PINACOL COUPLING

OF CARBONYL COMPOUNDS

Zhi-Ping Lina and Ji-Tai Li

b

aDepartment of Chemistry and Biology, Baoding University,

Hebei Province, Baoding 071000, P. R. China; bCollege of Chemistry and Environmental Science,

Hebei University, Key Laboratory of Analytical Science

and Technology, Hebei Province,

Baoding 071002, P. R. China

ABSTRACT

Carbon-carbon bond formation is one of the most important topics in organic

synthesis. One of the most powerful methods for constructing a carbon-carbon bond is

the reductive coupling of carbonyl compounds giving 1,2-diols. Of these methods, the


vicinal diols. 1, 2-Diols obtained in the reaction were very useful synthons for a variety

of organic synthesis, and were also used as intermediates for the construction of

biologically important natural product skeletons and asymmetric ligands for catalytic

asymmetric reaction. In particular, pinacol coupling has been employed as a key step in

the construction of HIV-protease inhibitors.

Generally, the reaction is effected by treatment of carbonyl compounds with an

appropriate metal reagent and/or metal complex to give rise to the corresponding alcohols

and coupled products, The coupling products can have two newly chiral centers formed.

Threo, erythro mixtures of diols are usually obtained from reactions. As a consequence,

efficient reaction conditions have been required to control the stereochemistry of the 1,2-

diols. Recent efforts have focused on the development of new reagents and reaction

systems to improve the reactivity of the reagents and diastereoselectivity of the products.

In some of the described methods, anhydrous conditions and long reaction time are

required to get satisfactory yields of the reaction products, some of the used reductants

are expensive or toxic; excess amounts of metal are needed. Sonication can cause metal

in the form of a powder particle rupture, with a consequent decrease in particle size,

Zhi-Ping Lin and Ji-Tai Li 106

expose new surface and increase the effective area available for reaction. It was effective

in enhancing the reactivity of metal and favorable for single electron transfer reaction of

the aldehydes or ketones with metal to form diols. Some recent applications of ultrasound

in pinacol coupling reactions are reviewed. The results are mostly from our research

group.

INTRODUCTION

During 1980s, like thermal chemistry, photochemistry, and electrochemistry,

sonochemistry as a new branch of chemistry is a new cross-discipline whose use of

ultrasound to accelerate chemical reactions, improves the chemical production rate. It has

become a frontier area in chemical research, and its development has been brought to the

attention of chemistry academics.

It has been recognized for many years that power ultrasound has a great potential for uses

in a wide variety of processes in the chemical and allied industries. Reported applications

include cleaning, sterilization, flotation, degassing, defoaming, filtration, homogenization,

extraction, crystallization and of course as a stimulus for chemical reaction [1-21].

The technology is expected to bring major changes in pesticides, synthetic drugs, plastics

and the microelectronic devices industry, and has been attracting increasing interest in

chemical and related industries.

Carbon-carbon bond formation is one of the most important topics in organic synthesis.

One of the most powerful methods for constructing a carbon-carbon bond is the reductive

coupling of carbonyl compounds giving olefins and/or 1,2-diols. Of these methods, the


vicinal diols [22, 23]. The corresponding products of this reaction can be used as

intermediates for the preparation of ketones and alkenes. 1,2-Diols obtained in the reaction

were very useful synthons for a variety of organic syntheses and also used as intermediates

for the construction of biologically important natural product skeletons and asymmetric

ligands for catalytic asymmetric reactions [24].

Particularly, pinacols with a chiral are important raw materials for the synthesis of chiral

natural products and drugs, such as Pradimicinone, -blockers (S)-Propranolol, leukotriene

antagonist SKF104353, Paclitaxel, and C2-symmetric HIV protease inhibitors [25, 26]. The

chiral pinacols have been applied to synthesize chiral diamines, chiral crown ethers, chiral

diphosphine ligands, and especially to synthesize asymmetric catalyst [27]. So, study on the

pinacol synthetic method is very meaningful.

The 1,2-diol unit is one of the most ubiquitous functional groups in nature, and

consequently a lot of methods leading to its synthesis have been developed. Foremost in this

arsenal are the reductive coupling of aldehydes or ketones [22], double-hydroxylation of

olefins [28], ring opening of epoxides [29], reduction of -hydroxy/alkoxy carbonyls [30] and

alkylation of -hydroxy/alkoxy carbonyls and so on [31].

Among them, the pinacol-coupling reaction of aldehydes/ketones is the most classical

and effective methods, but also is one of the frequently used methods for the formation of

carbon-carbon bonds in organic reactions [27]. Although 140 years have passed from the first

synthesis of pinacol [32] up to now, the topic is still one of the hottest research fields in

organic synthesis. Generally, the reaction is effected by treatment of carbonyl compounds

Some Applications of Ultrasound Irradiation in Pinacol Coupling… 107

with an appropriate metal reagent and/or metal complex to give rise to the corresponding

coupled product. This reaction is affected by many factors, such as the reductant used, solvent

type, and reaction pH, etc. The coupling products have two chiral centers, threo, erythro

mixtures of diols are usually obtained from reactions, which increases the difficulties to

improve chemoselectivity and stereoselectivity for the synthesis of pinacol. Therefore,

seeking new metal reagents, a new reaction system and new approaches have been the focus

of attention and study. Before the 1970s, the method of synthesis of pinacol was mainly

electrochemical methods, and reductive coupling of aldehydes and ketones induced by alkali

metals such as Na, Mg and other active metals in the non-proton-media. In the last two

decades, the introduction of low-valent transition metal and lanthanoid based reducing

systems, especially those based on titanium, have provided dramatic advances in efficiency

and selectivity. It is now possible to select appropriate conditions for efficient coupling of all

types of carbonyl compounds, often with high chemo-, region- and stereo-selectivity [33].

The formation of 1,2-diol [34] has been attempted using a number of regents such as Mg

[35], Al [36], Ga [37], In [38], Ti [39-68], V [69-72], Cr [73], Mn [74-75], Fe [76-77], Ni

[78], Zn [79-81] and their compounds, rare earth metals Ce [82-83], Sm [84-89], others [90-

92] and their compounds, non-metallic tellurium [93], the organic metal hydride of Tin

(Bu3SnH) [94].

In addition to using the reagents above by traditional thermodynamic approach to

synthesis of pinacol, microwave technology in solvent-free conditions [95] can also be used.

Coupling also can be initiated photochemically [96-103], electrochemically [104-105],

ultrasonically [106-110], or with the combination of ultrasound irradiation and

photochemistry or electrochemistry techniques [111-113].

However, in some of the described methods, anhydrous conditions and long reaction time

are required to get satisfactory yields of the reaction products. Some of the used reductants

are expensive and these reactions are often associated with the toxic reagents and heavy

metals, which would lead to economical and environmental concerns.

The coupling products can have two newly formed stereocenters.Threo, erythro mixtures

of diols are usually obtained from reactions. As a consequence, efficient reaction conditions

have been required to control the stereochemistry of the 1,2-diols. Recent efforts have

focused on the development of new reagents and reaction systems to improve the reactivity of

the reagents and diastereoselectivity of the products.

Ultrasound has increasingly been used in organic synthesis in the last two decades.

Applications of sonication achieve a number of beneficial effects, for example, accelerate a

reaction, permit the use of less forcing conditions, reduce the number of steps required—

favouring one-pot syntheses, enhance radical reactions, enhance catalyst efficiency, etc.

Compared with traditional methods, this technique is more convenient. A large number of

organic reactions, such as pinacol coupling reaction [114-140], Biginelli reaction [141-150],

Michael addition [151-155], Knoevenagel condensation [156-163], Claisen-Schmidt

condensation [164-169], Cannizzaro reaction [170], Vilsmeier Haack reaction [171],

Reformatsky reaction [172-174], and many other reactions [175-193], can be carried out in

higher yields, shorter reaction time and milder conditions under ultrasound irradiation.

In this chapter, some recent applications of ultrasound in the synthesis of pinacol were

reviewed, the results are mostly from our research group.


MECHANISM

As ultrasonic irradiation has been widely employed in chemistry and chemical

technology, a number of exhaustive monographs [194] and reviews [195] have been

published, there is no need for us to expatiate on the topic. However, a short explanation of

current concepts of sonochemistry seems to be useful for introducing the matter of this paper.

Ultrasonic irradiation differs from traditional energy sources in duration, pressure, and

energy per molecule. It is certain that sonochemical effects cannot be caused by direct impact

of the acoustic field on the reacting molecules since the energy of ultrasonic irradiation is too

low to alter their electronic, vibrational, or rotational states. Cavitation is the origin of

sonochemistry. Irradiation of liquids by power ultrasound leads to cavitation phenomena:

microbubbles present in a liquid are submitted to formation, growth, and finally implosion

[3, 19].

Cavitation is the production of microbubbles in a liquid when a large negative pressure is

applied to it. If a sufficiently large negative pressure is applied to the liquid that cavitation

bubbles will form. Once formed, small gas bubbles irradiated with ultrasound will absorb

energy from the sound waves and grow. Once the cavity has overgrown, it can no longer

absorb energy efficiently. The surrounding liquid rushes in, and the cavity implodes. In

succeeding compression cycles these cavities can collapse violently with the release of large

amounts of energy in and around these microbubbles.

The ‗hot-spot‘ theory suggests that temperatures of up to 5000K, pressures of several

thousand atmospheres, lifetime considerably less than a microsecond, and heating and cooling

rates above 10 billion oC per second are produced during this collapse. For a rough

comparison, these are, respectively, the temperature of the surface of the sun, the pressure at

the bottom of the ocean, the lifetime of a lightning strike, and a million times faster cooling

that a red hot iron rod plunged into water!

Because of the immense temperatures and pressures and the extraordinary heating and

cooling rates generated by cavitation bubble collapse, ultrasound provides an unusual

mechanism for generating high-energy chemistry.

1. Homogeneous Reactions Involving a Single Liquid Phase

The mechanical and chemical effects of the collapsing bubble will be felt in three distinct

regions: the inside of the bubble; at the interface between the bubble and the bulk liquid; in

the bulk media. High temperatures and pressures generated during collapse in the cavity. Less

forcing conditions pluses shock wave on collapse at the interface. Shock wave on collapse

induces shear forces in the bulk media [3, 19].

2. Heterogeneous Reactions Involving Immiscible Liquids

When the reactions were carried out in immiscible liquids such as water and an organic

solvent, the synthetic chemist will induce those reactions by the use of phase transfer catalyst

(PTC). However, in spite of their potential utility, some of the specialized PTC reagents are

suffer from drawbacks such as expensive and all PTCs are potentially dangerous since they


can transfer chemicals from water into human tissue. Sonication can be used to produce very

fine emulsions from immiscible liquids because disruption at phase boundary by cavitational

collapse causes emulsification.

3. Heterogeneous Reactions Involving a Solid and a Liquid

Cavitation can effect two types of reaction involving solid/liquid interfaces: (i) in which

powder is a reagent and trapped gas on surface or in defects cause nucleation and cavitational

collapse resulting in fragmentation and (ii) in which solid is a catalyst such as pinacol

coupling reduced by metal and collapse near solid surface in the liquid phase causes microjet

to hit surface.

A general problem during the preparations of organometallic compounds is that the metal

surface is easily ‗poisoned‘ by the presence of moisture and other impurities. Ultrasonic

irradiation has made it possible to prepare some of these reagents even with technical-grade

chemicals, conditions unheard of in classical methodology. Because sonication can cause

particle rupture, with a consequent decrease in particle size and increase in surface area

available for reaction.

The coupling of ketones to give pinacols is a very old, well-established reaction in

organic chemistry. The synthesis of pinacols from carbonyl compounds is generally thought

of as taking place via reduction of the carbonyl to a radical anion, followed by radical

coupling to give a pinacol dianion, which is subsequently protonated by the medium or upon

quenching [136,138].

The reaction is generally thought to proceed via single electron transfer (SET)

mechanism. Coupling is propagated by single-electron reduction of the carbonyl group from

metal reagent and /or metal complex to form a ketyl radical anion (I), which either undergoes

radical-radical coupling (route a), or is further reduced to the corresponding dianion (II) and

then nucleophilically attacks a second carbonyl group (route b), so leading to pinacol

formation, while dianion (2) can be protonated by H+ to generate alcohol (Scheme 1)

[22,

110]. By far the majority of pinacolic couplings occur via radical-radical coupling and

generally afford a mixture of dl and meso diols [196-197].

H+

R

R

R

RO

O

OH

OH

R

R

R

R

H+

+e

+e

R R

O

.

a

b

R

O

R

R

O

RO

RR

R R

O

.

R

R

R

RO

OOH

RR

H+

(I)

(II)

Scheme 1. Mechanism of pinacol coupling


PINACOL COUPLING UNDER ULTRASOUND

The recent interest in green chemistry has posed a new challenge for organic synthesis in

that new reaction conditions need to be found which reduce the emission of volatile organic

solvents and the use of hazardous toxic chemicals. In this connection, organic reactions in

water or aqueous media have attracted increasing interest currently because of the

environmental issue and understanding biochemical processes. Water offers many practical

and economic advantages as a reaction solvent, including low cost, safe handling and

environmental compatibility. Recently, pinacol coupling reaction in aqueous media has been

described in the literatures [36, 107, 109, 119, 125], however, these methods so far suffer

from harsh reaction and workup conditions, using an excess amount of metal or a long

reaction time.

1. Metal (or Ion) Induced Reductive Coupling of Aromatic Aldehydes and

Ketones in Aqueous Media

In the classic method the reaction of metal induced reductive coupling of aromatic

aldehydes and ketones is usually carried out in organic solvents which are usually flammable,

explosive or hazardous toxic chemicals, and easy to pollute the environment. On the other

hand, the increasing cost in production process is forced to search for environmentally

friendly solvents. In recent years, organic reactions in aqueous solution have attracted

considerable attention. Compared with the organic solvent, water is simple, safe, inexpensive,

and environmentally friendly and so on. The classical active metals used in the reductive

coupling reaction such as Li, Na, etc. due to the role of violent react with water which is very

dangerous. Lim et al. reported the reaction of aromatic aldehydes with indium in aqueous

media using sonication to afford the corresponding diols in moderate to good yield [107]. We

have choosen to examine the potential of Mg, Al, Zn, Mn and other metal in view of their

suitable catalytic activity, cheap and ready availability in pinacol coupling reaction.

1) Zinc Powder Induced Reductive Coupling of Aromatic Aldehydes and Ketones in

Aqueous Media

The Zn-ZnCl2 reagent is not sensitive to oxygen. In 1990, Tanaka et al. reported that the

reductive coupling of aromatic aldehydes and ketones in Zn-ZnCl2 aqueous solution, but the

main product is single-molecule alcohols, the pinacol product was isolated as a by-product in

a very low yield [117]. Delair et al. described the pinacol coupling reaction used of Zn-Cu

alloys under ultrasound irradiation and found that ultrasound accelerates considerably the

aromatic aldehydes and ketones‘ conversion in 1989 [118]. Mecarova and Toma described

the pinacol coupling reaction in aqueous media under ultrasound irradiation and found that

ultrasound accelerates considerably the benzaldehydes‘ conversion with zinc powder in 0.1 M

aq. NH4Cl [119]. Our laboratory has also reported the pinacol coupling of aromatic aldehydes

and ketones induced by Zn-ZnCl2 in aqueous THF media under ultrasound irradiation and the

results showed that ultrasound irradiation can not only accelerates considerably the

chemoselectivity of the reactions but also can significantly increase the yield of diol [120]. In

the classical method, 1,2-bisphenyl-1,2-ethanediol and 1,2-bis(p-chlorophenyl)-1,2-ethanediol


were prepared in only 11% and 16% yield respectively. Whereas under ultrasound irradiation,

they were increased to 49.6% and 51.5% respectively in the same time, 1,2-bis(o,p-

dichlorophenyl)-1,2-ethanediol was up to 77.5% yield in this procedure. Aromatic ketones are

much less efficient for this reaction. Aromatic ketones with electron donating groups such as

m-aminoacetophenone and p-methoxylacetophenone, and steric hindered ketones such as

dibenzyl ketone and benzophenone did not give any pinacol products.

In recent years, catalysts and reagents supported on inorganic substrates have received

increasing attention because of their high level of chemoselectivity and environmental

compatibility as well as simplicity of operation. When certain chemicals are absorbed onto

solid supports their reactivity is enhanced over the reagent itself. Montmorillonite K10 is

known to behave as Bronsted acids in organic reactions. The use of K10 as solid support has

become very useful in synthetic organic chemistry because of its enhanced selectivity due to

its lamellar swelling structure and large surface area. Up to now, the catalyst has been used as

acidic catalyst for many organic reactions. The advantages of the catalyst are easy handing,

chemical inertness, and lower cost, environmentally friendly and easy modification of acidity

by exchanging the cations in the interlayer space. We examined the pinacol coupling

catalyzed by ZnCl2 supported on Montmorillonite K10 instead of ZnCl2, some aromatic

aldehydes such as o,p-dichlorobenzaldehyde, m-chlorobenzaldehyde, cinnamaldehyde and

furfural gave the desired 1,2-diols with 87%, 74%, 75% and 61% yield respectively.

Compared with the reaction catalyzed by ZnCl2 only, the pinacols yield increased about 10%-

30%, and the supported reagent was very easy separation and recycling [121]. In the present

system, the ratio of dl and meso of the corresponding 1,2-diols is about 1:1. The K10-ZnCl2

could be recycled for 3 times without significant losing activity.

Pinacol coupling of aromatic aldehydes in aqueous H2NSO3H or H3PO4 mediated by zinc

powder under ultrasound irradiation could lead to the corresponding pinacols in 14%~88%

yields within 2.5 h. Aromatic aldehydes with electronwithdrawing groups increase the

reactivity. For system Zn-H2NSO3H(aq., 1N), when the substrates are o-chlorobenzaldehyde,

m-chlorobenzaldehyde and m-bromobenzaldehyde, the corresponding pinacol products were

obtained in 70%, 74% and 63% yield respectively for 2.5 h ultrasound irradiation; while using

Zn-H3PO4(aq., 3N), the yield of pinacols were 85%, 88% and 79% respectively, which also

indicated that higher meso-stereoisomer can be obtained and higher yield of pinacols also

obtained under system Zn-H3PO4 when compared to those of Zn-H2NSO3H [122].

If Zn-(COOH)2 (aq.) instead of H2NSO3H or H3PO4 during the reactions, high yields of

pinacol could be obtained when o,p-dichlorobenzaldehyde (78%) and p-chlorobenzaldehyde

(65%) as the substrate compared with the reaction using H2NSO3H or H3PO4 aqueous (the

corresponding yields are 54% and 42% using H2NSO3H, 42% and 34% using H3PO4,

respectively) after 2.5 h ultrasound irradiation [123]. Ultrasound irradiation frequency had

little effect on this reaction system. No coupling of m-chlorobenzaldehyde was observed

when Zn powder was replaced by Mg or Al powder.

Zinc is a amphoteric metal，the reductive coupling of aromatic aldehydes in a basic

system such as Zn-NaOH (10% aq.)-MeOH under ultrasonic irradiation was observed. The

results showed that the basic situation not only further improved the pinacols yield of

aromatic aldehydes with electron-withdrawing substituents in the benzene ring, but also

aromatic aldehydes with electron-donating substituents in the benzene ring such as p-

methylbenzaldehyde and p-methoxybenzaldehyde yielded the pinacols in 92% and 80%


respectively [124]. Compared with the acidic medium, the coupling in basic medium was

significantly improved on the chemoselectivity and shortened the reaction time from 2.5-3 h

to 25-60 min. When piperonaldehyde was used as substrate, the meso-pinacol was obtained

only, while other aldehydes as the substrates, the ratio of dl and meso was about 1:1.

2) Reductive Coupling of Aromatic Aldehydes and Ketones Induced by Magnesium

Powder in Aqueous Media

In 1999, Zhang and Li reported a simple and effective method for pinacol coupling

reactions of various aromatic aldehydes and ketones in aq. NH4Cl (0.1 N) mediated by

magnesium with stirring [125]. The yields of pinacols were 56-96%, but it needed a long

reaction time (12-24 h) and the molar ratio of ArCHO:Mg turning as high as 1:20. Mečiarová

et al. [109] studied the influence of reaction time, quality and quantity of magnesium, and

reaction conditions on pinacol coupling of benzaldehyde by Mg-NH4Cl (or H2O). They

decreased the molar ratio of PhCHO:Mg to 1:10, and found that ultrasound irradiation can

accelerate the pinacolization of benzaldehyde using magnesium turning, the conversion of

benzaldehyde up to 100% determined by 1H NMR and the corresponding pinacol was

obtained in 95% yield within 90 min. It is clear that the ultrasound can accelerate the metal-

induced pinacolization of benzaldehyde, but the pinacolization of other aldehydes did not

involve in the paper. They proved also that reaction could be carried out in pure water

(without addition of ammonium chloride) in very good yields. Later on they found also that

very good yields of pinacols are formed with zinc powder in aqueous NH4Cl, but no reaction,

even under sonication, was observed with iron, nickel and tin powders.

We studied the coupling of aromatic aldehydes induced by magnesium powder in the

NH4Cl (0.1M) aqueous within 3 h at room temperature under ultrasonic irradiation. The data

were shown that this system was very effective for the aromatic aldehydes with electron-

donating substituents in the benzene ring such as p-methylbenzaldehyde and p-

methoxybenzaldehyde yielded the pinacols both in 95% and the ratio of dl/meso of

corresponding pinacol were 9/1 and 2/1 respectively. When benzaldehyde and

piperonaldehyde were used as the substrates, the corresponding pinacols were obtained in

75% and 72% respectively, while the ratios of dl/meso were about 1:1 [127].

The pinacol coupling of aromatic aldehydes and ketones was carried out in 20-62% and

10-91% yield respectively with Mg and Mg-MgCl2 in water under ultrasound irradiation at

r.t. for 3-4 h. For example, 1,2-diphenyl-1,2-ethanediol, 1,2-di(p-methylphenyl)-1,2-

ethanediol and 1,2-di(p-methoxyphenyl)-1,2-ethanediol were obtained with 85%, 90% and

91% yield respectively using Mg-MgCl2 at r.t for 3 h under ultrasound and higher than those

in stirring condition. Furthermore, when aromatic aldehydes with electron-withdrawing such

as the p-chlorobenzaldehyde, m-chlorobenzaldehyde and furfural were used as the substrates,

the pinacols were obtained in 70%, 65% and 60% respectively, while in Mg-NH4Cl the yield

of pinacols decreased dramatically. By comparison, the reaction activity induced by Mg in

pure water significantly lower than in MgCl2 aqueous [128]. The coupling of aromatic

ketones showed very lower reactivity in the above two systems. While insteaded to Mg-

NH4Br (aq.), the coupling of acetophenone was successful coupling in 66% yield for 3 h with

sonication. The ratio of dl and meso of the corresponding 1,2-diols is 71/29, and no alcohol

was found during the reaction. The similar coupling yields of aromatic aldehydes were

obtained in the Mg-MgCl2 (aq.) syntem, but the meso-isomer was increased [129]. It was


shown that Lewis acid not only affects the chemoselectivity of the reaction, but also the

diastereoselectivity of the pinacol.

At room temperature, the Mg-MgI2 system has high chemoselectivity especially for

benzophenone, furaldehyde, α-acetonaphthone and β-acetonaphthone, to give the desired 1,2-

diols with 99%, 96%, 90% and 91% yields respectively within 20-60 min in a mixed solvent

of ether-benzene. The dl/meso ratio of the coupling products of α-acetonaphthone and β-

acetonaphthone were 8/2 and 7/3. We also observed the effect of different frequency of

ultrasound irradiation on this reaction. When the frequency was 25 kHz, the coupling of

benzaldehyde resulted the desired product in 56% yield in ethanol, while under 40 kHz and

59 kHz ultrasound irradiation, the pinacolization were completed with 49% and 35% yields

respectively [130]. It is shown that lower frequency of ultrasound irradiation improved the

result. The reason may be that the lower frequency condition creates the better cavitation.

3) Reductive Coupling of Aromatic Aldehydes and Ketones Induced by Aluminum

Powder in Aqueous Media

Khurana et al. reported pinacol coupling of aromatic aldehydes and ketones promoted by

aluminum in KOH aqueous solution in 1994 and 1996 respectively, and the corresponding

1,2-diols were produced in high yields [131]. But excess amount of alkali (the molar ratio of

substrate /KOH = 1/9) was used during the reaction. In 1999, Mečiarová et al. reported that

benzaldehyde can coupled into pinacol at a lower concentration of KOH aqueous solution

using ultrasonic probes [109]. The reaction time was shortened but the yield did not meet

Khurana‘s result. Furthermore, the yield was determined by 1H NMR analysis, not the

isolated yield.

Under ultrasound irradiation the pinacol coupling reaction of aromatic aldehydes and

ketones was carried out in 60%-98% yield with aluminum in aqueous NaOH-MeOH at r.t.

within 20-30 min [132]. Among them, benzaldehyde, p-methoxybenzaldehyde and p-

methylbenzaldehyde were reacted smoothly with aluminum in aqueous NaOH-MeOH. The

corresponding pinacol coupling products were obtained in 91%, 89% and 88% yield

respectively, m-chlorobenzaldehyde and p-chlorobenzaldehyde gave nearly quantitative yield

of 1,2-diols. But o,p-dichorobenzaldehyde gives 61% pinacol only. The reason may be that

the steric hindrance around carbonyl group inhibits the coupling during the reaction. When

aromatic ketones such as p-methoxyacetophenone and p-chloroacetophenone were used as the

substrates, less pinacols and more alcohols were obtained, while the reaction with

cinnamaldehyde was unsuccessful in the same conditions.

Metal aluminum has a low first ionization potential (5.986 eV) and the presence of trace

fluoride ion can accelerate corrosion of aluminum. In 2000, Chen et al. reported that the

conversion of benzaldehyde was 100% (measured by 1H NMR) by stirring for 16 h in the Al-

KF aqueous solution, and yield of pinacol was obtained in 87% [36d]. While under

ultrasound irradiation, benzaldehyde conversion will reach 99% within 1.5 h, 1,2-diphenyl-

1,2-ethanediol yield can reach 82%. But the coupling product of other aromatic aldehyde was

not as good as stirring conditions [198].


4) Manganese Powder Induced Reductive Coupling of Aromatic Aldehydes and

Ketones in Aqueous Media

The pinacol coupling of aromatic aldehydes was carried out in 40-90% yield with

manganese in aqueous NH4Cl at r.t. for 2 h under ultrasound irradiation. The reactions in Mn-

MnCl2/THF:H2O(1:4) system gave pinacols in 30-95% yield at r.t. for 2-3 h under ultrasound

[133]. Compared with classical method, the main advantages of the present procedure are

shorter reaction time, less reagent quantity and higher yield. For example, 1,2-diphenyl-1,2-

ethanediol, and 1,2-di(p-chlorophenyl)-1,2-ethanediol were prepared previously in 65% and

64% yield respectively using manganese in aqueous NH4Cl at r.t. for 16 h, whereas under

ultrasound irradiation, 1,2-diphenyl-1,2-ethanediol and 1,2-di(p-chlorophenyl)-1,2-ethanediol

were obtained in 70% and 90% yield respectively at r.t. for 2 h. Compared with Mn-NH4Cl

(aq.)-THF system, the Mn-MnCl2 (aq.)-THF system can lead to the higher chemoselectivity.

It is indicated that the aromatic aldehydes with electron-withdrawing substituents in the

benzene ring show higher reactivity and higher yield than those electron-donating

substituents. The coupling has lower stereoselectivity in the Mn-MnCl2 (aq.)-THF system and

the ratio of dl/meso is about 3/2.

The Mn-HOAc-H2O system was less effective in pinacol coupling as the above-

mentioned two systems [199].

5) Reductive Coupling of Aromatic Aldehydes Induced by Vanadium (II) in Aqueous

Solution

Vanadium (II) complexes have been recognized to be highly capable of one–electron

reduction, thereby including radical reactions such as reduction of several organic substrates

[200] and pinacol-type reductive coupling [201]. In 1926, Conant reported the dimolecular

reduction of carbonyl compounds by vanadium and chromous salts [202], but some aromatic

aldehydes were slowly reduced by vanadium salts in the presence of acid, alcohol or acetone.

In 1989, Pedersen reported the stereoselective coupling of two different types of substrates,

yet electronically similar aldehydes employing the well-characterized vanadium (II) complex,

[V2Cl3(THF)6]2[Zn2Cl6]; the major diastereomer in all of the cross coupling reactions is a

threo diol [203]. Hirao reported highly diastereoselective pinacol coupling of secondary

aliphatic aldehydes induced by Cp2VCl3/R3SiCl/Zn [69a]; and using VOCl3/Me3SiCl/Al

system [69c], six aromatic aldehydes gave desired pinacols in 49%-89% yields, their

diastereoselectivities were high (dl:meso≧9:1).

Pinacol coupling of aromatic aldehydes by aqueous vanadium (II) solution under

ultrasound irradiation at 15-35 oC can lead to the corresponding pinacols in 78%-93% yields

within 15-30 min. The substituent group in the benzene ring has no significant effection on

the reactivity but the stereoselectivity. For example, when p-methylbenzaldehyde, p-

methoxybenzaldehyde, and piperonaldehyde as substrate, the ratio of dl and meso of 1,2-diols

was dl isomer, 91/9 and 92/8. While benzaldehyde and m-chlorobenzaldehyde as the

substrate, the ratio of dl and meso was about 72/38 and 67/33 respectively [134].

The optimization reaction condition and yield of pinacol coupling in aqueous under

ultrasound irradiation were summarized in Table 1.

Should be clear is that eithor the classical method or ultrasound, Sn can not reduce

aromatic aldehydes and ketones to vicinal diol. When the aldehydes and ketones with nitro-

group in the benzene ring as substrates, there were no pinacol obtained, but the nitro was


reduced to amino group. The pinacol coupling was difficult to go on under the above

mentioned conditions when the aldehydes and ketones with amino groups in the benzene ring.

Table 1. The optimization reaction condition and yield of pinacol coupling in aqueous

under ultrasound irradiation

Entry ArCHO Met/ (aq.)/ time Isolated

yield, % *dl/meso

1 C6H5CHO Al/NaOH-MeOH(aq.)/20 min 91[132]

VCl2/EtOH(aq.)/15 min 91[134] 72/28

Mg/MgCl2(aq.)/3 h 85[128] 31/69

2 4-CH3OC6H4CHO Al/NaOH-MeOH(aq.)/20 min 89[132]

Mg/NH4Cl(aq.)/3 h 95[127]

VCl2/EtOH(aq.)/30 min 89[134] 91/9

Mg/MgCl2(aq.)/3 h 91[128] 7/93

3 4-CH3C6H4CHO Al/NaOH-MeOH(aq.)/20 min 88[132] 71/29

Mg/NH4Cl(aq.)/3 h 95[127] 90/10

VCl2/EtOH(aq.)/30 min 85[134] 100/0

Zn/NaOH-MeOH(aq.)/30 min 92[124] 51/49

Mg/MgCl2(aq.)/3 h 90[128] 92/8

4 4-ClC6H4CHO Al/NaOH-MeOH(aq.)/20 min 98[132] 71/29

VCl2/EtOH(aq.)/25 min 92[134]


Mn/MnCl2/THF(aq.)/2 h 95[133] 50/50

5 3-ClC6H4CHO Al/NaOH-MeOH(aq.)/20 min 98[132]

VCl2/EtOH(aq.)/15 min 86[134] 67/33


Mn/MnCl2/THF(aq.)/2 h 86[133] 61/39

6 2-ClC6H4CHO VCl2/EtOH(aq.)/15 min 88[134] 61/39

7 2,4-Cl2C6H3CHO Zn/K10-ZnCl2(aq.)/THF/3 h 87[121] meso

8 3-BrC6H4CHO VCl2/EtOH(aq.)/15 min 93[134] 37/63

9 piperonaldehyde VCl2/EtOH(aq.)/25 min 87[134] 79/21


10 furaldehyde Mg-I2/ether-benzene/60 min 96[130] 57/43

11 PhCOPh Mg-I2/ether-benzene/30 min 99[130]

12 α-acetonaphthone Mg-I2/ether-benzene/60 min 90[130] 79/21

β-acetonaphthone Mg-I2/ether-benzene/60 min 91[130] 70/30

*dl/meso were determined by 1H NMR.

2. Reductive Coupling of Aromatic Aldehydes and Ketones Using Low-

Valent Titanium

Low valent titanium is a highly reactive reagent and attracts increasing interest in

carbonyl-coupling reactions. High valent titanium reagent or complexes could be reduced by

some metal to corresponding low valent titanium complexes, which can induce some

aromatic aldehydes and ketones occurred the pinacol coupling reaction. In 1973, Mukaiyama


firstly reported that TiCl4-Zn reduced aromatic aldehydes and ketones to produce the

corresponding 1,2-diols in high yield [135], but the stereoselectivity was not reported. With

improved of the preparation of low-valent titanium, the in-depth study was underwent and

there are many reports on the synthesis of 1,2-diol using low-valent titanium complex in

recent years [22, 39, 42, 43, 51, 136]. However, in spite of their potential utility, some of the

reported methods suffer from drawbacks such as expensive catalysts and critical reduction

conditions. McMurry et al. [33] reported that coupling reaction induced by low valent titanium gave

pinacols at 0 oC, but at reflux temperature, it gave alkenes through deoxygenation. In the

presence of ultrasonic irradiation, the coupling was carried out at room temperature, gave

pinacol in high yield, and improved the chemoselectivity and stereoselectivity. Besides, the

competing Cannizzaro reaction, giving alcohol and carboxylic acid, was not observed and

there was also no olefin formation arising from McMurry reactions.

1) Reductive Coupling of Aromatic Aldehydes Induced by TiCl4-M (Zn, Mg, Al)-THF

in CH2Cl2

In 2001, Yamamoto et al. reported diastereoselective pinacol coupling of aldehydes

promoted by monomeric titanocene (III) complex Cp2TiPh [61]. Five aromatic aldehydes

given desired pinacol in 54-96% yields within 1-4 h. In 2000, Li et al. reported the 1,2-diols

were obtained in pinacol coupling mediated by TiCl4-Mg with a high stereoselectivity [42].

These systems of TiCl4-THF-Zn, TiCl4-THF-Al, TiCl4-THF-Mg can quickly reduce a

number of aromatic aldehydes to pinacol with high yields and high stereoselectivity under

ultrasound. Without ultrasound, the pinacols were obtained in lower yield. For example, using

TiCl4-THF-Zn under Ar stirring for 30 min, 1,2-diphenyl-1,2-ethanediol was previously

prepared in 57% yield [51]; using TiCl4-Et2O-Al and stirring for 38 h gave 1,2-diphenyl-1,2-

ethanediol in 50% yield [43]. Whereas under ultrasonication for only 5 min, in the presence

of TiCl4-THF-Zn, replacement of Ar by N2, 1,2-diphenyl-1,2-ethanediol was obtained in 96%

yield. TiCl4-THF-Al system provided 1,2-diphenyl-1,2-ethanediol in 90% yield for 20 min

[137]. Unfortunately, this method applies only to aromatic aldehydes with electron-donating

substituents in the benzene ring.

As shown in Table 2, the type of reduce metal has some effects on the reaction speed,

yield and product stereoselectivity. Al was proven to be more diastereoselective (dl/meso)

than Zn. The reaction in TiCl4-THF-Zn reduction system can be carried out in higher yields

and shorter reaction time, but lower stereoselectivity. While in TiCl4-THF-Al system, pinacol

coupling can be carried out in higher yields within 15-20 min, and the stereoselectivity was

also improved. Compared with the previous two systems, TiCl4-THF-Mg system was not

efficient to the reaction [137].

2) Reductive Coupling of Aromatic Aldehydes Induced by TiCl3-M (Al, Mg, Mn, Zn)-

EtOH

In 1982, Clerici et al. reported pinacol coupling of aromatic aldehydes and ketones

promoted by aqueous titanium trichloride in basic media [138]. The reaction was completed

in few minutes, but the reducing power of Ti3+

/Ti4+

system is strongly pH dependent, the

method has some limitations with respect to some aromatic aldehydes and ketones.


Clerici et al. again reported pinacolization of aromatic aldehydes mediated by titanium

trichloride in dichliromethane in 1996 [139]. The reaction was completed in high dl-

stereoselectivity, but aromatic aldehydes bearing an electron-donating group showed lower

reactivity. Recently, we reported the pinacolization mediated by TiCl4-M (Zn, Mg, Al)-THF

in CH2Cl2 at room temperature under ultrasound irradiation. Eight pinacols were obtained in

33-98% yield within 4-35 min. All of the results stated above prompted us to study the

possibility of the pinacol coupling of aromatic aldehydes mediated by TiCl3-Mn-EtOH,

TiCl3-Mg-EtOH, TiCl3-Al-EtOH and TiCl3-Zn-EtOH systems under ultrasound [140]. The

results were summarized in Table 3.

As shown in the Table 3, the coupling of some aromatic aldehydes was carried out in

good yields and diastereoselectivity using TiCl3-M-EtOH under ultrasound irradiation.

Compared with the classic stirring for 30 min in TiCl3-CH2Cl2, it could smoothly undergo that

the pinacol coupling of aromatic aldehydes carrying electron-withdrawing, and 1,2-diol was

obtained in higher yield in this procedure. Furthermore, aromatic aldehydes carrying electron-

donating substituents could also couple to pinacol in higher yield, which those did not

successed in TiCl4-THF-M system, but lower diastereoselectivity than those in latter system.

Improved diastereoselectivity has been observed in our system compared to the

analogous process in THF at room temperature [61]. When p-methylbenzaldehyde, p-

methoxybenzaldehyde as a substrate, the ratio of dl and meso of the 1,2-diols is 74:26 and

72:28 respectively in Yamamoto et al., report. In the TiCl3-Al-EtOH system, the ratio of dl

and meso of the corresponding 1,2-diols is 91:9 and 8:92 respectively.

Table 2 The reductive coupling of aromatic aldehydes using TiCl4-THF-M (M: Zn、Al

or Mg) at r.t. in CH2Cl2 under ultrasound irradiation

Entry Substrate Reduction

systema

Time,min Isolated yield,

% dl/meso*

a C6H5CHO A 5 96 76/24

B 20 90 93/7

C 20 68 77/23

b 3-ClC6H4CHO A 4 98 82/18

B 15 96 97/3

C 30 87 92/8

c 4-ClC6H4CHO A 4 98 69/31

B 15 98 96/4

C 20 89 85/15

d 2,4-Cl2C6H3CHO A 6 98 51/49

B 25 92 97/3

C 25 84 32/68

e 2-ClC6H4CHO A 5 97 74/26

B 20 92 34/66

C 20 79 52/48

f 3-BrC6H4CHO A 5 98 82/18

B 15 95 97/3

C 20 84 90/10 aA: TiCl4-THF-Zn; B: TiCl4-THF-Al; C: TiCl4-THF-Mg; *dl/meso were determined by

1H NMR.


Table 3 The reductive coupling of aromatic aldehydes using TiCl3-M (Mn、Mg、Al or

Zn)-EtOH under ultrasound irradiation

Entry Substrate Systemsa Time, min Isolated

yield, %

dl/meso*

a C6H5CHO A 40 64 73/27

B 40 75 68/32

C 60 75 63/37

D 40 89 63/37

b 2-ClC6H4CHO A 40 56 37/63

B 30 56 27/73

C 45 68 20/80

D 35 88 45/57

c 3-ClC6H4CHO A 40 67 41/59

B 20 85 28/72

C 40 79 66/40

D 35 92 53/47

d 2,4-Cl2C6H3CHO A 50 86 18/82

B 30 75 20/80

C 30 71 21/79

D 40 79 18/72

e 4-ClC6H4CHO A 40 69 38/62

B 30 92 25/75

C 30 79 66/34

D 40 92 65/35

f 3-BrC6H4CHO A 60 63 49/52

B 15 95 44/56

C 35 82 60/40

D 35 91 46/54

g 4-CH3C6H4CHO A 40 70 84/16

B 30 89 63/37

C 80 69 91/9

D 35 87 47/53

h 4-CH3OC6H4CHO A 40 72 74/26

B 40 81 66/34

C 80 62 8/92

D 45 83 60/40

i 3,4-(OCH2O)C6H3CHO A 50 62 89/11

B 15 86 86/14

C 50 69 66/34

D 30 87 66/34

j Furaldehyde B 30 86 55/45

C 50 58 59/41 aA:TiCl3-Mn-EtOH, B:TiCl3-Mg-EtOH, C:TiCl3-Al-EtOH, D:TiCl3-Zn-EtOH; *dl/meso were

determined by 1H NMR.


The coupling of some aromatic aldehydes was also carried out in good yield using TiCl3-Al in water under ultrasound irradiation. For example, 1,2-bis(p-methylphenyl)-1,2-ethanediol was previously prepared in 35% yield using TiCl3-CH2Cl2 under stirring for 30 min [139], whereas under ultrasonication, 1,2-bis(p-methylphenyl)-1,2-ethanediol was obtained with 52% yield. In Bhar and Panja’ [36b] report, 1,2-bis(o-chlorophenyl)-1,2-ethanediol was prepared in 62% yield using Al-NaOH-H2O under stirring for 120 min, whereas in this procedure, 1,2-bis(o-chlorophenyl)-1,2-ethanediol was obtained with 72% yield within 45 min. It is noteworthy that the reagents used are readily available inexpensive and stable to air oxidation, and the method is easier and more convenient compared with those so far reported.

It was shown that lower frequency of ultrasound irradiation improved the yield of pinacol coupling. The type of reducing-metal is a very important factor in the reaction, which related to the reduction of high valent titanium ability of metal. Furthermore, the type of solvent or ligands has a significant impact on the stereoselectivity of the products.

The following sequence of reaction appears to be a reasonable rationalization for the formation of the products [136, 138] (Scheme 2, The reductive coupling of aldehydes induced by TiCl4-M). The dl-diastereoselection could be explained by the initial generation of intermediate radical species whose oxygen atoms of the two ketyl radicals are linked side by side to the low valent titanium species and their alkyl groups are located anti each other to minimize the steric interaction. That is dl-pinacols are preferentially formed by an internal carbon-carbon coupling of 'titanium-bridged' intermediate A which is formed readily due to the highly coordinating ability of low valent titanium species. In the absence of 'titanium-bridged', the radicals of intermediate B, for which steric and polar effects appear to be important, lead predominantly to the meso dimmers.

R H

O++ M

R

TiO

H.

O Ti

.HR

Ti-O interaction

R

TiO

H.

OTi

.H

R

intermediate B

"Ti-bridged"intermediate A

R

RH

HOH

HO

R

RH

H

OHHO

dl selective

meso selective

TiCl4

Scheme 2.


CONCLUSION

Cavitation produces an unusual method for fundamental studies of chemistry and physics

under extreme conditions, and sonochemistry provides a unique interaction of energy and

matter. One may be optimistic that the unusual reactivities caused by ultrasound will find

important industrial application in the years to come.

Ultrasound irradiation as being of great value in pinacol coupling reaction has been

amply demonstrated by the many examples presented in this chapter. It improved the

chemoselectivity significantly of the reactions, but the stereoselectivity of the reaction was

expected to be further improved. The largest current drawback of the reaction is its

mechanism, diastereoselectivity, chiral synthesis and resolution of chiral isomer. As a

consequence, efficient reaction conditions have been required to control the stereochemistry

of the 1,2-diols. Recent efforts have focused on the development of efficient and

environmentally friendly reagents and reaction systems to improve the reactivity of the

reagents and diastereoselectivity of the products. We look forward to increasing the uses for

ultrasound irradiation in organic synthesis reaction.

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Chapter 4

ULTRASOUND AND HYDROPHOBIC

INTERACTIONS IN SOLUTIONS

Ants Tuulmets, Siim Salmar and Jaak Järv Institute of Chemistry, University of Tartu, Ravila 14A, 50411

Tartu, Estonia

ABSTRACT

Sonochemistry and solution chemistry have been explicitly brought together by

analyzing the effect of ultrasound on kinetics of ester hydrolysis and benzoin

condensation, measured by the authors, and similar kinetic data for the solvolysis of tert-

butyl chloride, compiled from literature. For the first time the power ultrasound, reaction

kinetics and linear free-energy relationships were simultaneously exploited to study ionic

reactions in water and aqueous-organic binary solvents and the importance of

hydrophobic ground-state stabilization of reagents in aqueous solutions was discussed.

This approach has opened novel perspectives for wider understanding of the effect of

sonication on chemical reactions in solution, as well as on solvation phenomena in

general.

1. INTRODUCTION

Because ultrasound promotes or accelerates a wide range of chemical and physical

processes [1-4], it has been used for a variety of purposes in areas as diverse as surface

cleaning, food technology, medical diagnostics and therapy, sewage treatment and chemical

synthesis. The latter applications, commonly described by the term ―sonochemistry ‖, have

proven to be invaluable and unique tools for making nanomaterials [5], in green technologies

[6], and certainly in organic synthesis [4,7,8].

As many homogeneous and heterogeneous reactions are initiated or accelerated by

ultrasound through generation of free radicals, which give rise to chain reactions [1,4], this

mechanism has been canonized in sonochemistry, and homogeneous ionic reactions have long

been confined to a marginal place in this vast domain of chemical reactivity. However, more

Ants Tuulmets, Siim Salmar and Jaak Järv 130

recent applications of quantitative methods like reaction kinetics and the linear free-energy

analysis (also known as ―correlation analysis‖), have lead to a better understanding of

sonochemical effects also in ionic reactions, as well as in the solvation phenomena taking

place in binary solvents in general.

In parallel to the analysis of sonication effects in kinetic data and their reliability,

measured in aqueous-organic binary solvents, the physical meaning of thermodynamic

activation parameters and other relevant issues will be discussed in this chapter.

Sonochemically investigated reactions implicated in this work as model processes are

solvolysis of tert-butyl chloride, acid-catalyzed, neutral, or base-catalyzed hydrolysis of

esters, and the benzoin condensation of benzaldehyde. These model processes have been

kinetically studied in water (if possible) and in aqueous-organic solvent mixtures involving

various alcohols and 1,4-dioxane as co-solvents.

Many of the conclusions reviewed in this chapter reach out beyond the conventional

sonochemistry actually contributing to solution chemistry and physical organic chemistry.

Among the most significant inferences from the results of these investigations, the

paramount importance of hydrophobic ground-state stabilization of reagents in aqueous

solutions, taking place independent of the reaction mechanism has to be stressed, an effect

mostly overlooked in conventional analysis of solvent effects [9]. An experimental

demonstration of formerly predicted ultrasonic retardation of reactions, reluctantly accepted

by the sonochemical community, provided a conclusive evidence for occurrence of

homogeneous ionic sonochemical reactions in the bulk solution instead of the cavitation

bubbles as commonly believed.

2. CURRENT VIEWS OF THE SONOCHEMISTRY IN SOLUTIONS

It is certain that sonochemical effects cannot be caused by direct impact of the acoustic

field on the reacting molecules, since the energy of ultrasound is too low to alter their

electronic, vibrational, or rotational states [1-4]. Therefore most often the effect of ultrasound

has been explained by the "hot spot" theory that assumes the involvement of cavitation

bubbles [10]. The nucleation, growth and collapse of these bubbles constitute the cavitation

phenomenon. According to the "hot spot" theory, each cavitation bubble acts as a localized

micro-reactor in which high temperatures and pressures are generated, reaching several

thousand degrees and hundreds atmospheres, thus effectively concentrating within "hot spots"

the diffuse energy of sound wave [1-3]. As the nearly adiabatic bubble collapse will thus

enhance molecular energy by almost nine orders of magnitude, it is no wonder that ultrasound

can affect chemical reactions.

The sonochemical process is usually thought to be localized either inside the cavitation

bubble or in the liquid shell surrounding it, or in both simultaneously. A general model

developed by Reisse et al. [11] considers the cavitating liquid as heterogeneous: each

collapsing bubble, acting as a closed microreactor, presents a physical environment that is

quite different from that of the bulk liquid phase.

Sonochemistry in solutions has been often rationalized in terms of this theory: solvents

that are volatile enough can be vaporized into the bubble where they will undergo pyrolytic

cleavage to form radicals or excited chemical species. These may induce subsequent reactions

Ultrasound and Hydrophobic Interactions in Solutions 131

with less volatile substrates at the bubble shell or, perhaps most frequently, in the bulk

medium.

Ionic reactions are extremely rare in the gas phase because separated ions are unstable

when not solvated. In other words, the dissociation of a molecule into ions is a process of

very low probability in the gas phase. Even chemical reactions where the activated complexes

are characterized by high dipole moments are uncommon in gas phase chemistry. Thus it is

hardly possible to conceive a neat heterolytic reaction in the gas phase of the bubble.

However, such reactions can take place in the liquid shell.

It is important to mention that intense shock waves form upon the collapse of cavitation

bubbles causing various mechanical actions. These are the mainstays of the explanation of

sonochemical effects on heterogeneous processes leading to enhanced reaction rates and

yields, which, however, often do not differ from those obtained by the use of a high-speed

stirrer [12].

Chemical effects of ultrasound will only occur if a particular reaction is the sonication

sensitive step of the process or when the active species released from cavitational collapse

participate as reaction intermediates. Luche et al [13] have distinguished sonochemical

applications resulting from "true" or "false" effects. The former are real chemical effects

induced directly by cavitation ("true sonochemistry"), while the latter can be mainly ascribed

to the mechanical impact of bubble collapse. A set of empirical rules has been established by

Luche [4,13]. While Rules II and III concern heterogeneous reactions, the Rule I states that

homogeneous reactions activated by sonication are those proceeding via radical or radical-ion

intermediates. Thus, according to Luche, homogeneous ionic reactions should not be affected

by sonication.

However, examples of ultrasonic acceleration of homogeneous ionic reactions had been

reported already before formulation of the Rules. Although they provided a challenging

puzzle, little attention was paid to them by sonochemists until recently, perhaps because these

reactions do not profit from the use of sonication in comparison with synthetically important

ones.

3. IONIC REACTIONS ACCELERATED BY ULTRASOUND

Following the current principles of sonochemistry (vide supra), it can be concluded that

an ionic reaction which is not switchable to a radical pathway, should not be susceptible to

ultrasound effect. However, several examples of homogeneous polar reactions accelerated by

ultrasound have been found, mostly hydrolysis and solvolysis reactions that have been

kinetically investigated for sonication effects. In the first paper of this kind [14], published

already in 1953, the acid-catalyzed hydrolysis of ethyl acetate in aqueous solution was

studied. The sonication effect was small but exceeded the experimental error.

Later, the acid-catalyzed hydrolysis of methyl acetate has been investigated by three

groups [15-17]. In all these works similar experimental conditions were used and the kinetics

was followed by titration of the formed acid. The reaction was performed in water without

sonication and under sonication and also in a water-acetone binary solvent [17]. The

sonication effect was from low to moderate, the rate enhancement not exceeding 60%.

Under conditions affording more pronounced sonication effects, a many-fold acceleration

of the acid-catalyzed hydrolysis of ethyl acetate in water was attained at 22 kHz [18].


However, detailed investigations revealed sonication effects not exceeding 30% in water

medium [19-21].

Still lower acceleration effect (14-15 %) by ultrasound has been reported for the alkaline

hydrolysis of 4-nitrophenyl esters of several aliphatic carboxylic acids in a water-acetonitrile

mixture [22]. Kinetics of this reaction was followed by spectrophotometric monitoring of 4-

nitrophenol formation. Similarly, for the base-catalyzed hydrolysis of 4-nitrophenyl acetate in

water accelerations in a range 10 to 12% were found [23]. Moderate sonication accelerations

were found over the 4-13 pH range for the hydrolysis of phthalic acid esters in aqueous

solution [24].

In contrast to these findings, an ultrasonic acceleration by two orders of magnitude was

reported by Hua et al. [25] for the hydrolysis of 4-nitrophenyl acetate in aqueous solution

over the pH range of 3-8 at 20 kHz. However, the reliability of their experimental procedure

has been questioned and just a moderate ultrasonic acceleration of the reaction was found by

Ando et al [26]. Later on, the comparative use of titanium and quartz immersion horns for

sonication [23] enabled rationalization of the sonication effects reported by Hua et al.

Evidently the large sonication accelerations observed were not merely caused by direct effects

of ultrasound but also involved a considerable contribution from catalytic effects probably

due to metal traces from titanium horns [23].

It is remarkable that only negligible to small sonication effects have been found in water

or in mixtures with organic solvents of low ability to form hydrogen bonds.

In contrast to this large sonication effects up to 20 times were observed for the solvolysis

reaction of 2-chloro-2-methylpropane (tert-butyl chloride) in ethanol-water [27-30],

isopropanol-water [28] and tert-butanol-water [28] mixtures by Mason‘s group. (Scheme 1,

a). Kinetics of the solvolysis was followed conductometrically. Surprisingly, the effect of

ultrasound showed nonlinear dependences on the composition of aqueous binary mixtures.

For example, at 10° C the solvolysis rate in 20 wt% of ethanol in the presence of ultrasound

was twice that in the absence of irradiation, whereas at 40% and 60% of ethanol the rate

increases were six- and 20-fold, respectively. The solvolysis of 1-bromo-1-phenylethane in

alcohol-water mixtures has been studied by another group [31] and also an ultrasonic

acceleration of the reaction was observed, however, the stereoselectivity was unaffected in all

cases.

Kinetic investigations of sonication effects in water-organic binary mixtures were

recently reopened by our group [19-21,23,32-35]. The acid-catalyzed hydrolysis of alkyl

esters (Scheme 1, b) in water-ethanol and in water-1,4- dioxane binary mixtures were studied

in these works [19-21,32,35]. Also the base-catalyzed hydrolysis of 4-nitrophenyl acetate

(Scheme 1, c) [23] as well as the benzoine condensation of benzaldehyde in water and in

ethanol-water binary mixtures [33,34] were investigated for sonication effects.

Results of this extensive research allowed to draw a number of important conclusions

about the mechanism of sonication effects for homogeneous ionic reactions as well as on the

nature of solvation phenomena in aqueous-organic systems. The most important results

concern the hydrophobic interactions in solutions and their role in determining the reactivity

in solutions.


CH3

C

CH3

CH3

Cl

CH3

C+

CH3

CH3

Cl

OH2

CH3

C

CH3

CH3

OH

OH3

+

CH3

C

O

OR

OH

CCH3

OR

OH2

+

CH3

C

O

OH ROH H+

H+

OH2

CH3

CO

NO2

O OC

OH O

CH3

NO2

CH3

C

O

O

OH

NO2

OH

+ +

,

+ +

+

a)

b)

c)

Scheme 1. a) Solvolysis of 2-chloro-2-methylpropane, b) acid-catalyzed hydrolysis of an alkyl acetate,

c) base-catalyzed hydrolysis of 4-nitrophenyl acetate.

4. WATER AND HYDROPHOBIC INTERACTIONS

Water occupies a special place in chemistry because of its role as the solvent for all of the

chemical reactions of life. Water is also a desirable solvent for industrial chemical reactions

for reasons of cost, safety, and environmental concerns [6,36]. Moreover, this interest arises

from the fact that reactivity of some compounds benefit from the unique properties of water,

resulting inter alia from hydrophobic interactions to which species are subjected when

dissolved in water [37-39].

Although water is not frequently the solvent of choice because it is a poor solvent for

nonpolar compounds, solubility of these compounds in water can be improved by additions of

miscible with water organic solvents. This largely expands the range of feasible reactions.

Among such additives the lower alcohols distinguish in all aspects as green solvents.

Solvation of reactants is one of the most important factors governing the rates of polar

reactions [9]. In binary solvents this dependence is complicated by the occurrence of

preferential solvation. This means that the composition of the solvation shell around reacting

species is different from that of the bulk solvent. In solvents that can form hydrogen bonds

the structure of the medium is also of great importance [40]. This seems to be the main reason

why quantitative solvent effects on organic reactivity have been extensively studied mainly

for pure solvents, and great numbers of correlation equations have been suggested for

description of these effects [9]. Alongside of this mainstream, investigations into solvent

effects in binary solvents, including water-solvent mixtures, have lead to results not as

ambiguous if compared with those for pure solvents [41].

Clearly, involvement of water in a binary mixture brings forth a number of specific

interactions with the co-solvent, including the hydrophobic interaction. The hydrophobic


interaction is the tendency of apolar species to aggregate in aqueous solutions to reduce their

contact surface with water [37-39]. Hydrophobic interactions between apolar molecules or

apolar parts of molecules in water are important noncovalent driving forces for inter- and

intramolecular binding and assembling processes, taking place in aqueous chemistry and

biochemistry [36-39]. In aqueous systems these interactions can strongly influence chemical

equilibria and reaction rates [37-39,42,43]. In the hydrolysis of esters enforced hydrophobic

interactions stabilize the ground state and make the ester less reactive [42-44]. On the other

hand, the Diels-Alder reaction and the benzoin condensation are dramatically accelerated

because of the packing of hydrophobic surfaces of the reagents in the transition state when the

reaction is carried out in water rather than in organic solvents [37,45,46].

Solute-solvent and solute-solute interactions have attracted interest of investigators for a

long time. Since the pioneering work from the Engberts and Blandamer group [47] on

quantitative interpretation of the co-solute-induced rate effects, many papers about reactions

in mixed aqueous solvents have appeared (for reviews see, e.g. Refs [38] and [39]). However,

the most definite results have been obtained for water-rich media, i.e. at concentrations of co-

solvents about few mole percents. At lower concentrations of water, complications of

different origin have been met (see, e.g. [44]).

Attempts to describe the solvent effects in binary solvents have been done also by means

of empirical multiparameter correlation equations [9] or basing on simplified solvent

exchange models concerning the solvation shell of a solute [41,48]. Thermodynamic

considerations [40] and several theoretical calculations, e.g. of the Kirkwood-Buff integral

functions [49] have shown that many binary mixtures are micro-heterogeneous, consisting of

microdomains composed of organic solvent molecules surrounded by water, and of water

solvated by the organic solvent.

These ideas have been well supported by recent spectroscopic, X-ray diffraction, and

mass spectrometric investigations of alcohol-water solutions [50-52]. It has been concluded

that small additions of ethanol in the range of 0<XE <0.08 (XE is the ethanol mole ratio) exert

a strong structure-making effect accompanied by an increase in the self-association of water

molecules. Indeed, the partial molar volume of ethanol is a minimum at XE= 0.08 [53], and

the excess solvatochromic parameters distinctly show an enhancement in the structure of

water in this region [54]. Further additions of the alcohol begin to prevent water from

organizing into three-dimensional structures. The structural behavior of these solutions is

strongly modified at XE>0.15. In this region a large number of ethanol-water bonds are

formed and water-water bonds are broken. The resulting structure is described by a cluster

model, envisaging a stacked ethanol core and a thin water shell. The region shifts to lower

alcohol contents for more hydrophobic alcohol-water mixtures. On the contrary, in aqueous

methanol the cluster region stretches at XMeOH > 0.4 [52], however, in an alcohol-water

mixture, concentrated in regard of the alcohol, most of the water molecules exist as clusters in

the alcohol medium [55].

Although hydrophobic interactions can be studied by a large variety of experimental and

computational techniques, the determination of chemical reactivity has an important position

among them [38,39,45,46]. Further we show that application of power ultrasound to kinetic

investigations into polar homogeneous reactions revealed important features of hydrophobic

interactions in solutions.


5. SONOCHEMICAL EFFECTS IN REACTION KINETICS

A comprehensive investigation of sonication effects on polar homogeneous reactions was

first performed by Mason‘s group [27-30]. An unexpectedly complicated dependence of the

sonication effect (kson/k) on the composition of alcohol-water binary solvents was found for

the solvolysis reaction of tert-butyl chloride. The authors supposed that the application of

ultrasound to the reaction disrupted the binary solvent structure, thus permitting a better

solvation of the substrate and resulting in enhanced reaction rates.

That pioneering work inspired us to extend the investigation to a mechanistically

different reaction, viz. to the acid-catalyzed hydrolysis of esters in aqueous binary mixtures to

elucidate more details of the sonication effect on polar reactions. Whereas in the case of tert-

butyl chloride the matter of sonication-induced radical processes can still be raised, it is

almost excluded when alkyl esters are used. Furthermore, experiments carried out in 1 M HCl

solutions prevent possible pH changes due to water sonolysis or nitrogen oxidation products

[56,57].

For the acid-catalyzed hydrolysis of ethyl acetate we observed a similar trend to the work

of Mason on the dependence of the sonication effect on ethanol-water solvent composition

(see, e.g. Figure 1). These results initially led us to think that sonication effects were merely

related to perturbation of the solvent system. However, on replacing ethyl acetate with more

hydrophobic esters, we observed a dramatic change in the dependence of the sonication effect

on solvent composition, which obliged us to revise our early point of view. Solute-solvent

interactions in these complicated systems proved to be particularly important in clarifying the

matter [21,34].

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

X

ks

on/k

no

n

Figure 1. Rate inhancements induced by ultrasonic irradiation in water-organic binary solvents (X -

molar fraction of the organic component in the mixture).

□ - acid-catalyzed hydrolysis of ethyl acetate in 1,4-dioxane -water mixtures at 18 °C [21]

○ - acid-catalyzed hydrolysis of ethyl acetate in ethanol-water mixtures at 18 °C [20]

● - solvolysis of tert-butyl chloride in ethanol-water mixtures at 20 °C [29]


For the effect of ultrasound on the rate of the reaction in ethanol-water and 1,4-dioxane-

water mixtures, non-linear dependences involving extreme points were found (Figure 1).

Because many physicochemical properties of binary systems depend on the composition

nonlinearly, it should be clearly determined how much of the ultrasonic energy is absorbed by

the system at any component ratio to ensure a confident interpretation of the results.

Several methods are available to estimate the amount of ultrasonic power entered into a

sonochemical reaction [3,58]. Many authors have suggested determining the thermal effect of

ultrasound as a means of obtaining the effective power. This is based on the assumption that

almost all the cavitational energy produces heat, and thus the output power can be obtained

via calorimetry. The other method involves a chemical dosimeter, which monitors the

sonochemical generation of a chemical species. The yields of the reaction after an adequate

sonication time are regarded as a measure of the power of the ultrasound.

Although chemical dosimetry is generally believed to be the most straightforward method

for determination of the ultrasonic power in a sonochemical reaction, it cannot be applied to

binary solvent systems, because the reaction rate as well as the ultrasonic acceleration

depends on the solvent composition. However, many authors [59-62] have shown that the

results from a chemical dosimeter were directly and linearly related to the calorimetrically

determined ultrasonic power. In addition, it is important to notice that a chemical dosimeter

may not describe the true acoustic power, but describes the sonochemical efficiency for the

reaction induced under certain experimental conditions [62].

Ultrasonic power determinations were performed in the 0-60 wt.% region of ethanol-

water and 1,4-dioxane-water binary mixtures (For details see [21]). It appears that the

calorimetric sonication effect depends insignificantly on the solvent composition (Figure2).

The power of ultrasound in this system did not exceed 1.6% relative to that for pure water and

thus remained within the experimental error limits. Similar results were obtained for the 1.4-

dioxane-water system [21].

0 10 20 30 40 50 60 7037

38

39

40

41

42

wt% EtOH

P (

W)

Figure 2. Ultrasonic power in a 500 cm3 calorimeter filled with water or ethanol-water mixtures.

If the assumption that almost all the cavitational energy produces heat that is measurable

via calorimetry is valid, it follows that at least for the solvent systems under consideration the

solvent properties show an insignificant effect on the number of cavitational events as well as

the cavitational intensity. This result is somewhat unexpected in the context of the complexity


and microheterogenity of alcohol-water binary systems. However, our results indicate that

dependences of the ultrasonic rate enhancement on solvent composition do describe changes

in the sonochemical efficiency.

Data for the hydrolysis of ethyl acetate and for the solvolysis of tert-butyl chloride

(Figure 1) show a distinct maximum in the region about 50 wt. % of ethanol. Mason et al.

[29,30] have pointed out a coincidence of the maximum in their data with the maxima found

in the viscosity, enthalpy of mixing and sound absorbtion versus solvent composition curves

[40]. These properties of the binary liquid mixture show the existence of a structurally critical

region at 0.2-0.3 mol fraction (40-50 wt.%) of ethanol. This is also reflected in the volumes of

activation ΔV#. All the available data for a variety of solvolysis reactions in ethanol-water

mixtures show a decrease in ΔV# when passing from water to ethanol-water mixtures and a

minimum in the region between 0.2 and 0.3 mol fraction of the alcohol [63].

Recent spectroscopic, X-ray diffraction and mass spectrometric investigations [50-52]

have shed light on the structure of ethanol-water solutions (see previous section). Based on

these findings the application of ultrasound to the reaction would, by disrupting the binary

solvent structure, result in the enhanced rates of reaction. However, solute-solvent

interactions in these complicated systems can be particularly important, since the replacement

of ethyl acetate by more hydrophobic esters changed beyond recognition the dependence of

the sonication effect on the solvent composition (Section 7).

Engberts, Blandamer et al. [38,64,65] have developed a versatile quantitative approach to

reactions in binary solvent systems including ester hydrolyses based on an idea about

equilibrium formation of encounter complexes between reactants and hydrophobic co-

solvents. The more hydrophobic the reagents and the co-solvents, e.g. alcohols, the more

extensively the reagents are included in the encounter complexes and thus rendered

unreactive. From the rate constants for the neutral hydrolysis of 4-methoxyphenyl-2,2-

dichloroalkanoates in dilute aqueous solutions of short- chain alcohols, the molar energies of

hydrophobic interactions between the components of the solutions have been estimated to be

as small as 1 kJ or less [65]. Nevertheless, two-fold and greater rate decreases in solutions

that are about 2 mol % in alcohol and 10-5

M in ester were plausibly assigned to hydrophobic

interactions.

Kinetic sonication data for the hydrolysis in the 1,4-dioxane-water solvent system are

usefully complementary to the reasonings above (Figure 1). It has been pointed out that the

structure enhancement of long-range order in water-alcohol systems appears to be absent in

mixtures of dioxane and water [66,67]. Moreover, in solutions ranging from pure water up to

0.2 mole fraction, dioxane gradually breaks down the structure of water [68].

Consequently, in the region beyond 5 mol % of dioxane (Figure 1), the sonication effect

can be attributed to the breakdown of ester-1,4-dioxane encounter complexes, the efficiency

of irradiation decreasing with an increase in the content of the hydrophobic co-solvent in the

mixture.

In contrast to the sonication effects and despite considerable changes in the solvent

structure, rate constants for the hydrolyses without ultrasonic irradiation decrease slightly and

monotonously with increasing organic co-solvent content. The same was observed for the

solvolysis of tert-butyl chloride in ethanol-water mixtures. Winstein and Fainberg [69] have

shown that the activation free energy of tert-butyl chloride solvolysis increases smoothly with

increasing ethanol content, while the enthalpy and entropy of activation show mirror-imaged

extremes in the region of 15 mol % of ethanol. This is also the region of the maximum


solvation energy of the initial reagent, tert-butyl chloride [70]. A similar compensation effect

has been observed for the hydrolysis of ethyl acetate in water-DMSO and water-acetone

systems [71].

Thus, ultrasonication is able to reveal subtle interactions and particular effects of entropic

or enthalpic origin, which remain hidden in conventional kinetics.

6. MODE OF ACTION OF ULTRASOUND ON REACTIONS IN SOLUTIONS

From mean velocities for the first half-lives of reactions without sonication and under

ultrasound, the sonochemical efficiency of our experimental equipment was estimated to be

1.3 10-9

molJ-1

for the hydrolysis of butyl acetate in 40 wt.% 1,4-dioxane and 2 109

molJ-1

for ethyl acetate in 50 wt.% ethanol [21].

These numbers are comparable with those reported for OH radical formation in water (3

1010

molJ1

) [72], the sonolysis of 4-nitrophenylacetate (5.7 10-9

molJ-1

) [73], KI

oxidation (6 109

molJ1

) [62], and Fricke dosimeter (3 1010

molJ1

) [62]. This

comparability is somewhat amazing because all these data are related to radical formation or

degradation reactions, i.e. to high-energy processes, while in our case only weak interactions

in the solution are perturbed by the irradiation. This means that similar molar efficiency is

apparently associated with a lower energetic efficiency in the case of these polar reactions.

Hence, the question, of how ultrasound acts upon homogeneous ionic reactions still needs to

be answered.

Cavitation is now generally accepted as the origin of the chemical effects of ultrasound.

The sonochemical reaction is thought to occur in the cavitation bubble or in its immediate

vicinity (see Section 2). Extremely harsh conditions are produced by the collapse of a

cavitation bubble. Under these conditions standard solvents are in the supercritical state, thus

providing a promoting medium for certain reactions [25,74].

Three regions in which a reaction can take place exist in a cavitating liquid: the gaseous

phase inside the bubble, the limit shell around it, and the bulk solution [11,74,75]. Therefore,

a cavitating reaction medium is considered to be a pseudo-heterogeneous system. This is the

concept that sound energy is focused in small regions and is not able to process into the rest

of the material, and thus its effect is felt only at certain points in the medium.

If the sonochemical acceleration or promotion of a non-radical reaction occurs

exclusively in the cavitational sites of the reaction medium as generally expected (see Section

2), the rate of a first-order reaction under sonication can be expressed as follows:

v = kson,obs c = ksilent c + xk°sonc,

where x is the fraction of the reaction medium under perturbation by cavitation at any

instant, and k°son is the rate constant of the reaction inside the cavitational site.

It should be noticed that the observed rate of the reaction consists of the rate in

cavitational sites and of the rate in bulk solution presumably not affected by sonication.

The intrinsic sonochemical rate constant, i.e. that for the reaction inside the cavitational

sites, can thus be calculated as:


.,

x

kkk

silentobssonoson

While the observed sonochemical acceleration is

,,

silent

obsson

obsk

ka

the intrinsic sonochemical acceleration is

.11 ,

silent

obsson

silent

osono

k

k

xk

ka

Whereas rate constants kson,obs and ksilent can be routinely determined, the values for x are

not available in most cases. However, void fractions of 104

[76] or 2.9 × 105

to 4.2 × 105

[77] have been calculated for water under sonication. Actually, the active volume including

the shell around the bubble may be greater, e.g. Hua et al.[25] used a heat-transfer model for

the estimation of the lifetime and spatial extent of alleged supercritical water (SCW) during

the cavitational bubble collapse. A value for x, equal to 1.5 × 103

in pure water was proposed

[25].

Thus, depending on how rigorous conditions the reaction requires, x can take different

values, however, it should not exceed 103

. In other words, 0,1% of the reaction solution or

less is under cavitation simultaneously.

This means that the intrinsic ultrasonic acceleration a° required to produce an observed

rate enhancement (aobs) by a factor of two is about 103 times or more. In other words, the

reaction located in the cavitational sites has to proceed up to several thousands times faster

than in the bulk solution. Such rate enhancements have been reported for only a few reactions

and require substantial changes in solvent properties [9]. On the contrary, quenching of a

reaction in the cavitation zone leads to a rate decrease by 0.1% or less and therefore cannot be

ascertained experimentally.

Although the intrabubble gas phase is an inconceivable site for ionic reactions to proceed,

the liquid shell, particularly in the supercritical state, can provide a favorable medium for

reactions. However, the low density, low polarity and cluster formation indigenous to

supercritical water [78] counteract ester hydrolysis reactions. The bubble-bulk interface can

also be a site of accumulation for hydrophobic molecules [72-74], however, estimated

concentration limits of species are far too low to provide the required rate enhancements.

Moreover, the observed sonication effect increases in the opposite direction to the

hydrophobicity of the esters (see next section).

Although an extension of the linear Arrhenius equation up to the supercritical water or

hot-spot region temperatures may be acceptable in the case of cleavage or degradation

reactions, the same approach is not valid for extremely solvation-dependent solvolysis or


hydrolysis reactions. Moreover, the occurrence of high-temperature zones in cavitating

solution provides no adequate explanation of the observed effects in polar reactions because

the absence of a sonication effect for a reaction with a positive activation energy has been

documented [26] and has also been found in our work [21].

From the definition of the activation volume of the reaction,

,ln

RT

V

dP

kd

T

#

the acceleration caused by pressure can be calculated. Assuming an activation volume equal

to –20 cm3mol

-1, the rate of the reaction can be doubled by applying a pressure of 800 atm to

the reaction solution at the standard temperature. At higher temperatures, e.g. in the cavitation

bubbles, considerably greater pressure must be applied. If the reaction is accelerated only at

cavitational sites with x = 0.001, then the same rate increase can be attained under a pressure

greatly exceeding 7500 atm, which is hardly accessible even in the hot spots. Thus, also the

kinetic pressure effects should be ruled out.

It follows, that the observed acceleration ratios for polar homogeneous reactions,

particularly those for ester hydrolyses, cannot be accounted for directly by the phenomena

occurring in the cavitation bubbles. It seems to be necessary to take into consideration the

bulk solution or at least an essential part of it.

Evidently, ultrasonic waves passing through the medium cause changes in the

translational energy of species. The same may occur because of shock waves produced by

collapsing cavitational bubbles in the medium. An acoustically induced motion of the water

of crystallization in the crystal lattices leading to changes in the melting points of compounds

has been pointed out [79]. The perturbation of normal molecular motion in the liquid phase by

ultrasound has been detected through its effects on NMR spin-lattice relaxation times [80,81].

From NMR-spectra, it has been found that the introduction of 20 kHz ultrasound to a liquid

sample induces conformational changes to appropriate constituent molecules of the sample

[81]. It has been accepted for a long time that the equilibria involving aggregates present in

solution are perturbed by pressure changes produced by sound waves (for recent reviews see

[82]) and that extensively exploited relaxation processes in liquids are caused by the re-

establishment of the equilibria perturbed by sound waves [83,84]. Our results corroborate this

concept pointing at a highly probable action of ultrasound in the bulk solution.

However, current results do not permit to discern the true acoustic-field effects from

those caused by pressure waves due to the cavitation phenomena. In many cases, the indirect

contribution of cavitation is evident, since the efficiency of ultrasound increased when

hydrolysis was performed under argon [2-4,18,25] or decreased with elevation of the reaction

temperature [2-4].

7. INFERENCES FROM SONICATION EFFECTS

Our approach to a better understanding of solvent effects stemmed from the study of

kinetic sonication effects in aqueous binary solvents. We were able to relate ultrasonic


acceleration of ester hydrolysis to a perturbation by power ultrasound of hydrophobic solute-

solvent interactions.

As discussed in Section 5, these kinetic data were in line with the current idea of the

structure of ethanol-water solutions. According to it (see section 4), in mixtures with XEtOH >

0.15 a large number of ethanol-water hydrogen bonds are formed at the expense of water-

water bonds, a result that led to a cluster model envisaging a stacked ethanol core and a thin

water shell. This model allowed a straightforward interpretation of our results: a hydrophobic

reagent could be hidden inside the clusters and thus made unavailable for the reaction. If such

interaction with the hydrophobic interior of the cluster can be overcome by ultrasound, the

reaction will be accelerated accordingly.

In our recent study [21] ethyl, n-propyl and n-butyl acetates were used as probes of

reagent inclusion within the clusters. In fact the sonication effects (kson/k) for hydrolysis,

determined in the XEtOH > 0.15 range, matched in reverse order the hydrophobicity of the

esters. Sonication had the smallest effect in the case of butyl acetate, the substrate that should

be most powerfully trapped within the clusters.

We were able to conclude that the regular decrease in the rate of ester hydrolyses in

ethanol-water mixture was mainly due to hydrophobic interactions, i.e. to ground-state

stabilization by this solvent system [21,34].

A logical inference from the results of kinetic sonication experiments with esters was that

ultrasound would decrease, rather than increase, the rate of reactions promoted by

hydrophobic interactions, similar to the Diels-Alder reaction, the benzoin condensation, etc.

[37-39]. As early as in 1997 we have predicted an ultrasonic retardation for chemical

reactions [85], an effect that has so far been ignored in the sonochemical literature.

It cannot be excluded that some researchers have encountered the same phenomena,

however taking it for an experimental error or discarding as nonsense. Therefore we consider

the rationalization of the sonochemical retardation of the reaction rate as particularly

important.

Our choice for the model reaction was the benzoin condensation of benzaldehyde [33,34],

a reaction of well established mechanism [86] (Scheme 2), investigated in detail for the

hydrophobic effects by Breslow et al. [45,87].

It has been shown [45,46] that hydrophobic packing of reactants in the transition state

promotes the benzoin condensation. In the rate-determining step of the reaction two benzene

rings become stacked, an interaction that in an aqueous solvent is favored by a hydrophobic

effect. In ethanolic solutions stacking effects should be greatly reduced; as a matter of fact the

reaction is much slower than it is in water [87].

As expected, the reaction was slowed down by ultrasound in pure water and in ethanol-

water mixtures up to an ethanol content of 45 wt % (XEtOH = 0.25, Figure 3).

The good linear fit of these data to second-order kinetics proves that ultrasound affects

the rate-limiting condensation step of the reaction exclusively. Sonochemical degradation of

benzaldehyde would have led to an apparent acceleration of the reaction instead of the

observed retardation. A loss of benzoin by decomposition could lower the apparent reaction

rate; in this case however a curvature of the second-order kinetic plot should be observed.

Moreover, GLC analysis of solutions of benzaldehyde and benzoin in the absence of catalyst

did not reveal any degradation products after they had been sonicated longer than required by

kinetic experiments. If the reaction was switched to a chain mechanism under sonication, no


rate reduction could be observed and unexpected by-products should have appeared.

However, this was not the case.

C-

CN

OH

CO

H

Ph C

O

HC

-N

Ph C-

OH

CN

PhC

O

H

+C

-N-+

Ph C

O

PhC

OH

H

A

B

Scheme 2. Mechanism of the benzoin condensation of benzaldehyde (A); stacking of the benzene rings

in the rate-determining step (B).

The retardation effect of ultrasound was most pronounced in pure water and gradually

decreased with increasing ethanol content up to about XEtOH = 0.25 (Figure 3), when

sonication turned to a promoting factor. The last finding can be interpreted in terms of the

structure of aqueous ethanol binary system. Additions of ethanol up to 25 mol% modify the

structure but evidently do not entirely prevent the favorable hydrophobic effects which are

disturbed by ultrasound.

Ethanol clusters in this region and more extensively in that of higher XEtOH bring about

different consequences at different ethanol concentrations. The condensation reaction is

favoured when ethanol clusters host complexes of the reagents. On the other hand, if ethanol

clusters host single reagent molecules, the reaction is slowed down. These effects obviously

compete with one another; the small accelerating effect of ultrasound for XEtOH > 0.20

indicates the prevalence of the latter.

The observed statistically significant decrease of the rate of benzoin condensation means

that the reaction was quenched in 20 % of the total volume of water solution, or was hindered

in a larger part of the solution. This provides a direct and unambiguous evidence for the

occurrence of non-radical sonochemical processes in the bulk solution of homogeneous

systems, i.e. outside of cavitational sites (see the discussion in Section 6).

Apart from this fundamental conclusion, kinetic investigations into the sonochemical

effect in water-organic binary mixtures led to an important generalization which can be called

the Fourth Rule of sonochemistry (a sequel to the Rules by Luche [4,13]): if sonication breaks

down stabilization of the encounter complexes between reagents, it decreases the reaction

rate; on the contrary, if sonication perturbs the solvent stabilization of the initial state of the

reagents, it accelerates the reaction.


0.0 0.1 0.2 0.3 0.40

2

4

6

8

without sonication

under ultrasound

XEtOH

kII

I

10

3

Figure 3. Plot of third-order rate constants, kIII (L2mol

2s

1) for the benzoin condensation of

benzaldehyde vs ethanol content in the aqueous solution under ultrasound and without sonication at 65

°C catalyzed with KCN [33,34].

8. SOME QUANTITATIVE CONSIDERATIONS

8.1. LFE Analysis

It has been concluded that in aqueous-organic binary solvents the sonochemical effect in

ionic reactions may be related with the destruction of hydrophobic solute-solvent interactions

[21,23]. However, the conclusion has been a qualitative deduction based on the observed

sonication effects in reaction kinetics (Section 7). To obtain a quantitative proof of this

conclusion the sonication effects were further related to the Hansch-Leo hydrophobicity

parameter log P [34,88], where P is the partition coefficient of the substrate between 1-

octanol and water [89,90], and the data were subjected to the linear free energy analysis (the

correlation analysis) [91,92].

In Figure 4 the linear free energy (LFE) relationships show how the kinetic sonication

effect for ester hydrolyses are related to the hydrophobic interaction of reagents with the


solvent system. The plot in Figure 4A represents the relationship at XEtOH = 0.28 in the region

of ethanol clusters, providing a convincing quantitative proof of the conclusions made

intuitively above. Plotting of sonication effects at XEtOH = 0.04 and 0.09 against

hydrophobicity parameters (Figure 4B) reveals also linear relationships.

The LFE test indicates that the mechanism of the sonication effect is the same for 4-

nitrophenyl acetate and the alkyl acetates independent on the hydrolysis reaction mechanism,

in this particular case the base-catalyzed vs acid-catalyzed reactions. In light of the sonication

effects one can admit now that independent on the reaction mechanism the esters interact

similarly with the solvent system. Subsequently it can be concluded that the regular decrease

of the rate of ester hydrolysis with the increasing content of the alcohol in aqueous binary

solvents is mainly caused by hydrophobic interactions, i.e. by the ground-state stabilization

by the solvent system. Likewise this reaffirms conclusions made by other groups for water-

rich solvent systems [38,39,47].

0.5 1.0 1.5 2.00.0

0.1

0.2

0.3

0.4

0.5

EtOAc

PrOAc BuOAc

0.0

0.1

0.2

0.3

0.4

0.5

EtOAc

PrOAc

BuOAc

XEtOH = 0.28Log (

ks

on/k

no

n)

4-NO2PhOAc

4-NO2PhOAc

A

B

XEtOH = 0.04

XEtOH = 0.09

Log P

Lo

g (

ks

on/k

no

n)

Figure 4. Linear Free Energy Relationships between sonication effects for ester hydrolyses and the

hydrophobicity parameter (log P) for the substrates. Acid-catalyzed hydrolysis of ethyl, n-propyl, and

n-butyl acetate; base-catalyzed hydrolysis of 4-nitrophenyl acetate. Data from Refrs. [21], [23], and

[35], normalized for sonication intensities. A - XEtOH = 0.28, B - XEtOH = 0.04 and 0.09.


In the region XEtOH < 0.15 (e.g., Figure 4B) the order of the sonication effects is reverse

to that found for the region of clusters. Such dependence of sonication effects was related to

the weak solvation of esters in this region. Obviously, an enforced cluster formation occurs

when a hydrophobic substrate is introduced to a solvent system not comprising common

alcohol-water clusters. Undoubtedly, these clusters or encounter complexes are weaker than

the clusters present in the region XEtOH > 0.15. Thus, a greater hydrophobicity of the substrate

leads to stronger solvation and consequently to the decreased reactivity. However, ultrasound

breaks down the weak hydrophobic interactions almost entirely, thus providing paradoxically

large sonication effects for more hydrophobic esters.

Recent experimental data [35] corroborated this conclusion straightforwardly. The

neutral hydrolysis of 4-nitrophenyl chloroacetate was studied. While the observed rate of the

hydrolysis decreased in the presence of 1 mol% of aliphatic alcohols and this effect was

parallel with the increasing hydrophobicity of the co-solvents, the rate constants were not

different under ultrasound (Figure 5). Thus, the dependence of the apparent sonication effect

upon co-solvent hydrophobicity (Figure 6) was similar to the plot shown in Figure 4B. In

other words, the applied acoustic power appeared to destroy completely the ester-cosolvent

encounter complexes, regardless of hydrophobicity of these compounds.

Figure 5. Diagrammatic representation of rate constants for the neutral hydrolysis of 4-nitrophenyl

chloroacetate without sonication and under ultrasound at 20 °C [35]. The alcohols were present as co-

solvents in 1 mol% amount. The initial concentration of the ester was 10-5

M. The ultrasonic power at

25 kHz was 8.1 W/100 mL.


0 1 2 3 4 51.0

1.2

1.4

1.6

1.8

The number of carbon atoms in the alcohols

ks

on/k

no

n

Figure 6. Sonication effects for the hydrolysis of 4-nitrophenyl chloroacetate vs the number of carbon

atoms in the alcohols used as co-solvents in 1 mol% amount.

It is noteworthy that extrapolation of the dependences in Figure 5 and 6 to the point for

methanol predicts a very small sonication effect close to that found in pure water. However,

this is not too surprising if the similarity between water and methanol is considered.

8.2. Solvolysis of Tert-Butyl Chloride

Solvolysis of tert-butyl chloride, investigated by Mason‘s group under sonication [27-

30], has important theoretical implications for understanding both solvation phenomena and

sonication effects. The authors clearly concluded that ultrasound caused perturbation of

molecular interactions taking place in the reacting system [30].

Now a more detailed interpretation of the results can be developed. The solvolysis of tert-

butyl chloride is accelerated in polar and protic solvents, as these stabilize the dipolar

transition state. However, in water the reaction is much faster than would be expected on the

basis of the polarity and hydrogen-bonding ability of water. Abraham et al. [93] showed that

owing to the hydrophobic character of the reagent its ground state is destabilized in water in

comparison to other polar protic solvents. Addition of ethanol to the solvent system causes an

effective hydrophobic stabilization of the ground state leading to a dramatic decrease of the

reaction rate (Figure 7).

The sonication effects for tert-butyl chloride solvolysis confirm the suggestions above. In

Figure 7 the data compiled from literature are compared with reaction rates under sonication,

obtained by extrapolation to the zero degree (0 oC) of Arrhenius plots from the paper by

Mason et al. [29]. The sonication effects are large and increase with the increasing ethanol

content in the binary solvent.


0.0 0.2 0.4 0.6-7

-6

-5

-4

-3

-2

XEtOH

Lo

g k

Figure 7. A compilation of literature data [94] for the solvolysis of tert-butyl chloride in aqueous

ethanol at 0 °C. Vertical arrows indicate the rate enhancement by sonication extrapolated from Mason‘s

kinetic data [29].

However, the reaction rate under ultrasound depends only slightly on the solvent

composition, which indicates that sonication suppresses hydrophobic ground-state

stabilization leaving little play for speculations on medium polarity effects. Extrapolation of

sonication data to pure water results in an almost negligible sonication effect, in accordance

with the highly destabilized ground state of tert-butyl chloride in water (vide supra).

It is remarkable that sonication-accelerated rate constants in Figure 7 mainly fall into the

range corresponding to lower ethanol content without sonication. Similarly, the rate constants

determined at 25 °C correspond to those found at 0 °C for considerably lower ethanol content

(Figure 8) As far as the temperature effect on reaction rates in condensed media comprises

inter alia changes in the solvation of reagents, both sonication and temperature effects can be

assigned to the shifts of solvation equilibria in the reaction system. Indeed, El Seoud [41] has

concluded that hydrogen bonding of water with the substrate ground state is less susceptible

to temperature increase than that of the organic component. This leads to a measurable

depletion of the organic co-solvent in the substrate solvation shell as a function of increasing

temperature.

The moderate influence of ultrasound on the reaction at 20 °C (Figure 1) thus cannot be

explained in conventional terms of solvent vapor pressure, but should be considered as an

evidence of a large difference in solvation at these temperatures. Moreover, Lorimer and

Mason [30] failed in establishing a distinct relationship between the reaction rate under

sonication and the solvent vapor pressure in this experiment. In some way or another (cf.

Section 6), sonication leads to a rise in the effective temperature of the species in solution,

resulting either in solvent structure break or in shift of the solvation equilibria.

This conclusion seems to be supported by the thermodynamic parameters of activation

for hydrolyses reactions. For the acid-catalyzed hydrolysis of methyl acetate only a little

variation in the activation energy was produced by the use of ultrasound. However, the

Arrhenius plots showed notably different intercept values for the irradiated and non-irradiated

reactions [15, 95] (Figure 9).


0.0 0.2 0.4 0.6-7

-6

-5

-4

-3

-2

-1

25oC

0oC

XEtOH

Lo

g k

Figure 8. Rate data for the solvolysis of tert-butyl chloride in aqueous ethanol at 0 °C and 25 °C [94].

For the meaning of dotted lines see Text.

30 32 34 36 38

-8

-7

-6

-5

-4

-3

without sonication

under ultrasound

104

/ T

Ln

k

Figure 9. Arrhenius plots for the hydrolysis of methyl acetate. Data from [15].


Explanations of the sonochemical effect based on the shift of the frequency factor in the

Arrhenius equation under sonication have been suggested [15, 30, 95]. However, the reported

changes in thermodynamic activation parameters under sonication may largely be artifacts,

because in their calculation sums of rate constants (knonson + Δkson) appear under logarithm.

Indeed, if these are expanded into a logarithmic series we have

....2

2ln)ln(ln

sonnonson

son

nonsonsonnonsonsonkk

kkkkk

Limiting the sum to the first two terms we obtain

nonsonson

nonsonson

nonsonsonkk

kkkk

)(2lnln

and if the sonication effect is small the difference in the frequency factor is

1)(2

nonson

son

nonsonson

nonsonson

k

k

kk

kkA

,

with practically no difference of activation energy for the sonicated and non-sonicated

reaction, as was actually observed [15, 95].

However, if Δkson >> knonson , which is the case for the data from Mason‘s group [30], the

calculated activation parameters can reflect the real proportions. For the solvolysis of tert-

butyl chloride under sonication the values obtained for the activation entropy are largely

negative (Table 1). The most substantial decrease observed was nearly 500 J mol-1

K-1

in 60

wt% ethanol. Even if some systematic error could be suspected in such large numbers, the

established trend in the data indicates a large electrostriction effect in the activation process

inherent for polar reactions in low-order media. Because ultrasound cannot affect the

transition state, these activation entropy values reflect a great disorder in the solvation of the

ground state brought about by ultrasound.

Table 1. Activation energies and entropy values for the solvolysis of tert-butyl chloride

in ethanol-water mixtures under sonication [30].

Ethanol

( wt%)

Eson

(kJ mol-1)

∆S≠son

(J mol-1 K-1)

20 62 -75

30 30 -193

40 10 -270

50 -21.6 -386

60 -49.5 -491


In contrast to the hydrolysis reactions [15, 95], a notable decrease in the observed

activation energy for solvolysis of tert-butyl chloride was found under sonication (Table 1).

For systems with higher ethanol content the decrease resulted in negative activation energies.

This appearance is reminiscent of those found for reactions of low intrinsic activation energy

whose mechanism comprises pre-equilibria that can account for the negative ΔH≠

obs values

[96-99]. In such cases, if log kobs = log K + log k, and since -ΔHo > ΔH

≠ it appears that

∆H≠

obs = ∆Ho + ∆H

≠ < 0.

The formal analogy between those processes and that under consideration stems out of

the similar procedure for determination of the activation energy. Since the reacting system

(M) undergoes a perturbation prior to the reaction,

M + H M* products,

the activation energy determined from the dependence of log kobs vs 1/T, appears as

Ea = ∆∆H + ∆H≠.

Whereas the sonication effect diminishes with the increasing temperature, the amount of

acoustic energy absorbed by the reacting system accordingly decreases. Therefore ∆∆H < 0

which in some cases results in Ea < 0.

9. THE SOLVENT STRUCTURE AND SONICATION EFFECTS

The rate of solvolysis of tert-butyl chloride in a binary solvent is nonlinear but smooth in

respect of the solvent composition and therefore does not reflect known features of the

solvent structure, e.g. those in aqueous alcohols. However, log k values of tert-butyl chloride

solvolysis in water-methanol and water-ethanol solutions plotted against each other (Figure

10A) show a distinct deviation of points in the region of OH2X between 0.45 and 0.85. This

is just the region of cluster formation in aqueous ethanol found by physical investigations

[50-52]. In aqueous methanol the cluster formation is much weaker and can be observed only

at OH2X < 0.6. As we have shown [21], the clusters are able to seize the reagent molecules

rendering them less reactive.

It was instructive to superpose kinetic sonication effects from the work by Mason‘s group

[29] on the graph. In Figure 10A arrows represent kinetic sonication effects in water-ethanol

solution at the same temperature. Evidently ultrasound breaks down the hydrophobic solvent

clusters and thus brings the solvation patterns of reagents closer to each other in the solvent

systems. It is not clear whether more intensive sonication would further shift the points in

Figure 10A; however, feeble sonication effects in aqueous methanol can be expected (see

Section 8.1). Nevertheless, available experimental data reflect well the impact of the

hydrophobicity driven solvent structure in aqueous binary solvents.


-6 -5 -4 -3 -2-6

-5

-4

-3

-2

-6

-5

-4

-3

-2

A

B

Log k (H2O - EtOH)

Lo

g k

(H

2O

-MeO

H)

Lo

g k

(H2 O

- dio

xa

ne

)

Figure 10. Plot of log k for the solvolysis of tert-butyl chloride [69] A - in water-methanol and water-

ethanol binary mixtures, B - in water-1,4-dioxane and water-ethanol mixtures. The straight lines were

plotted to guide the eye and represent ideal solvation of the reagents. In A arrows represent kinetic

sonication effects in water-ethanol solution at the same temperature from Ref [29]. In B arrows

represent expected sonication effects in 1,4-dioxane-water solution.

Using the same approach as above, data for the tert-butyl chloride solvolysis in water-

1,4-dioxane and water-ethanol solutions were plotted in Figure 10B. In this case a deviation

in the opposite direction can be observed. Not much is known about the structure of water-

1,4-dioxane mixtures, however, it has been pointed out that the structural enhancement of

long range order in water-alcohol systems appears to be absent in mixtures of 1,4-dioxane and

water [66,67]. Moreover, in solutions ranging from pure water up to 0.2 mol fraction, 1,4-

dioxane gradually breaks down the structure of water [68]. Evidently, the solvent-structural

effects on the reactivity operating in water-1,4-dioxane mixtures are opposite to and exceed

those in the water-ethanol solvent system. As a result, somewhat unexpected deviations seen

in Figure 10B appear.

Sonication effects on the solvolysis of tert-butyl chloride in water- 1,4-dioxane mixtures

have not been determined, however, one can speculate upon two available facts. First, in

water-ethanol mixtures sonication effects for the solvolysis and for acid-catalyzed hydrolysis

of ethyl acetate show very similar dependences on composition of the solvent being small in

the region up to 15 mol% of the alcohol [21,30] (Figure 2). Second, rate of the acid-catalyzed

hydrolysis of ethyl acetate in water-1,4-dioxane solution exhibits remarkable susceptibility to

sonication just in this region [21]. Consequently, if the latter is valid also for the solvolysis

reaction, sonication effects would be significant for the reaction in the water-1,4-dioxane

solution shifting the points in Figure 10B upwards and thus reducing the differences between

the straight line and the experimental curve.


10. THE DOMINO EFFECT IN UNDERSTANDING OF SOLVATION

PHENOMENA

The progress in understanding of solvation phenomena in ethanol-water binary mixtures

attained through the LFE-analysis of sonication data (Section 8.1) allows us to expand the

analysis to different reactions in various water-organic solvents [100].

For rate data processing, the free-energy relationship can be used in the form

log k = Csim log kst + b,

where Csim is the similarity coefficient [92] and kst is the rate constant of the standard reaction,

the solvolysis of tert-butyl chloride in this case.

Rate constants of reactions in aqueous organic binary solvents were correlated with those

for the solvolysis of tert-butyl chloride in the same solvents. Relying on principles of the

LFE-analysis, the solute-solvent interactions for the standard process and for the process

under consideration must be closely related [9]. In reverse, if the LFE relationship holds,

similarity between the solute-solvent interactions is greatly plausible.

Correlations found were good to excellent for binary solvents, ranging from water-rich

systems up to mixtures with prevailing organic co-solvent. Numerical values of the similarity

coefficients reflect the susceptibility of reaction rates to changes in the solvent composition

relative to the solvolysis of tert-butyl chloride in the same binary solvent.

The most impressive conclusion from the results was the fact that good correlations had

been found for such definitely different reactions as ester hydrolyses, various reactions with

ionic and non-ionic reactants, a Menshutkin reaction and SN2 replacements included. It is

remarkable that all these reactions provided linear relationship with kinetic data for tert-butyl

chloride solvolysis, an SN1 reaction, in a wide range of the co-solvent content. Thus, it can be

inferred that independent of substrate and the reaction mechanism, the nature of this

phenomenon is caused by hydrophobic interactions of reagents with the aqueous reaction

medium.

It has to be mentioned that the similarity determination procedure has some similarity to

correlations with the well known Y values by Grunwald and Winstein [101]:

Y = log kt-BuCl

(solvent) - log kt-BuCl

(80 vol% EtOH-H2O).

However, the approach of this work [100] was more straightforward because only the

values for log kt-BuCl

in different solvents were involved as standard systems, and these plots

clearly point to the prevalent contribution of hydrophobic interactions in water-organic binary

systems. This means that the Y parameters, as derived by using the log k values in water-

ethanol mixture, should also contain this influence. The latter aspect has, however, never been

discussed before.


CONCLUSION

Application of quantitative methods, including kinetic measurements and correlation

analysis, to the study of homogeneous ionic reactions under sonication in aqueous and

aqueous-organic solutions has opened new perspectives for better understanding of the

mechanism of these reactions and solvation phenomena in general. Quantitative correlation of

kinetic sonication effects with substrate hydrophobicity has shed more light on details of the

solvation of reagents in aqueous-organic binary solutions. Therefore ultrasound may now

become a useful tool for physico-chemical investigations to reveal subtle hydrophobic

interactions that remain hidden in conventional kinetic analysis.

An analysis of sonication data has revealed that independent of the reaction mechanism

the decrease in reaction rate with increasing content of hydrophobic co-solvent is mainly due

to the ground-state stabilization of reagents and this phenomenon is largely of hydrophobic

origin. Following this concept, if ultrasonication suppresses the hydrophobic stabilization of

reagents, it accelerates the reaction. On contrary, if ultrasonication perturbs the stabilization

of encounter complexes between the reagents, sonication hinders the reaction. Notably, the

first experimental evidence of this phenomenon has been obtained.

The detailed knowledge about the mechanism of ultrasonic acceleration and retardation

of reactions can be useful for chemical technology, based on application of aqueous solvent

systems as ―green‖ media. The same conclusion can be drawn for biotechnology, if control of

chemical modification of proteins is needed in water-based media. Moreover, the capability

of ultrasound to control reactions by affecting weak interactions between reacting species in

water solutions also indicates that the impact of ultrasound on living organisms may have

much more complex nature than the physical and chemical destructive effects caused by

cavitation phenomena.

In summary, many of the conclusions drawn so far reach beyond the conventional

sonochemistry, giving for the first time some more information about solution chemistry and

physical organic chemistry. In this way, investigations into homogeneous polar reactions have

provided clear evidence that sonochemistry is not merely a random method for the

improvement of reaction yields by few percentage or a tool for sludge degradation in the

sewage industry. It is a useful probe for solution chemistry that can reveal information not

easily obtained by any other method.

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Joseph, P. F. J. Phys. Chem. A 2003, 107, 306-320.

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[87] Kool, E. T.; Breslow, R. J. Am. Chem. Soc. 1988, 110, 1596-1597.

[88] Tuulmets, A.; Järv, J.; Salmar, S.; Cravotto, G. J. Phys. Org. Chem. 2008, 21, 1002-

1006.

[89] Hansch, C. Acc. Chem. Res. 1969, 2, 232-239.

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Steric Constants; ACS: Washington DC, 1995.

[91] Shorter, J. Correlation Analysis in Organic Chemistry; Clarendon Press: Oxford, 1973.

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[93] Abraham, M. H.; Gullier, P. L.; Naschzadeh, A.; Walker, R. A. C. J. Chem. Soc. Perkin

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[94] Palm, V. A.; Ed.; Tables of Rate and Equilibrium Constants of Heterolytic Organic

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Reviewed by Professor Timothy J. Mason

Director of the Sonochemistry Centre

Faculty of Health and Life Sciences

Coventry University, UK



Chapter 5

SYNTHETIC METHODOLOGIES

USING SONINCATION TECHNIQUES

Ziyauddin S. Qureshi, Krishna M. Deshmukh

and Bhalchandra M. Bhanage1 Department of Chemistry, Institute of Chemical Technology, N. Parekh Marg,

Matunga, Mumbai-400 019. India

ABSTRACT

Ultrasound generates cavitation, which is "the formation, growth, and implosive

collapse of bubbles in a liquid. Cavitation collapse produces intense local heating (~5000

K), high pressures (~1000 atm), and enormous heating and cooling rates (>109 K/sec)"

and liquid jet streams (~400 km/h), which can be used as a source of energy for a wide

range of chemical processes. This review will concentrate on theory, reactions and

synthetic applications of ultrasound in both homogeneous liquids and in liquid-solid

systems. Some recent applications of ultrasound in organic synthesis, such as, Suzuki

reaction, Sonogashira reaction, Biginelli reaction, Ullmann coupling reaction,

Knoevenagel condensation, Claisen-Schmidt condensation, Reformatsky reaction,

Bouveault reaction, Baylis-Hillman reaction, Michael addition, Curtius rearrangement,

Diels-Alder reaction, Friedal-Craft acylation, Heck reaction, Mannich type reaction,

Pechmann condensation and effect of ultrasound on phase transfer catalysis, oxidation-

reduction reactions, ionic liquids and photochemistry are reviewed. Ultrasound found to

provide an alternative to traditional techniques by means of enhancing the rate, yield and

selectivity to the reactions.

INTRODUCTION

Chemicals reactions are typically performed using conventional thermal energy sources

such as oil baths, sand baths and heating jackets. These sources can develop temperature

1 Tel.: +91 22 24145616; fax: +91 22 24145614, Email address: [email protected];

[email protected].

Ziyauddin S. Qureshi, Krishna M. Deshmukh and Bhalchandra M. Bhanage 158

gradient within the sample. In addition to this over heating, which occurs many times can lead

to product, substrate and reagent decomposition. Owing to these, there are various constraints

for use of conventional energy sources for organic reactions. Hence, increasing efforts have

been made during the last decades to replace conventional energy sources with other non-

conventional techniques such as Ultrasound and Microwaves. The non-conventional sources

often have upper hand in terms of selectivity, reaction time and operational simplicity. For

instance, many reactions can be made to go completion at ambient temperature under

ultrasound irradiation. Sonochemistry is the study of the effect of ultrasound on chemical

reaction [1-5]. On the basis of the frequency, sound is divided into three ranges (Figure 1).

Thus, ultrasound is defined as any sound a frequency beyond the level to which human

ear can respond, i.e. 20 KHz. The sound audible to the human ear falls between 16 Hz (cycles

per second) to 18 KHz and it has no effect on chemical reactions. Ultrasound is again divided

into two regions namely, high frequency ultrasound having frequency in the range of 1-10

MHz and power ultrasound with frequencies between 20-100 MHz. the upper limit of

ultrasound frequency is one which is not sharply defined but is taken to be 5 MHz for gases

and 500 MHz for liquid and solids.

The use of ultrasound within this frequency range may be divided broadly into two areas.

The first area may involve low amplitude (higher frequency) propagation, which is concerned

with the effect of the medium on the wave and is commonly referred to as low power or

higher frequency ultrasound. It is used in the medicinal scanning, chemical analysis and the

study of relaxation phenomenon. The second area involve high energy (low frequency) wave

known as power ultrasound, which is between 20-100 KHz used for cleaning, plastic welding

and more recently to effect the chemical reactivity [1]. The application of high frequency

ultrasound concerned essentially with the measurement of degree to which the sound is

absorbed as it passes through medium.

Infrasound: Frequencies below 10 Hz fall in the category of infrasound.

Sonic: The human hearing range (10 Hz-18 KHz) and is termed as sonic range.

Ultrasonic: Frequencies greater than 20 KHz are called ultrasonic waves.

Figure 1. Ultrasound range diagram.

Synthetic Methodologies Using Sonincation Techniques 159

This effect is known as ‗attenuation‘. High frequency ultrasound is used in medicine for

fetus imaging, in under water range finding (SONAR) and in non-destructive testing of metals

for flaws. For chemist, ultrasound is a form of energy that would be considered for the

acceleration of chemical reactions.

In many chemical reactions, both homogeneous and heterogeneous applications of

ultrasound is known to increases the reaction rates, change in chemical reaction pathway and

assist in conducting the reaction under less severe conditions. There are a few examples of

reactions which occur only upon irradiation with ultrasound.

Most modern ultrasonic devices rely on transducer which use the inverse effect i.e.

production of an electrical potential across the opposite faces. If the potential is alternated at a

high frequency, the crystal converts the electrical energy into the sound energy [6]. Different

types of transducers are used for generating ultrasound waves. These are piezoelectric,

magnetostrictive, mechanical, electromagnetic, electrostatic and miscellaneous which

includes thermal, chemical and optical transducers Figure 2.

Figure 2. The energy transformation chain in an ultrasonic apparatus.


Ultrasonic waves can be focused, reflected and refracted, but they require a medium of

elastic properties for propagation. When these waves propagate, particles in the elastic

medium oscillate and transfer the energy through the medium in the direction of prppagation.

The marked effect of ultrasound actually arises from the way in which sound propagates

through the medium. In solid, both longitudanl and transverse waves can be transmitted

where as in gas and liquids only longitudinal waves can be transmitted. In liquids,

longitudinal vibrations of molecule generate compressions and rarefactions, i.e. alternating

zone of high pressure and low pressure. The low pressure gives rise to formation of cavities

or bubbles which expand and finally, during the compression phase collapse violently

generating shock waves. The phenomenon of bubble formation and collapse is generally

known as cavitation and is generally responsible for most of the ultrasonic physical and

chemical effects in solid/liquid or liquid/liquid.

Cavitation: The principle phenomenon behind all the effect of ultrasound is cavitation.

First reported in 1895 [7], cavitation is defined as phenomenon of formation, growth and

eventual collapse of small bubbles within a liquid [8-9]. A cavity or bubble is grown by

reducing the ambient pressure by static or dynamic means. The word formation is also refers

to the excitation of the cavities or microbubbles that are already present in the medium under

the influence of the pressure variation. Cavitation is classified in many ways. The one which

is based on the method of its generation is given below.

1. Acoustic cavitation: In this cavitation the growth of the cavity is induced by the

pressure variation by the passage of ultrasound.

2. Hydrodynamic cavitation: This type of cavitation is induced by pressure variation in

the system by changing the flow geometry of the flow system. This can be achieved

by passing the fluid through a reducing cross-section, like a venture or an orifice.

3. Optic cavitation: This cavitation is produced by passing photons of high intensity

light (laser), rupturing the liquid bonds.

4. Particle cavitations : It is produced by bombarding a liquid with high intensity

particles, like proton, rupturing the liquid of the four types listed above, only acoustic

cavitation and hydrodynamic cavitation have the potential for commercially

exploitation. In the present work, acoustic cavitation has been used to carry out

organic reactions.

Since liquids are not elastic, successive cycles of compression and rarefaction lead to

non-uniform translational motion of individual molecules within the solution, which enhance

the rate of the transport processes. As the power is increased, more efficient mixing is

typically observed. In addition, applied above a critical intensity, ultrasonic irradiation can

also induce oxidation and other chemical reactions.

Factors affecting cavitation:

a) Frequency of ultrasound: Using moderate power, any common liquid (generally

water) can be made to undergo cavitation in the frequency range of 20-50 KHz which

is usually to carry out sonochemical reactions. In a sonochemical reaction, as the

frequency of irradiation is increased, more power is required to maintain an

equivalent amount of cavitation in a liquid. In a high frequency region, the cavitation

is difficult as the rarefaction and compression cycles are so rapid, that sufficient time


is not available to pull the molecule of the liquid part, thereby generating the bubble

and further growth of it.

b) Intensity of ultrasound (power input): As the intensity of sonication is increases,

sonochemical effect increases due to increase in the amplitude of vibration of the

source of ultrasound. But, due to certain factors, such as damage to the transducer,

loss in efficiency of the transfer of the power from the source to the medium and

formation of more number of bubbles, which collapse to form stable bubbles

ultrasonic energy input to the system, can not increase. These may dampen the

passage of sound energy through the liquid and also remove many of the smaller

bubbles, which would have collapsed to give sonochemical effects. Luche has

illustrated the importance of the use of proper intensity (power) [10].

Now a days, ultrasonic bath and ultrasonic probe/horn are the most commonly used

source of ultrasonic irradiation in the chemical laboratory Figure 3.

a b

Figure 3. (a) Ultrasonic bath (b) Ultrasonic probe.

There are three different types of reactions susceptible to sonochemical enhancement.

1) Homogeneous sonochemistry: homogeneous systems that proceed via radical or

radical-ion intermediates. This implies that sonication is able to affect reactions

proceeding through radicals and, furthermore, that it is unlikely to affect ionic

reactions. In the case of volatile molecules, the bubbles (or cavities) are believed to

act as a microreactor; as the volatile molecules enter the microbubbles and the high

temperature and pressure produced during cavitation break their chemical bonds,

short-lived chemical species are returned to the bulk liquid at room temperature, thus

reacting with other species. Compounds of low volatility, which are unlikely to enter

bubbles and thus be directly exposed to these extreme conditions, still experience a

high energy environment resulting from the pressure changes associated with the

propagation of the acoustic wave or with bubble collapse (shock waves);

alternatively, they can react with radical species generated by sonolysis of the

solvent.

2) Heterogeneous sonochemistry (liquid–liquid or solid–liquid systems): heterogeneous

systems that proceed via ionic intermediates. Here, the reaction is influenced

primarily through the mechanical effects of cavitation, such as surface cleaning,


particle size reduction, and improved mass transfer. When cavitation occurs in a

liquid near a solid surface, the dynamics

3) of cavity collapse change dramatically. In homogeneous systems, the cavity remains

spherical during collapse because its surroundings are uniform (Figure 4a). Close to a

solid boundary, cavity collapse is very asymmetric and generates high-speed jets of

liquid (with velocities of approximately 400 Km/h; Figure 4b). These jets hit the

surface with tremendous force. This process can cause harsh damage at the point of

impact and produce newly exposed highly reactive surfaces.

4) Sonocatalysis (overlap homogeneous and heterogeneous sonochemistry):

heterogeneous reactions that include a radical and ionic mechanism. Radical

reactions will be chemically enhanced by sonication, but the general mechanical

effect described above may very well still apply. If radical and ionic mechanisms

lead to different products, US should favor the radical pathway, potentially leading to

a change in the nature of the reaction products.

Figure 4. (a) Cavitation bubble in a homogeneous system; (b) cavitation bubble in a heterogeneous

system.

ULTRASOUND IN ORGANIC SYNTHESIS:

The application of ultrasound in organic synthesis has gained considerable attention in

recent years and several organic transformations are effected using ultrasound. It has been

observed that they reduce the reaction temperature and higher reaction rates can be achieved

at ambient conditions. Some of important contributions are summarized bellow.

Suzuki Reaction

The Suzuki reaction is one of the most studied carbon-carbon bond forming reaction and

useful for the synthesis of several symmetrical/unsymmetrical biaryls (Scheme. 1) [11].

Couple of the methods has been reported for Suzuki reaction under ultrasound and microwave

irradiation [12-13].

B(OH)2

H3CO

Br

H3CO+Pd, base

Solvent

)))

Scheme. 1. Ultrasound-assisted Pd catalyzed Suzuki–Miyaura cross-coupling reaction.


Palladium (0)-catalyzed cross-coupling reactions between potassium aryl- and

inyltrifluoroborate salts and aryl- and vinylic tellurides proceeds readily to afford the desired

stilbenes in good to excellent yields (Scheme. 2) [14-16]. Stilbenes containing a variety of

functional groups can be prepared.

R

X Te R1R

X R2R

R2R

R2 R2R2BF3K, Pd(PPh3P)4

Ag2O, Et3N, MeOH

)))

+ +

Scheme 2. Cross-coupling reaction between potassium organotrifluoroborate salts and the

organotellurium compounds.

Reaction of 3-bromo-4-hydroxycoumarin with aryl boronic acid under high-intensity

ultrasound with Pd/C heterogeneous catalyst was carried out. 3-Arylation with the Suzuki

procedure had failed, exclusively affording the homocoupling products, symmetric biaryls

(Scheme. 3) [17]. Besides offering a number of operational advantages, the use of HIU

broadens the field of application for the Suzuki reaction.

O O

OH

Br

O O

OH

O O

OH

Ar

Ar-Ar

Ar-B(OH)2

Scheme 3. Suzuki homocoupling of arylboronic acids in the presence of 3-bromo-4-hydroxycoumarin.

Sonogashira Coupling

Srinivasan et al. have reported firstly a copper-, ligand- and amine-free one-pot synthesis

of benzo[b]furans via palladium acetate catalyzed tandem Sonogashira coupling-5-endo-dig-

cyclization under ultrasonic irradiation at ambient temperature (Scheme. 4) [18].

OH

I

Ph

O

PhPd(OAc)2, Base

Solvent, )))

+

Scheme. 4. Synthesis of 2-substituted benzo[b]furan/nitro benzo[b]furan under ultrasonic irradiation.


Similar concept was applied for ligand-, copper-, and amine-free one-pot synthesis of 2-

substituted indoles via Sonogashira coupling 5-endo-dig cyclization (Scheme. 5) [19].

NHTs

I

Ph

NTs

PhPd(OAc)2, Base

Solvent, )))

+

Scheme. 5. Synthesis of indole derivatives under ultrasonic irradiation.

Biginelli Reaction

The Biginelli reaction was first described more than a century ago, as a one-pot

multicomponent reaction providing low yield (20-50%) of the product [20]. Biginelli reaction

is a condensation of 1,3-dicarbonyl compounds with aldehydes and urea or thiourea in the

presence of a catalytic amount of an acid (Scheme. 6).

R1

R2

OO

R3

H

O

H2N NH2

X

NH

NH

R1

R3

R2

O

X

+ +

X = O, S

Scheme 6. Biginelli reaction

The product dihydropyrimidinones (DHPMs) are serving as skeleton in many natural or

synthetic biologically active materials and its derivatives are applied in various

pharmaceuticals and biochemicals fields (Figure 6) [21-23].

O

Et

NH

N

HO

O

H

N

O

NH2

NH2

OH

NH O

Me

X-

+

Crambescidin 800

( )12


OH

C2H5O O

HN NH

S

Monastrol

Figure 5. Some biologically active DHPMs.

Dihydropyrimidin-2-ones (thiones) (DHPM) were also recently prepared under

ultrasound irradiation in solvent-free conditions [24]. The Biginelli reaction was catalyzed by

HCl (1 mol %) or trifluoroacetic acid (5 mol %) and completed within 15–45 min in reactions

involving urea and 60–90 min in reactions involving thiourea. Srinivasan and co-workers [25]

discovered that Biginelli reactions can also be performed in the absence of any catalyst. The

reaction between aldehydes, ethyl acetoacetate and urea or thiourea was carried out in 1-n-

butylimidazolium tetrafluoroborate [Hbim]BF4, a non-volatile ionic liquid, in the presence of

sonic waves in a very short reaction time. DHPM(s) were easily isolated and in a high yield,

by simple dilution and filtration procedure. The aqueous filtrate was then distilled to remove

water and leave behind quantitative yields of [Hbim]BF4. The recovered ionic liquid could be

reused in the same reaction at least three times without decrease in yield. Based on the

spectral data, the authors were able to postulate that the ionic liquid plays an important role in

this multicomponent reaction, acting as an inherent Brønsted acid.

Li and co-workers [26-27] used ultrasound to promote the Biginelli reaction between

aldehydes, β-keto esters, and urea to obtain DHPM in good to excellent yields. The reaction

was catalyzed by aminosulfonic acid or iodine and, in both cases; the reaction was very

tolerant of aromatic aldehydes carrying either electron-withdrawing or electron-donating

substituents. However, when aliphatic aldehydes were employed, the iodinecatalyzed reaction

was not very successful.

Yadav and co-workers [28] showed that ceric ammonium nitrate (CAN) can also be used

as a catalyst in ultrasound promoted Biginelli reaction. The reaction was carried out in

methanol under ultrasonic waves. Heteroaryl, aromatic (electron poor or electron rich),

aliphatic, and α,β-unsaturated aldehydes were used and, in all cases, compounds were

obtained in high yields and with high purity. The authors suggest a radical mechanism for the

reaction, in which a single-electron transfer from CAN to the β-keto esters and latter radical

adds to the imine intermediate.

DHPM(s) have been produced by utilization of inexpensive ammonium chloride as a

mediator of the reaction under ultrasound irradiation [29]. The Biginelli reaction was carried

out in methanol and irradiated for 3–5 h in a cleaning bath. The antioxidant activity of these

DHPM(s) was evaluated, and some of these compounds exhibited strong activity against lipid

peroxidation induced by Fe and EDTA.


Ullmann Coupling Reaction

The Ullmann condensation between 2-chlorobenzoic acid and 2-aminopyridine

derivatives using ultrasound has been described [30-31]. The reaction was carried out in the

presence of anhydrous potassium carbonate and copper powder using DMF as solvent

(Scheme 7). In comparison with conventional conditions (stirring for 6 h at reflux

temperature), the ultrasound irradiated reaction demonstrated a shorter reaction time (20 min)

and greater yields

N

R1

R2

H2N

R5

R4

R3

Cl

OH

O

N

N

O

R1

R2

R3

R4

R5

Cu, K2CO3, DMF

)))

+

Scheme 7. Ultrasound mediated Ullmann condensation between 2-chlorobenzoic acid and 2-

aminopyridine derivatives.

Mason and group reported ultrasound assisted methods for Ullmann coupling reaction of

halonitro benzene in presence of copper powder which enhances the reaction reactivity to a

50 fold [32-33]. Ultrasonic irradiation of a mixture of picryl bromide and copper powder at or

below room temperature result in the formation of hexanitrobiphenyl (I) or a 1,3,5-

trinitrobenzene/picric acid mixture, depending on the solvent and relative amounts of picryl

bromide and copper [34].

NO2O2N

NO2

NO2 NO2

NO2

(I)

Applications of ultrasound in the Ullmann reaction were successfully employed for the

synthesis of several N-arylanthranilic acids [35], diaryl ethers [36] and substituted [37]

quinazolin-12-ones by using copper catalyst.


Knoevenagel Condensation

Application of ultrasound has been found to greatly assist the Knoevenagel aldol

condensation reaction of activated methylenes with aromatic aldehydes under mild conditions

(Scheme 8) [38]. The outcome of the ultrasound-promoted reaction depends upon the

electronic nature of the aromatic aldehyde, the solvent employed and the addition of acids,

bases or ammonium salts.

Ar

NO2

OH

RRCH2NO2ArCHO

R-C-CH2CO2H

O

Ar

NO2

R

Ar R

O

R = OH, alkyl, aryl

Scheme 8. Knoevenagel aldol condensation reaction of activated methylenes with aromatic aldehydes.

Whereas condensation of ketones with ethylcynoacetate catalyzed by ammonium acetate-

acetic acid and alkaline-promoted clays (Li+- and Cs

+-exchanged saponites) results in ethyl

alkylidene α cyanoacetate in 31-89% and 97% yields respectively under ultrasound irradiation

(Scheme 9) [39-40].

R1

O

R2

CN

CO2Et

CN

CO2EtR2

R1NH4OAc / AcOH

40-50 oC, )))

Scheme 8. Condensation of ketones with ethylcynoacetate catalyzed by ammonium acetate-acetic acid

and alkaline-promoted clays.

Surfactant ethyltrioctylammonium chloride (Aliquat 336) [41] and KF-Al2O3 [42]

assisted Knoevenagel condensation of active methylene compounds with arylaldehydes were

also effectively catalyzed under ultrasound.

Claisen-Schmidt condensation

Chalcones are important intermediates in the synthesis of many potential anti-

inflammatory and cancer chemopreventive agents [43]. They are commonly synthesized via

the Claisen-Schmidt condensation between acetophenone and benzaldehyde (Scheme 10).


CHO C

O

H3CR RCH

CH

C

O

Base+

Scheme 10. Claisen-Schmidt condensation between benzaldehyde and acetophenone to yield the

chalcone.

Improved synthesis of chalcones under ultrasound irradiation were successfully catalyzed

by alumina-supported potassium fluoride [44], barium hydroxide [45-46], basic activated

carbons (Na and Cs-Norit) [47], KF-Al2O3 [48-49]. In order to develop a greener protocol

Martin et al. developed a green, solvent free procedure for the preparation of chalcones using

a new type of amino grafted zeolites under ultrasound activation [50].

Reformatsky Reaction

Bartsch and co-workers [51] have reported the synthesis of β-lactams 15 via the US-

promoted Reformatsky reaction using ‗not activated‘ zinc dust and a catalytic amount of

iodine. The reactions were subjected to high-intensity ultrasound (HIU) from a direct

immersion horn. A previous work [52] also demonstrated the formation of β-lactams, but

under low intensity ultrasound (LIU). However, in this case the zinc dust was activated by

washing with nitric acid in order to achieve high yields. The reactions were performed in a

cleaning bath in the presence of catalytic I2 in dioxane and products were obtained in 70–95%

yields. Under these conditions the formation of β-amino esters was not observed.

Comparing this work with that described by Bartsch and coworkers [51], the importance

of the intensity of ultrasound in sonochemistry becomes clear; zinc activation was not

necessary in HIU, however, inactivated zinc leads to an almost 50% reduction in yield using

LIU. Another remarkable difference between the uses of different intensity US is the reaction

time; in HIU, the reaction requires only 5 min, while 4–10 h is necessary in LIU (Scheme 11).

Ar2

N

Ar1

BrH2C-CO2Et

N

O

Ar1

Ar2

i

i: Zn "activated", I2 (cat), LIU, 4-10 hr

+

(Scheme 11).

β-Hydroxy esters were prepared via Reformatsky reaction by sonication technique using

different metals such as Indium [53], zinc dust with catalytic amount of iodine [54-56].


Bouveault Reaction

Alkyl, cycloalkyl, and aryl halides were converted to corresponding aldehydes in high

yield by irradiation with ultrasound in the presence of Li and DMF. As ultrasonic irradiation

of Butyl bromide in DMF containing Li sand at 40 kHz for 5 h gave 88% pentanal [57].

Einhorn and coworker studied the effect of solvent on Bouveault reaction [58] from

DMF, N-(2-Dimethylamino-ethyl)-N-methyl-formamide, and 4-methyl-1-piperazinecarbo-

xaldehyde and the effect of THF, tetrahydropyran, and Et2O under ultrasonic irradiation

effect. In the case of Et2O, the results were strongly dependent on the wave frequency. Thus,

PrBr and DMF in THF containing Li sonicated for 10 min at 50 KHz gave 81% PhCHO.

Baylis–Hillman Reactions

Fernando et al. studied the effect of ultrasound radiation on Baylis–Hillman reaction with

several aldehydes (aromatics and aliphatics) and different α,β-unsaturated reactants [59]. For

all aldehydes tested, the utilization of ultrasound sources augmented the reaction rate and the

chemical yields. The use of ultrasound with 1,4-diazabicyclo[2.2.2]octane [DABCO]) is

much more effective for catalyzing a Baylis–Hillman (Scheme 12).

EWG R H

O

REWG

OH

N

N

EWG = CO2R, CN, CHO, COCH3, SO2Ph

)))

Baylis-Hillman adduct

Scheme 12. Formation of α-methylene-β-hydroxy compounds by the Baylis–Hillman reaction.

Michael Addition Reaction

Ceric ammonium nitrate efficiently catalyzes the Michael addition of indole to α,β-

unsaturated carbonyl ketones by means of alkylation of indole under ultrasonic irradiation to

afford the corresponding adduct in excellent yields (Scheme 13) [60]. Interestingly it was

observed that substitution on the indole nucleus occurred exclusively at the 3-position, and N-

alkylation products have not been observed.

NH

R1 R2

O

R1 R2

O

HN

CAN / )))

r.t.+

Scheme 13. Ultrasound assisted Michael addition of indole with α,β unsaturated carbonyl compounds


Curtius Rearrangement

Vommina and co-worker [61] reported the synthesis and isolation of isocyanates of

Fmoc-amino acids by means ultrasonications as Curtius rearrangement and their utility for the

synthesis of dipeptidyl ureas (Scheme 14).

HN C

N3

R H

O

Fmoc- Fmoc- HN NCO

R H

Toluene, )))

15-20 min.

(Scheme 14)

Diels-Alder Reaction

Ultrasound irradiation accelerates hetero Diels-Alder reactions between 1-

dimethylamino-1-azadienes and electron-deficient dienophiles [62]. Besides the lower

reaction times and increased yields, other advantages of the sonicated reactions are the

possibility of isolating previously unknown adducts due to the milder reaction conditions and,

in some cases, the decrease in side reactions.

Martin et al. [63] studied the insights reaction mechanism for Diels-Alder cycloadditions

of masked o-benzoquinones with furans by means of thermal and sonochemical aspects.

Friedel–Crafts Acylation

The Friedel–Crafts acylation of 2-methoxynaphthol is generally carried out by using

highly polluting acids such as HF, AlCl3, BF3 which are used in more than stoichiometric

quantities and are neutralized at the end creating large quantities of waste, corrosion problems

and hazard. To overcome these disadvantageous Yadav et al. reported the use of acid treated

clays such as K-10, Filtrol-24 and cation exchange resins such as Amberlyst-36, Amberlyst-

15 and Indion-130 as catalysts at 25 oC (Scheme 15) [64]. In the presence of ultrasound, the

activities were found to increase by more than a factor of 3 in the case of large porous resins

than the clays. The selectivity to 1-acyl-2-ethoxynaphthalene was found to remain the same.

Utrasonics did not promote isomerisation or direct conversion to 6-acyl-2-

methoxynaphthalene.

O

O

O

OCH3 OCH3

O OCH3

CH3COOH+ +Catalyst

Scheme 15. Acylation of 2-methoxynaphthalene with acetic anhydride.


Acylation reactions of various aromatic and heterocyclics were successfully done with

pivaloyl chloride in the presence of catalytic amtount of iodine [65], without any added

solvent and at room temperature giving excellent yields of the respective pivalophenones in a

short reaction time.

Heck Reaction

Samant et al. reported the Low temperature recyclable catalyst for Heck reactions using

ultrasound [66]. The Heck reaction of iodobenzene with methyl acrylate in NMP as a solvent

has been studied using Pd/C as a catalyst in the presence of ultrasound at room temperature.

(Scheme 16) It was observed that ultrasound increases the rate of the reaction and reaction

only takes place in the presence of ultrasound.

I

CH2=CHCOOCH3

Et3N

CH=CHCOOCH3

+NMP, Pd/C

Scheme 16. Heck reaction of iodobenzene with methyl acrylate using Pd/C in the presence of

ultrasound.

Zhang and co-worker reported an aqeous Heck reaction by Pd(0) nanoparticles under

ultrasonic irradiation at the ambient temperature (25 °C) [67]. It was found that catalyst for

the reaction palladium forms nanoparticles in-situ can be recycled. Furthermore, the Heck

reaction under such mild and environmentally friendly conditions offers excellent

regioselectivity of para- over ortho-substitution in phenyl iodides especially with electron-

donating groups.

Mannich-Type Reaction

Zeng et al. for the first time reported the one-pot three-component Mannich-type

reactions of aldehydes with ketones and amines using sulfamic acid as an efficient,

inexpensive, non-toxic and recyclable green catalyst ultrasound irradiation [68]. This

ultrasound protocol has advantages of high yield, mild condition, no environmental pollution,

and simple work-up procedures. Most importantly, β-aminocarbonyl compounds with ortho-

substituted aromatic amines are obtained in acceptable to good yields by this methodology.

Pechmann Condensation

Ultrasound was found to synergistically accelerate the Pechmann condensation of phenol

with β-ketoesters in the presence of bismuth (III) chloride [69]. In the absence of ultrasound,

under the same conditions, the reaction was found to be slow. Thus, the reaction can be

carried out in the presence of ultrasound at room temperature (28-30 °C), with a considerable

reduction of reaction time, with high yield and high purity of coumarins.


Allylation of Aldehydes and Ketones

The allylation reactions of aromatic aldehydes and ketones using SnCl2-H2O under

ultrasound irradiation at room temperature gave homoallyl alcohols in 21-84% yield within 5

h, whereas the same system desired 24 h for completion. Compared with traditional stirring

methods, ultrasonic irradiation is more convenient and efficient [70].

Epoxidation Reaction

Ultrasound-assisted epoxidation of cyclohexene [71], α,β-unsaturated ketones [72] and

unsaturated fatty esters [73] were successfully carried out with oxygen on ultrasound air-lift

loop reactor, hydrogen peroxide and m-chloroperoxybenzoic acid (MCPBA) respectively.

This general and selective protocol is relatively fast and is applicable to a wide variety of

substrates.

Effect of Ultrasound and Phase Transfer Catalysis

The rate of reacting two immiscible reactants is low because of poor mass transfer. To

increase the reaction rate, strong agitation is essential. Phase ransfer catalyst (PTC) is of help

in such cases. It transfers the active species from one phse to the other. Ultrasound produces

either extremely fine emulsion of immiscible liquids or assists mass transfer and surface

activation (in solid/liquidsystem). These factors enhance PTC catalyzed heterogeneous

reactions or even replace PTC. A number of such reactions are reported.

Ultrasound accelerates the reaction of oxime with dichloromethane in the presence of

sodium hydroxide in combination with benzyldimethyltetradecylammonium chloride as a

PTC to give methylene dioxime (Scheme 17) [74].

R1

R2

NOH

CH2Cl2

R1

R2

N O CH2

+

NaOH/PTC

)))

2

(Scheme 17)

Wang studied the effect of different quaternary ammonium salts with potassium

hydroxide in a synthesis of 4-ethoxynitrobenzene by nucleophilic substitution reaction of p-

chloronitrobenzene with ethanol (Scheme 18) [75], ultrasound is found to enhance the

reaction with quarternary ammonium salt, as compared to the reaction carried out under silent

condition.


NO2

Cl

EtOH KOHPTC

NO2

Et

H2O KCl+ +)))

++

(Scheme 18)

Ultrasound along with microwave has been used for the synthesis of ethers through

Williamson synthesis reaction, which usually involves the employment of organic solvent or

PTC for several hours. The simultaneous use of ultrasound and microwave irradiation

(SUMI) results in reduction or reaction time with good yield of the desire product without

using (Scheme 19) PTC [76].

Ar-OH R-Cl Ar-O-R+

R = benzyl, aryl

NaOH / H2O

SMUI

60-150 S

SMUI: Simultaneous microwave

and ultrasound irradiation

(Scheme 19)

Ultrasound accelerates the Cannizzaro reaction of 4-chlorobenzaldehyde under PTC,

using benzyl triethylammounium chloride as PTC to give 4-chlorobenzoic acid [77]. Perfume

material 2-naphthyl ether is synthesized in high yield and better purity by means of the

ultrasound-PTC method [78]. Vegetable oils can be saponified by sonically using aqeous

KOH and various PTCs at room temperature [79]. It is observed that heterogeneous liquid-

liquid phase saponification of vegetable oils is remarkably accelerated by ultrasound.

Formation of benzoylbenzyl cyanided and benzoic acid from benzyl cyanide and potassium

superoxide in the presence of 18-crown-6 and ultrasound is reported [80].

The application of ultrasound to the N-alkylation of a variety of amines (indole,

carbazole, Ph2NH) by alkyl halides under phase transfer conditions (polyethylene glycol

Methyl ether and alkylammonium compounds as catalysts) leads to a decrease in the time

required to effect reaction [81].

Ultrasound in Oxidation and Reduction Reaction

Ultrasonic irradiation of a biphasic system consisting of substrate, CH2Cl2, H2O, CH3CN,

NaIO4 and catalytic amounts of RuCl3.nH2O, accelerated the oxidation reaction of aromatic

and heteroaromatic compounds to afford the desired products in good yields [82]. In the

presence of ultrasound various mono-, di-, and -unsaturated cyanides were reduced with

Cu-Al alloy in NaOD-D2O and THF to the corresponding deuteriated aliphatic amines, such

as nonylamines, Putrescine, and 1,6-hexanediamine, in high deuterium content [83].


Zinc reduction of -unsaturayed ketones in acetic acid has been efficiently

accomplished under sonochemical conditions [84]. Different -enone systems give two kinds

of products: olefins and allylic alcohols. Regio- and stereoselectivities are reported. Thus, a

mixture of enones I (R1 = H, Cl, R2 = OAc, R3 = H; R1 = R3 = H, R2 = Ac; R1 = H, R2R3 = O)

and Zn-AcOH was sonicated 15 min at 15° C to give ~quantity yields of reduced products

II as 5 :5 epimers.

O

R1

Me

MeR2

R3

Me

HR1

I II

Similarly, under ultrasonic irradiation deoxygenation of 3-oxosteroids I (X = O) with zinc

dust in acetic acid or acetic acid-water give rise to 90% androstanol I (X = H2), a new

Clemmensen-type reduction [85].

X

Me

MeOH

H

I

Sonication in Ionic Liquids (IL)

Ionic liquids (IL) are consisting of complex cations, usually imidazolium, pyridinium or

phosphonium cations and complex anions having negligible vapour pressure. Although the

correlation between vapour pressure and cavitational energy is not straightforward, rate of the

sonochemical reaction can be increased, within the limits by lowering the vapour pressure of

the solvent. It is extremely difficult to induce cavitation in an IL; however the reactant could

enter the cavitation bubbles or the superheated liquid shell surrounding it, to undergo strong

cavitational effect.

The ultrasound assisted preparation of several 1-butyl-3-methylimidazolium and

pyridinium cations salts (BF4, PF6, CF3SO3 and BPh4) ionic liquids (ILs) was carried out [86-

89]. The reaction yield increased, the reaction time decreased dramatically, and the quality of

the products improved. A short and simple method is used to recover the ILs without a

purification step.

Ionic liquids have favorable intrinsic properties that make them of interest as solvents for

various chemical reactions. The same properties that make the liquids effective solvents also


make them interesting liquids for studies involving sonochemical, acoustic cavitation, and

sonoluminescence. Recent interest in using ultrasound to accelerate chemical reactions

conducted in ionic liquids necessitates an understanding of the effects of acoustic cavitation

on these solvents [90].

A few coupling reactions have so far been achieved ubder sonication in ILs, such as 1,3-

di-n-butylimidazolium tetrafluroborate or bromide [bmim][BF4]. Suzuki reaction gives good

yield in an ultrasonic bath (50 KHz) under argon atmosphere at room temperature in an IL

and methanol as a co-solvent [91]. The ligands are not required and chlorobenzene could be

also taken as a substrate under these conditions. Sonochemical Heck reaction of aryl halides

and acrylate is carried out in IL with ultrasound to give the corresponding products (Scheme

20) [92].

XCOOR COOR

IL

)))

+

X = Halogen

(Scheme 20)

Pei and co-worker reported the synthesis of 3-naphthylcyclohexene by the Heck reactions

[93] of bromonaphthalene and naphthyl triflates with cyclohexene catalyzed by palladium and

nickel complex (promoted by ultrasonic and microwave in ionic liquid of [bmim][BF4]) with

high yield and good regioselectivity. This method has advantages of environmentally benign,

generality, simplicity and potential for recycling of ionic liquid and catalyst.

Potential of an inexpensive IL in catalyzing a rearrangement with ultrasound has also

been shown in the rearrangement of cyclopropyl carbinol derivatives to give aryl substituted

trans-conjugated butadienes (Scheme 21) [94]. This procedure offers marked improvements

such as operational simplicity, stereoselectivity (exclusively trans) and high yield of products,

considerably low reaction time and mild reaction conditions.

Ph R

OH

PhRIL

)))

(Scheme 21)

Moreover, in recent years ultrasonic irradiation in an IL has been used for

multicomponent synthesi of dihydropyrimidones [95], acetylation of alcohol [96], nitration of

phenols with para-selectivity [97], in the direct halogenations of alcohols with ter-butyl

halides [98] and synthesis of 4-azalactones [99].


Ultrasound and Photochemistry

Ultrasound considerably enhances and simplifies photochemical reaction of

cyclohexanone with cyclohexene [100]. The Paterno-Bachi reaction of acetone with ethyl

vinyl ether is enhanced by ultrasound and yields a different ratio of cis/trans oxetanes under

sonication by comparison with the silent reaction. Sonication appears to affect the first

reaction by homogenization of excited intermediates and by quenching the excited triplet state

at the second reaction.

The combination of ultrasound and photochemistry has been used for the oxidation of

unsymmetrical 1,4-dihydropyridines to the pyridine derivatives [101]. An ultrasonic probe of

24 kHz frequency and an Hg-lamp of 100 W have been used for this study. The effects of

parameters such as ultrasonic intensity, the presence of oxygen, argon atmospheres and also

the separate usage of one of these irradiation sources have been studied. Whereas sonication

of these compounds alone did not result in their oxidation, the use of ultrasound increases the

rate of photooxidation. The presence of oxygen decreases or increases the rate of reaction,

depending on the type of excited state of 1,4-dihydropyridines involved in the reaction.

Miscellineous

Bhanage and co-workers effectively demonstrated a simple and convenient methodology

for the regioselective nitration of phenols using dil. HNO3 as nitrating agent under sonication

(Scheme 22) [102]. The protocol eliminates the use of any additive and requires lower

concentration of HNO3 (9%). The effect of various reaction parameters such as agitation

speed, solvent, phase hold-up ratio, substrate concentration, HNO3 concentration and

temperature on the reaction system was studied. The present methodology shows a

considerable enhancement in the reaction rate along with improved para-selectivity compared

with the reactions performed under silent conditions. The kinetic analysis of nitration of

phenol both with and without sonication was studied by studying reaction parameters such as

substrate and HNO3 concentration. The increased rate of reaction and selectivity are

explained on the basis of ultrasonically generated cavitational effects.

OH

HNO3))))))

R

OH

NO2

R R

OH

NO2

+TBAB, 25

oC

+

R=CH3, Cl

Scheme 22. Ultrasound assisted nitration of phenol to o-nitrophenol using phase transfer catalyst.

Similar type of work eas reported by Kamal and group which uses nitric acid/zinc

chloride for the nitration of phenols under ultrasonic conditions [103].


The effect of cavitating ultrasound was studied, in heterogeneous aqeous hydrogenation

of cis-2-buten-1-ol and cis-2-penten-1-ol to obtain trans-2-buten-1-ol and trans-2-penten-1-ol

and saturated alcohols (1-butanol and 1-pentanol, respectively), using a com. Pd black

catalyst (Scheme 23) [104].

OHH2

OHHO

HO

trans-2-buten-1-ol

trans-2-penten-1-ol cis-2-penten-1-ol

cis-2-buten-1-ol

Pd black

(Scheme 23)

The hydrogenation, employing hydrogen gas, of cinnamaldehyde was performed using

Pd-black and Raney Ni catalysts at 298 ± 3 K in a water-cooled (jacketed) reaction vessel

[105]. Comparing the ultrasound-assisted and blank (stirred) experiments revealed that a

higher maximum relative concentration of the intermediate benzenepropanal was formed in

the ultrasound experiments compared to the stirred experiment. The activity of the ultrasound

experiments compared to blank was 9-fold and 20-fold greater for the Pd-black and Raney Ni

catalysts, respectively.

Bhanage and co-workers reported a simple and convenient methodology for selective

sulfonation of aromatic compounds using sulfuric acid under sonication (Scheme 24) [106].

The present methodology shows a considerable enhancement in the reaction rate along with

improved selectivity compared with the reactions performed under silent conditions. The

effect of various parameters such as agitation speed, sulfuric acid concentration, and

temperature on reaction system has been investigated and is explained on the basis of

ultrasonically generated cavitational effects.

SO3H

+ Conc. H2SO4

)))))

25 o

CRR

R = H, CH3, OCH3, -X, Ar etc

Scheme 24. Sulfonation of aromatic compounds under sonication.

Application of ultrasound shows significant rate enhancement for the synthesis of β-

enamino nitriles in the presence of base (Scheme 25) [107]. The role of various homogeneous

and heterogeneous bases/solvents was also studied for the reaction, and potassium t-

butoxide/t-Bu alc combination was found to give the best result at room temperature.


CN

R

t-BuOK

t-BuOHR

CN

H2N R1

+ R1CH 2CN

R= (a) -H (b) -CH 3 (c) -Cl

R1 =(a) -H (b) -Br (c) -Ph (d) -CH 2OCH 3

1 3 2

)))

Scheme 25. Ultrasound assisted synthesis of -Enaminonitriles

Naphthols were selectively coupled under sonication using Fe+3

impregnated pillared

Montmorillonite K-10 and TBHP as an oxidant. Considerable enhancement in the reaction

rate was observed under sonication as compared to the reaction performed under silent

condition (Scheme 26) [108].

Scheme 26. Symmetrical coupling of naphthalene.

Furthermore, in recent years effect of ultrasonic irradiation is found to be very important

tool in several organic reactions such as synthesis of diethyl ether without catalyst [109],

preparation of phenylalkyl ethers and phenyl esters from benzenediazonium [110], synthesis

of 1,5-bis(nitroaryl)-1,4-pentadien-3-ones[111], conversion of azides to carbamates and

sulfonamides using Fe:NH4Cl [112], synthesis of symmetrical vicinal diamines [113],

synthesis of imines [114], synthesis of propargylamines [115], synthesis of functionalized

arylacetylenes [116], one pot synthesis of α-amino phosphonates [117], cleavage of epoxides

with aromatic amines [118], ultrasound and microwave assisted bromination reactions of

substituted alkyl aromatics with N-bromosuccinimide [119], O-alkylation of 5-

hydroxychromones [120], synthesis of benzaldehyde from benzyl alcohol using H2O2 [121],

N-alkylation and N-acylation of 2,4-dinitrophenylamine [122], synthesis of N-

alkoxyphthalimides [123], N-alkylation of imidazole [124], N-Alkylation of acetanilide [125],

synthesis of N-alkoxyphthalimides [126].

CONCLUSION

This Review summarized the recent developments in the area of Sonochemistry:

Reactions and Synthesis, and applications. Sonochemistry is an expanding field of study that

continues to thrive on outstanding laboratory results that have even more significance with


the availability of the types of scale-up systems used in processing. Compared with the past,

there is now far greater contact and cooperation between the scientific disciplines interested

in the effects of cavitation. The future of sonochemistry is therefore bright, both from the

point of view of a greater interest in the fundamental principles of its action and in the

development of international programmes in applied research and technology.

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Chapter 6

SONOCHEMOTHERAPY AGAINST CANCERS

Tinghe Yu11, 2 and Yi Zhang

2

1 Institute of Life Science, Chongqing Medical University,

Chongqing 400016, China

2 Laboratory of Biomedical Ultrasonics,

Institute of Women and Children‘s Health,

West China Second University Hospital,

Sichuan University, Chengdu 610041, China

ABSTRACT

Sonochemotherpy is the use of ultrasound to enhance anticancer agents. Preclinical

trials have manifested this modality is effective against cancers including chemoresistant

lesions. Sonochemotherapy is a target therapy, in which cavitation plays the leading role.

Making the occurrence and level of cavitation under control improves the safety and

therapeutic efficacy. Sonosensitizers and microbubbles enhance cavitation, being a

measure to adjust the level of cavitation. Free radicals due to cavitation have the

potentials of restructuring a molecule and changing the conformation; thus the molecular

structure and anticancer potency of a cytotoxic agent must be investigated, especially

when sonosensitizer and microbubble are employed. A potential clinical model for

investigating sonochemotherapy is the residual cancer tissues when performing palliative

high intensity focused ultrasound treatment.

INTRODUCTION

Adverse events and the development of chemoresistance are the main barriers to clinical

applications of chemotherapy against tumors. Techniques, which potentiate anticancer agents

and decrease their toxicities to noncancerous tissues as well, are therefore urgently needed.

As a non-ionizing mechanical wave, ultrasound has a better tissue penetration and can be

focused on the predetermined volume within the body without harming overlying tissues.

1 Correspondence: Institute of Life Science, Chongqing Medical University, Chongqing 400016, China; E-mail:

[email protected].

mailto:[email protected]

Tinghe Yu and Yi Zhang 190

Ultrasound issues in structural and functional changes in exposed tissues and the insonation

level must be determined according to the therapeutic goal [1]. Biological responses due to

ultrasound result from thermal and nonthermal (mechanical effect and cavitation) effects, and

there are considerable differences between tissue types and between individuals [2, 3]. When

the temperature is <50 °C, this modality is ultrasonic hyperthermia. Ultrasonic hyperthermia

can enhance anticancer drugs, i.e., ultrasonic thermochemotherapy [2]. This technique will

not be discussed in this chapter, since it is actually thermochemotherapy. High intensity

focused ultrasound (HIFU) also employs heat, which leads to a temperature of above 56 °C in

few seconds. HIFU results in immediately coagulative necrosis, and has been used to ablate

solid tumors [3]. Therapeutic applications of nonthermal effects, especially cavitation, have

been investigating.

Sonochemotherapy is the use of ultrasound to enhance anticancer chemicals against

cancers [4]. This technique is once considered as ultrasound-mediated drug delivery, as

insonation facilitates the transmembrane transportation of drug molecules [5, 6]. However,

recent data have manifested that some events cannot be interpreted with the increase of

intracellular drug accumulation [2]. Sonochemotherapy is still at the preclinical stage despite

those stirring findings, which confuses scientists. Thus, this paper concentrates on the

limitations of this modality, and then possible solutions including personal perspectives are

brought forward.

AN OVERVIEW OF SONOCHEMOTHERAPY

Sonochemotherapy is the use of ultrasound to enhance anticancer agents [4]. This

modality equipotentially deactivates cancer cells with a lower dose of antitumor drug

compared with conventional chemotherapy; toxicities to normal tissues are decreased as well.

Cytotoxicity occurs only in the lesion via focusing ultrasound beams at the predetermined

volume. Thus, sonochemotherapy realizes a targeted therapy. The excellent tissue penetration

of ultrasound suggests that this technique can be applied for a lesion located deeply.

The dose-anticancer effect curve parallels the dose-toxicity curve in conventional

systemic chemotherapy. Sonochemotherapy leads to a left-shift of the dose-effect curve and a

right-shift of the dose-toxicity curve (Figure 1). Sonochemotherapy is a targeted therapy,

which is realized via releasing anticancer drugs into the lesion directly thus deactivating

cancer cells efficiently. Adjacent noncancerous tissues are therefore exposed to a very low

dose decreasing toxicities. Consequently, sonochemotherapy indicates higher therapeutic and

safe indexes.

Sonochemotherapy Against Cancers 191

Figure 1. Dose-anticancer effect and dose-toxicity curves in conventional chemotherapy (Left) and

sonochemotheapy (Right). Two curves are parallel in conventional chemotherapy. In

sonochemotherapy, the level of a cytotoxic agent is increased and confined within the lesion. These

lead to the deactivation of cancer cells directly and efficiently. Surrounding normal tissues are therefore

exposed to a much lower dose of anticancer drug. Thus, the dose-effect curve shifts left and the dose-

toxicity curve shifts right in sonochemotherapy compared with systemic chemotherapy.

Sonochemotherapy is a targeted therapy.

Sonochemotherapy here employs nonthermal effects of ultrasound. The use of ultrasound

increases the intracellular drug accumulation, which is usually mediated by cavitation [2, 5,

6]. Cavitation, the formation and/or activity of gas-filled bubbles in insonated medium, results

in localized high temperature (104-10

6 K) and high pressure (10

4 atmospheres). Such an

extreme condition leads to the formation of free radicals, microstreaming and microjetting

[5]. Those active species damage tissues, and shear forces resulted from microstreaming and

microjetting distort surrounding objects. These permeabilize cell membrane including pore

formation, thus facilitating the influx. The level of cavitation depends on the intensity,

frequency and exposure duration. A higher intensity and lower frequency favor the

occurrence of cavitation, and cavitation increases with prolonging exposure duration.

Cavitation in vitro can be adjusted by varying the physical acoustic parameters. The tissue

property is the biological determinant for cavitation. In vivo cavitation depends on both

ultrasound and tissues. The cavitation threshold, viz. the minimal intensity to trigger

cavitation, is low in loose and porous tissues, and is high in dense ones. The behavior of

ultrasound in tissues is so complex that it is difficult to predict/modulate cavitation in vivo.

Some agents, such as microbubbles and sonosensitizers, serve as cavitation nuclei decreasing

the threshold intensity in vitro and in vivo [2, 5]. Overpressure suppresses the expansion of

bubbles thus being a measure to inhibit cavitation, but this technique cannot be applied in vivo

[7]. Sonodynamic therapy (SDT) is also mediated by free radicals attributable to ultrasonic

cavitation, which differs from sonochemotherapy on the role of insonation (Tab. 1).

Toxicity

Anticancer effect

Dose

Effic

iency

Anticancer effect

Toxicity

Dose

Effic

iency


Table 1. Comparison of sonodynamic therapy and sonochemotherapy

Initiator Promoter

Sonodynamic therapy Ultrasound Sonosensitizer

Sonochemotherapy Anticancer drug Ultrasound

The increase of intracellular drug level does not occur sometimes, but a better anticancer

effect occurs [8]. This is because ultrasound has the potential of sensitizing cells. Insonated

cells are prone to being deactivated by other cytotoxic factors. Ultrasonic sensitization is one

of the reasons why the dose-anticancer effect curve in sonochemotherapy shifts left. The

addition of ultrasound lowers the thresholds for both apoptosis and necrosis [2]. Some cells

directly befallen necrosis when exposed to sonochemotherapy, which will be deactivated via

apoptosis pathway while exposed to an anticancer drug alone. This leads to an interesting

phenomenon in sonochemotherapy--the rate of cell death increases with increasing drug level

but with only a slight gain in the rate of cell apoptosis [9, 10]. Survival curves of human

ovarian cancer cells subjected to sonochemotherapy were evaluated with the ―target‖ model

in radiation biology (S=1-(1-Exp(-D/D0)N )), where S was the survival rate, and D0 and N

were cell-specific intrinsic parameters determining the sensitivity to a cytotoxic factor.

Insonation alone had no cytotoxicity, but changed both D0 and N [11]. This might be the

mechanism for ultrasonic sensitization.

That sonochemotherapy directly induces cell necrosis is a specific advantage.

Malfunction of apoptosis leads to chemotherapy resistance. One of strategies to overcome

chemoresistance is to deactivate cells independent of apoptosis [12]. This suggests that

sonochemtherapy can be developed to treat refractory lesions.

The Present Status

Not all antitumor drugs can be applied for sonochemotherapy, and not all cancers respond

to this modality [2]. Those anticancer agents and human cancer types with better therapeutic

outcomes are briefly summarized in Tab. 2.

Sonochemotherapy is effective against chemoresistant cancers. Adriamycin-resistant

human ovarian carcinoma cells SKOV3/ADR were exposed to ultrasound (0.24 MHz, 5.76

W/cm2) after adriamycin administration. Sonochemotherapy resulted in a lower surviving rate

compared with adriamycin. If cells were pretreated with 1 μg/ml verapamil for 24 h and then

subjected to sonochemotherapy, the deactivation rate was increased further [9]. The finding

suggests a measure for refractory cancers, i.e., the combination of a chemical modifier and

insonation. A chemical modulator cannot be clinically adopted just because the dose required

to overcome chemoresistance is beyond body tolerance. The use of ultrasound makes it

possible to decrease the dose of a modulator to a safe level. Adriamycin resistance usually

results from the overexpression of P-glycoprotein (P-gp), an ATP-depended membrane

transport protein, which pumps out intracellular drug molecules. A P-gp modulator

overcomes chemoresistance via down-regulating gene expression and quenching/inhibiting

enzyme [26, 27].


Table 2. A summarization of human cancer types and anticancer drugs, which have a

better response to sonochemotherapy

Anticancer agent Cancer Frequency

(Hz)

Intensity

(W/cm2)

Ref

In vitro

ADR Leukemia (HL-60) 80 k 3.6 13

Ovary (3AO) 0.24 M 5.12 8

Ovary (SKOV3/ADR)* 0.24 M 5.76 9

Breast (MCF-7WT) 1.765 M 0.25 14

Lymphoma (U937) 1.0 0.3, 0.5 15

ADR micelle P105 Leukemia (HL-60) 80 k 3.6 13

Ovary (A2780/ADR)* 69 k 3.2 16

DDP Uterine cervix (Hela) 28 k 17

Ovary (COC1/DDP)* 0.8 M 2.0 18

Ovary (HO-8910) 0.8 M 2.0 19

Ovary (HO-8910PM) 0.8 M 2.0 19

MMC Uterine cervix (Hela) 28 k 16

Ara C Leukemia (HL-60) 48 k 0.3 20

5-FU Breast (MCF-7) 3.0 M 3.0 21

Paclitaxel Ovary (HO-8910) 0.8 M 2.0 19

Ovary (HO-8910PM) 0.8 M 2.0 19

In vivo

ADR Ovary (SKOV3/ADR)* 0.24 M 7.84 22

Liposome-encapsulated

ADR

Colon (WiDr) 20 k 3.16 23

ADR micelle P105 Ovary (A2780) 1 M 1.2 24

5-FU Colon (KM20)* 20 k 25

Plutogel-encapsulated 5-

FU

Colon (WiDr) 20 k 3.16 25

ADR: Adriamycin, DDP: Cisplatin, MMC: Mitomycin, Ara C: Cytosine Arabinoside, 5-FU: 5-

Fluorouracil. * Chemoresistant cancer.

Reverse transcription-polymerase chain reaction assay did not support the hypothesis that

ultrasound inhibited the gene expression of P-gp in SKOV3/ADR cells [10]. Verapamil

competitively inhibits P-gp thus reducing the efflux, and ultrasound enhances the influx.

These increase both the intracellular adriamycin level and the retention. Anticancer effect is

therefore enhanced. In vivo investigations confirmed the effect of sonochemotherapy against

chemoresistant ovarian cancers, in that a smaller tumor volume was detected [22]. Cisplatin

resistance is the other type of chemoresistance, where a higher capacity of DNA repair plays

the leading role. Single cell comet electrophoresis assay (SCGE) revealed severer DNA

breaks in cells subjected to sonochemotherapy, compared with cisplatin alone [19].


Encapsulated adriamycin micelle P105 has been used in sonochemotherapy.

Investigations on human leukemia cell line HL-60, ovarian cancer cell line A2780 and its

chemoresistant subclone A2780/MDR, have demonstrated that the cytotoxicity of

encapsulated drug was stronger than that of free molecule [13, 16, 24]. Ultrasound enhances

the release of drug from micelles and the uptake of cells. The release of drug from P105

comprises four phases, i.e., micelle destruction, destruction of cavitation nuclei, reassembly

of micelle, and reencapsulation of adriamycin [28]. The recapsulation indicates the reuse of

drugs. Those drug molecules, which are not driven into cells, will be released into the cancer

tissues in following sonication cycles. Ultrasound (20 kHz, 3.16 W/cm2 in continuous wave)

enhanced liposome-encapsulated adriamycin (3 mg/kg or 6 mg/kg, i.p.) and Plurogel-

encapsulated 5-FU (100 mg/kg or 200 mg/kg, i.p.) against human colon cancers WiDr

transplanted in nude mice, in that tumor growth was delayed [23]. Considering a better

tolerance, sonochemotherapy with liposome-encapsulated drugs can be especially developed

for patients with poor health conditions.

The Present Limitations and Possible Solutions

Low-level ultrasound is preferred for sonochemotherapy; it can be performed

conveniently. Ultrasonic thrombolysis employing low-level ultrasound and cavitation has

been clinically introduced [29]. However, sonochemotherapy is still at an early stage.

Miller and Dou reported that high amplitude ultrasound (1.35 MHz, 5 MPa with 1 ms

burst, and ultrasound was emitted at a rate of 1 Hz) enhanced lung metastasis of melanoma

B16-D5, and cavitation was considered as the mechanism [30]. Cavitation detaches cancer

cells from the primary focus releasing them into circulation, the first step of metastasis.

Interestingly, cavitation also plays an important role in HIFU but clinical investigations have

not confirmed that metastasis is enhanced [3]. There must be some puzzles beyond the

present horizon. The level of cavitation increases with increasing intensity and prolonging

exposure duration. The cavitation threshold in vivo is actually low, 80 mW/cm2 at 0.75 MHz

induces detectable cavitation in limb of a guinea pig [31]. HIFU therapy employs an intensity

of above 1000 W/cm2, and is lengthy. Such a manner favors the occurrence of cavitation.

Theoretically, HIFU may lead to a higher rate of metastasis compared with low-level

ultrasound, but this is not the fact. Metastasis is a very complex process through a serial of

programmed events [32]. Intravasation of cancer cells does not indicate a spread necessarily.

The phenotype of metastasis is cell-specific; cancers with a high potential are prone to

metastasis. We therefore assume that the difference between low- and high-intensity

ultrasound results from the metastatic potantial of cancer cells; insonation is not the initiator

but only a promoter. If this is confirmed, a cancer with highly metastatic potential is a

contraindication to ultrasonic therapy. Effects of sonochemotherapy on adhesion, migration

and invasion were investigated in human ovarian cancer cells HO-8910PM, a subline with

highly metastatic potential. All of them were inhibited by insonaton and lower rates occurred

when applying sonochemptherapy, indicating that cancer spread was not stimulated. A low

level of paclitaxel produced no cytotoxicity, but inhibited cell migration and invasion, being a

potential measure to prevent cancer metastasis during ultrasonic therapy [19].

Only partial cancer types respond to sonochemotherapy [2]. However, those human

cancer types have not been screened systemically. Arthur et al manifested that human bladder


cancer cells TCC-SUP, T24 and RT4 poorly responded to sonochemotherapy with

doxorubicin [33]. Thus, sonochemotherpy cannot be employed for those cancer types.

Designing ultrasonic devices with a low-intensity output is not a challenge. However,

there is no a commercially clinical instrument for sonochemotherapy. In ultrasonic

thrombolysis, the intensity varies from <720 mW/cm2 to 35 W/cm

2 [34]. Sonochemotherapy

might also work under a wide range of acoustic intensity.

Cavitation is the mediator of sonochemotherapy. The occurrence and level of cavitation

in vivo should therefore be under control. In vivo cavitation is usually determined by detecting

passive cavitation. Free radicals attributable to cavitation are involved in sonochemotherapy.

Thus, those reactive radicals in tissues need to be measured. The cavitation level depends on

the intensity, frequency and insonatime time. Therapeutic ultrasound works in the range of

non-linear acoustics, and there are drastic differences between tissue types and between

individuals. Thus, it is difficult to control cavitation in vivo via just adjusting the acoustic

parameters. Employing other techniques might be a solution.

Polystyrene nanoparticles (with diameters of 100 nm and 280 nm) decrease the cavitation

threshold. In nude mice bearing KM20 colon cancers, 100-nm particles and 5-fluorouracil (5-

FU, 90 mg/kg) were injected before 20 kHz ultrasound irradiation. This modality resulted in

an enhanced antitumor effect, in that the cancer volume was reduced compared with that

attributable to 5-FU alone [25]. Microbubbles enhance ultrasonic cavitation, and can be used

for sonochemotherapy. Potential techniques include (i) anticancer agents are encapsulated

into/coupled with microbubbles, (ii) cytotoxic drugs and microbubbles are co-used, and (iii)

micelles containing anticancer chemicals are combined with microbubbles [2]. 5-FU, Optison

(microbubble agent for contrast ultrasonography), and ultrasound (3.0 MHz, 3.0 W/cm2) were

combined to deactivate human breast carcinoma cells MCF-7. The combination of Optison

and ultrasound induced immediate cell death, and cell deactivation more depended on 5-FU

after 24 h. With insonation (50 % duty cycle), there was a synergism between 5-FU (10

μg/ml) and Optison (10 %); however, cell deactivation attributable to 5-FU decreased with

increasing concentration of Optison without insonation [21]. Watanabe et al manifested that

ultrasound enhanced cisplatin against HT29-luc xenografts in the presence of microbubbles

[35]. These data confirm the role of cavitation in sonochemotherapy.

Sonosensitizers favor ultrasonic cavitation suggesting a measure for sonochemotherapy,

i.e., ―sonosensitizer-anticancer drug-ultrasound‖. Free radicals are generated when activating

the sonosensitizer. The level of cavitation is under control via adjusting the dose of

sonosensitizer. The alternative is that an anticancer agent is linked to a sonosensitizer.

Insonation is performed when the fusion molecule reaches the target lesion. Cavitation is

therefore confined to a definite volume, improving the therapeutic precision and efficiency.

Developing a drug responding to ultrasound specifically improves the safety and therapeutic

effects.

Microbubbles can be applied for aforementioned technique, i.e., the combination of

sonosensitizer, microbubble, antitumor drug and insonation. Sensitizers and cytotoxic agents

are encapsulated into microbubbles, and ultrasound is used to destruct microbubbles when

entering into the lesion. Anticancer agents are therefore released into cancer tissues directly.

The influx of anticancer drugs is facilitated as sensitizers and microbubbles enhance

cavitation permeabilizing cell membrane. Alternatively, micelles containing sonosensitizers

and anticancer chemicals are co-administrated with microbubbles. Microbubble-enhanced


cavitation accelerates the release of anticancer drugs and sonosensitizers from micelles, and

permeabilizes cancer cells as well.

In ―sonosensitizer-anticancer drug-ultrasound‖ and ―sonosensitizer-microbubble-

anticancer drug-ultrasound‖ modalities, the cavitation level can be adjusted by modulating the

dose of sonosensitizer and/or microbubble, while the insonation level is set at the threshold to

activate sensitizers/microbubbles. These techniques realize targeted cavitation in vivo.

Sonochemotherapy can be guided and monitored with ultrasound images. A device

integrated a diagnostic ultrasound unit with a therapeutic ultrasound one should be developed

to perform sonochemotherapy. Ultrasonography is a rapid imaging technique, so treatment

can be monitored in real time.

Sonosensitizers and microbubbles intensify acoustic cavitation. Sonochemistry

attributable to cavitation can decompose chemicals, restructure molecules, change

conformations, and increase the rate of chemical reactions [36-38]. Thus, the molecular

structure and anticancer potency of an anticancer agent must be investigated, especially when

sonosensitizers and microbubbles are used. On the other hand, some drugs, such as

cyclophosphamide (CTX) and ifosfamide (IFO), must be catalyzed in tissues producing active

forms. The rate of catalysis may be accelerated by cavitation, which increases the yield of

active forms resulting in better therapeutic outcomes. Effects of cavitation on the structure

and function of proteins need to be investigated when employing an antibody-linked

anticancer drug for sonochemotherapy. The denaturation of antibodies nullifies the affinity to

cancer cells.

Inducing apoptosis is involved in sonochemotherapy. Cytochrome C-mediated apoptosis

has been understood, in which the release of cytochrome C from mitochondria initiates the

cascade of cell suicide. As an indicator of permeabilization of outer membrane (a preparation

for releasing cytochrome), swollen mitochondria occur frequently [39, 40]. Apoptotic bodies

were detected in COC1/DDP cells after exposed to nonlethal insonation. However, the level

of cytochrome C in cytosol was decreased, and mitochondrial tumefaction only occurred in

some cells [41]. The data suggests that non-mitochondria apoptosis pathways participate in

ultrasound-induced cell apoptosis.

When chemoresistant cancers SKOV3/ADR were treated with sonochemotherapy,

necrosis was detected in the center of cancer tissues [22]. The finding suggests that

sonochemotherapy may impair microcirculation in cancers. Vascular endothelial growth

factor (VEGF), a pivotal cytokine for angiogenesis, was decreased by ultrasound (10 MHz, 40

W/cm2) either in cancer tissues or in serum [42]. VEGF is also involved in the formation of

metastatic lesions, and is a target for cancer treatment [43]. These suggest that ultrasound may

not stimulate the cancer spread.

Sonochemotherapy is an adjuvant therapy, which is performed after surgical debulking to

deactivate residual and chemoresistant cells. This technique will change the clinical practice

when chemotherapy becomes a radical cure for cancers. The development of this modality

also relies on the understanding of biological effects induced in tissues, and the advancement

of ultrasonic devices.


The Interaction between an Anticancer Drug and Insonation

The interaction between a cytotoxic drug and ultrasound in sonochemotherapy has not

been understood; it is not clear whether ultrasound synergizes or just adds a drug. We

evaluated the interactions in human ovarian cancer cells HO-8910 and HO-8910PM.

Insonation efficiently enhanced a combined chemotherapy. Combination indexes showed a

synergism or an addition when applying cisplatin, and a synergism, an addition or an

antagonism when using paclitaxel. Cell type was an important determinant for

sonochemotherap; sonochemotherapy realized a specific effect via a synergism and mediated

another specific response via an addition, for a specific cell type [19]. Sonochemotherapy

should not be applied when there is an antagonism. The data indicate the cell death (tumor

inhibition) rate may not reflect the panorama of the effects of sonochemotherapy. How many

fold the dose of an anticancer drug can be decreased in sonochemtherapy needs to be

investigated in following trials.

Exploring Sonochemotherapy from HIFU Therapy

HIFU treatment opens a window to investigating sonochemotherapy. The focal region

comprises 84 % and 71 % energies emitted from the transducer, when defined as -6 dB and -3

dB, respectively [44]. Such a high intensity induces a temperature of >56 °C thus ablating

tissues within the focus. Tissues outside the focus are therefore exposed to a limited

ultrasonic energy. Cavitation can still be triggered in those areas, as the threshold intensity is

low. HIFU is a palliative therapy sometimes (only segmental lesions are ablated). If those

cases receive anticancer drugs, residual cancer tissue may be a clinical model to explore

sonochemotherapy (Figure 2).

Figure 2. A clinical model for exploring sonochemotherapy. During HIFU therapy, ultrasonic energies

emitted from the therapeutic transducer are efficiently delivered into the focal tissues, resulting in

immediately coagulative necrosis. Tissues outside the focus are therefore exposed to a limited

ultrasonic intensity thus producing slight thermal effects, but cavitation can also be induced. When

HIFU is used as a palliative treatment, residual cancer tissues open a window to evaluating

sonochemotherapy.


SUMMARY

Sonochemotheapy is a promising modality against cancers, including refractory lesions.

This modality is at the preclinical stage yet. Ultrasonic cavitation plays the leading role in

sonochemotherapy. Making the occurrence and level of cavitation under control improves the

safety and efficiency. The use of sonosensitizer/microbubble enhances cavitation, and can

also adjust cavitation in vivo. The molecular structure and anticancer potency of a cytotoxic

drug must be investigated when combining anticancer agents, sonosensitizers and

microbubbles. The interaction between an anticancer agent and ultrasound in

sonochemotheray must be understood. Residual cancer tissues after palliative HIFU treatment

open a window to evaluating sonochemotherapy.

ACKNOWLEDGMENTS

The work in our laboratory is supported with grants from the Natural Science Foundation

of China (10774198) and the Natural Science Foundation Project of Chongqing (CSTC

2009BA5049).

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Chapter 7

APPLICATION OF ULTRASOUND FOR WATER

DISINFECTION PROCESSES

Vincenzo Naddeo, Milena Landi and Vincenzo Belgiorno Department of Civil Engineering, University of Salerno,

Fisciano SA, Italy

SONOCHEMICAL EFFECTS IN WATER TREATMENT

Ultrasound (US) is a sound wave of a frequency greater than the superior audibility

threshold of the human hearing. Sonochemistry is the application of ultrasound in chemistry.

It became an exciting new field of research over the past decade. Some applications date back

to the 1920s (Harvey and Loomis, 1929). The 1950s and 1960s subsequently represented the

first extensive sonochemical research years and significant progresses were made throughout

them. Then it was realized that ultrasound power has a great potential for uses in a wide

variety of processes in the chemical and allied industries (Brown and Goodman, 1965;

El‘Pilner, 1964). In these early years, experiments were often performed without any real

knowledge of the fundamental physical background about the US action. The situation

changed in the 1980s when a new surge of activity started and the use of US as a real tool in

chemistry began. It was in 1986 that the first ever international symposium on Sonochemistry

was held at Warwick University U.K. (Mason and Peters, 2002).

Therefore, ultrasound are considered as an novel alternative technologies able to reduce

the total processing cost while maintaining or enhancing product quality in an

environmentally benign manner. Cavitation, indeed, offers immense potentiality for

intensification of physical or chemical processing in an energy-efficient manner (Gogate and

Kabadi, 2008).

Today, we know that the reason why ultrasound power can produce chemical effects is

through the phenomenon of cavitation. Cavitation is the production of microbubbles in a

liquid that are formed when a large negative pressure is applied to it (Mason and Peters,

2002).

Compression and rarefaction waves rapidly move through the liquid media. If the waves

are sufficiently intense they will break the attractive forces in the existing molecules and

Vincenzo Naddeo, Milena Landi and Vincenzo Belgiorno 202

create gas bubbles. As additional ultrasound energy enters the liquid, the gas bubbles grow

until they reach a critical size. Once gas bubbles have reached the critical size, they will either

implode or collapse (Neppiras, 1980; Dehghani, 2005).

High power ultrasound produces strong cavitation in aqueous solutions causing shock

waves and reactive free radicals (e.g., •OH, HO

•2 and O

• ) through the violent collapse of the

cavitation bubble. These effects should contribute to the physical disruption of microbial

structures and inactivation as well as the decomposition of toxic chemicals (Furuta et al.,

2004).

Sonochemical benefits are caused by chemical and physical effects that arise from

ultrasonic cavitations.

CHEMICAL EFFECTS

The chemical effects of ultrasonic cavitation are caused by the formation of hydroxyl and

hydrogen radicals in the collapsing cavities (Makino et al., 1982). Radicals react with each

other and with solutes present in the liquid medium.

Sonochemical reactions in a cavitating liquid occur in three regions, the gas inside a

cavity, the interface between a gas cavity and a liquid and the bulk liquid (Él'piner, 1959;

Suslick et al., 1986). Figure 1 shows a diagram of these three regions in a cavitating liquid,

where sonochemical reactions and processes take place.

Figure 1. Schematic diagram of the three regions in a cavitating liquid in which the chemical reactions

take place.

Application of Ultrasound for Water Disinfection Processes 203

High temperatures and pressures occur inside the collapsing gas bubble, according to the

hot spot theory (Noltingk and Neppiras, 1950). They both cause the thermal dissociation of

water vapour into hydroxyl HO

and hydrogen radicals H. These radicals have been

identified by reaction with spin trapping compounds (Makino et al., 1982; Makino et al.,

1983). Hydroxyl and hydrogen radicals in the gas cavities react either with other radicals

producing water, hydrogen peroxide and hydrogen gas, as shown in Figure1a, or with other

gaseous components such as volatile organic solutes (Sehgal et al., 1982; Todd, 1970).

The second region where sonochemical reactions take place is at the interface between

the hot gas cavities and the bulk liquid. Large temperature and pressure gradients exist in this

region (Riesz and Kondo, 1992). The formation of H2O2, H2O and H2, as shown in Figure 1b,

also occur at the interface between the hot gas cavity and the bulk liquid.

Sonochemical reactions take place also in bulk liquid solution (Figure 1c) at ambient

temperature (Riesz et al., 1990). Radicals produced in collapsing gas cavity (a) and gas/liquid

interface (b) diffuse into this region and react with non-volatile solutes with kinetics similar to

that observed in aqueous radiation chemistry (Riesz et al., 1990; Sehgal et al., 1982).

Gutiérrez and co-workers estimated that less than 10 % of the hydroxyl and hydrogen radicals

formed in the gas cavity reach the bulk solution (Gutiérrez et al., 1991). Products, such as

hydrogen peroxide, from radical reactions occurring in the first two regions also diffuse into

this region and undergo secondary reactions.

Early ultrasonic investigations showed that the nature of a gas present during sonication

affected the sonochemical reactions. Weissler and co-workers found that the amount of iodine

liberated from a potassium iodide solution was dependent on the gas (air, oxygen, nitrogen,

helium or carbon dioxide) that was present during sonication (Weissler et al., 1950). Parke

and Taylor found that hydrogen peroxide formation differed when different gases (oxygen,

argon or nitrogen) were present during sonication (Parke and Taylor, 1956).

PHYSICAL EFFECTS

Sonochemical applications, such as catalysis, cleaning, emulsification and

depolymerization, are possible due to the physical effects of ultrasound. These effects, like

the chemical effects of ultrasound, are caused by the collapse of cavities in a liquid under the

action of a sound wave. The two important mechanisms responsible for the effects of

cavitation in solid-liquid mixtures are microjet impact and shock wave damage (Suslick et al.,

1990). These phenomena will occur when a cavitation bubble collapses near a solid surface,

asymmetric cavitaty collapse (Figure 2). During a collapse a cavity deformation is self-

reinforcing and at the same time a stream of fast-moving liquid directed towards the solid

surface is generated (Olson and Hammitt, 1969; Suslick et al., 1990). It is called microjet.

The speed of microjets is estimated to be around 100 m/s (Suslick et al., 1990). The

impact of microjets on a solid surface causes localised erosion or pitting. Ultrasonic pitting

has been recorded photographically (Numachi, 1965; Olson and Hammitt, 1969). The surface

area of a solid is increased and new surface material is exposed.

Shock waves formation also occurs during cavity collapse (Boudjouk, 1986). Shock

waves break apart loosely aggregated particles along existing cracks (increasing the surface

area) and remove loosely adhering particles to a solid surface (Crawford, 1963).


Figure 2 Schematization of asymmetric cavity collapse in bulk solution and microjet and shock wave

formation.

Reaction products are thus removed from a solid surface, leaving the surface clean and

available for further reaction (Mason, 1999). Turbulent flow and shock waves produced by

intense ultrasound cause small metal particles to collide with sufficiently high speed so as to

induce melting at the point of collision (Suslick et al., 1990).

The following paragraph overview several studies on the use of ultrasound to treat

several types of microorganisms in various aqueous matrices.

ULTRASOUND FOR WATER DISINFECTION

Ultrasound environmental applications include many processes on drinking water,

wastewater, sludge, waste, contaminated site remediation and air. Disinfection is one of the

applications of ultrasound at both drinking water and wastewater treatments.

It is well known that chlorination is the most common method for water disinfection. It is

also well known that during this process, however, chlorine and its compounds react with

some organic matter to form unwanted by-products, hazardous to human health, known as

DBPs (Disinfection By Products). In more countries very stringent limits for chlorination by-

products such as trihalomethanes were set for wastewater reuse. In accord with this, the use of

alternative oxidation/disinfection systems should be evaluated as possible alternative to

chlorine. Recently ultrasound were found to be effective for wastewater disinfection. An

alternative to chlorination is the use of multiple disinfectants which can enhance inactivation

of pathogens and reduces the chlorination by-products (i.e. trihalomethanes and haloacetic

acids).

Ultrasound is able to inactivate bacteria and de-agglomerate bacterial clusters through

both physical and chemical effects. Indeed, cavitation bubbles produce enough energy to

mechanically weaken or disrupt bacteria or biological cells through essentially these two

processes listed hereafter (Joyce et al., 2003a; Von Sonntag, 1986; Oyane, 2009; Furuta et al.,

2004).

Formation of mechanical forces due to surface resonance of the bacterial cell.

Pressures and pressure gradients, resulting from the collapse of gas bubbles, get

bacterial solution in or near bacterial cell walls. Bacterial cell damage results from

mechanical stress, over a period of time, which depends on frequency (Furuta et al.,


2004). As a effect of cavitation there is micro-streaming, that occurs within bacterial

cells, induces shear forces breaking bacterial agglomerates up into a greater number

of individual bacteria (bacterial splitting) (Oyane I., 2009).

Formation of radicals (•OH and

•H) during cavitation in the aqueous medium

generates a chemical attack. These radicals attack the chemical structure of bacterial

cell walls and weaken cell walls to the point of disintegration (Von Sonntag, 1986).

Hydrogen peroxide, which is a strong bactericide, is among final products of water

sonochemical degradation (Hua and Thompson, 2000).

The overall effect of applying ultrasound is thus the result of a competition between these

two processes in a water solution.

Disinfection efficiency is also strongly influenced by both irradiation time and intensity.

Generally, disinfection performances are in direct ratio to time and intensity. Sometimes it

may happen that low frequencies apparently provide to a higher performance at first (Figure

2). In these cases even if we have the same number of colonies at t0, more colonies could be

found at high intensities at short sonication time. This happen because for high frequencies

the splitting is higher. These is one limit that cannot be clear with used analytical methods.

Sonication alone can provide powerful disinfection. However, to achieve high

performances in a shorter time, using only ultrasound is necessary to use high ultrasonic

intensities.

Most studies focus on the inactivation of Escherichia coli in various aqueous matrices

(Table 1). E. coli inactivation exhibits pseudo-first order behaviour (Hua and Thompson,

2000; Oyane et al., 2009). Hua and Thompson (2000) investigated the effect of ultrasound at

a frequency of 20 kHz for E. coli inactivation. They observed that the extent of inactivation

increased with increasing intensity and became about 2.8 Log after 60 min at the highest

density of 470 W/L. Furuta et al. investigated the inactivation of E. coli along with hydrogen

peroxide formation due to water sonolysis in order to correlate the level of H2O2 formation to

the extent of inactivation (Furuta et al., 2004). A 27.5 kHz horn type sonicator was used

whose operation was based on the ―squeeze-film effect‖ (i.e. the film is defined as the space

between the end of the probe of the sonicator and the bottom of the reactor) and the maximum

power of this sonicator was 42 W/mL. When the amplitude on the vibration face was 3m,

inactivation was 6 Log at room temperature. They observed that the ultrasonic shock wave

was more important in killing microorganisms rather than the indirect effect of •OH radicals

formed by ultrasonic cavitation. Ultrasound waves at a frequency of 42 kHz were used to treat

aqueous suspensions of E. coli in the study by Dehghani (2005); the author reported a 2.7 Log

inactivation at a power density of 0.12 W/mL and a sonication of 90 min.

Ultrasound performances increase with smaller treatment volumes. Indeed the better

result was obtained by Furuta and co-workers that worked with a ―squeeze film‖ ultrasonic

system (Furuta et al., 2004).

It should be noted that, in almost all studies reviewed, the used systems are low

frequencies systems. However, it has been generally observed that these low frequency

technologies achieve the best performances concern to microorganisms inactivation.

Indeed, it can be said that less resistant microorganisms are Escherichia coli and Total

coliform, these showed the higher inactivation percentages.


Several types of reaction systems have been employed for ultrasound disinfection studies

and these are discussed in detail in recent reviews (Gogate, 2007; Gogate and Pandit, 2004),

while they are schematically shown in Figure 3. In brief, the ultrasonic probe is suitable to

treat relatively small volumes of water with the ultrasound irradiation being localized around

the emitting horn and not distributed as in the case of ultrasonic bath, plug-flow reactor and

the flow cell. The plug-flow reactor consists of immersed ultrasound transducers unlike the

bath where the transducers are not in direct contact with the liquid phase; moreover, it can

treat larger volumes than the probe or the bath. On the other hand, the flow cell is a unique

system that works under pressure and the water inside is sonicated all around.

There is a drive towards the use of ultrasound in disinfection as an adjunct to other

techniques because some microorganisms are becoming resistant to existing disinfection

techniques involving biocides, ultraviolet light, and heat treatment. In this view, recent studies

have dealt with the use of low frequency ultrasound (in the range 20–40 kHz) alongside

ozone (Belgiorno et al., 2007), ultraviolet irradiation (Naddeo et al., 2007), hydrodynamic

cavitation (El‘Piner, 1964), electrolysis (Brown and Goodman, 1965), chlorination (Mason

and Peters, 2002; Mason, 1976; Asher, 1987), and heterogeneous catalysts (i.e. activated

carbon, ceramic, zinc, and titanium dioxide) (Hoyler and Luke, 1994; Richards and Loomis,

1927), and reported that enhanced disinfection efficiencies could be achieved with the

combined treatments. Process efficiency may also be dictated by operating conditions such as

ultrasound intensity, frequency, and conduct time as well as the water matrix (i.e. pH,

alkalinity, suspended solids, dissolved gases) (Hua and Thompson, 2000; Madge and Jensen,

2002; Joyce et al., 2003; Dadjour et al., 2006; Antoniadis et al., 2007).

The following paragraph overview several studies on the use of ultrasound to treat

several types of microorganisms in various aqueous matrices.

Figure 2. Example about as analytical methods affect bacterial measurements (t0<t1<t2)


Table 1. Summarizes studies on the inactivation of various microorganisms induced by

ultrasound irradiation as the sole disinfectant; relevant operating conditions and

efficiencies are quoted

Microorganism Frequency

Density

[W/mL]

Ultrasonic

system

Sonication

time

[min]

Efficiency Reference

Bacillus

subtilis L 0.24 Probe 15 C

Joyce et al.,

2003

Escherichia

coli

L 0.47 Probe 60 C

Hua and

Thompson,

2000

L 42 Probe** 3 A Furuta et al.,

2004

L 0.12 Bath 90 A Dehghani,

2005

L 45 Probe 20 A Antoniadis et

al., 2007

L 0.005 Reactor 30 B Naddeo et al.,

2009

Pseudomonas

aeruginosa H 15 Bath 15 C

Phull et al.,

1997

Saccharomyces

cerevisiae L 0.16 Probe** 10 C

Tsukamoto et

al., 2004

Total coliforms

L 2.4 Probe 15 C Jyoti and

Pandit, 2004

L 0.06 Bath 15 C Jyoti and

Pandit, 2004

L 0.005 Reactor 30 B Naddeo et al.,

2009

Figure 3. Ultrasonic systems typically used for sonochemical treatment


ULTRASOUND USED IN COMBINATION WITH ULTRAVIOLET

Some authors have proposed a combined use with ultraviolet using cavitation for

disinfection as a pre-treatment rather than as a combined treatment. On the other hand,

Naddeo and co-workers (2009) found that the effectiveness of chlorination significantly

improves in combination with ultrasound.

A patent was granted to University of Salerno for the combined treatment of ultrasound

and ultraviolet for wastewater disinfection. A study was carried out for testing the

simultaneous combination of UV and US in terms of bacteria inactivation at pilot-scale.

The pilot plant was composed of two reactors (US–UV reactor and UV reactor) and set

up for use at the wastewater treatment plant (WWTP) of Mercato San Severino (Salerno,

Italy). Reactor 1 (US–UV reactor, Figure 4) was composed of ultrasonic transducer TD-US

1400 (CEIA S.p.A., Italy) at low frequency (39 kHz), variable power from 350 to 1400W and

two low pressure UV-C lamps (Trojan Technologies, Canada) of 150W each. Reactor 2 (UV

reactor) was composed of only two low pressure UV-C lamps (PROCOM s.r.l., Italy) of

200W each. In both reactors, a volume of 80L has been designed for disinfection zone. The

tests were carried out to evaluate the inactivation of two type of bacteria: Escherichia coli and

Total coliform.

An important enhancement of UV disinfection ability has been observed in presence of

US, especially. Sonication effects also increase the UV disinfection efficiency in terms of

reduction of big particles and cleaning lamps.

The tests were conducted with wastewater characterized by low transmittance, where

generally UV disinfection was not suitable. Instead, this innovative combined treatment is

able to guarantee high performances also with low transmittance wastewater. An important

enhancement of UV disinfection ability has been observed in presence of US, especially.

Sonication effects also increase the UV disinfection efficiency in terms of reduction of big

particles and cleaning lamps. What is more, the analyses show the effects of solar radiation on

UV lamps fouling formation and the specific possibility to remove fouling by US.

Thus the combined process US–UV can be considered as a valuable alternative to

conventional oxidation/disinfection processes when less expensive solutions such as

chlorination cannot be applied because of very stringent limits set by regulations (e.g.

trihalomethanes). Indeed, the combined process US–UV allowed decreasing the E. coli

colonies under 10 CFU/100mL wastewater reuse Italian limit) with a retention time of 15min.

The tests showed the influence of ultrasound on lamps fouling formation. In fact, while

during the tests, the lamps in UV reactor were becoming dirtier day by day, in US–UV reactor

the UV lamps were perfectly clean even after three days of treatment (Figure 2). The US

cleaning effects was guaranteed by the collapse of cavitation bubbles which produce liquid

jets on the lamps‘ surface. In this way, the US breaks the cake layer on the lamps making the

UV beans emission achievable in wastewater. US irradiation in combined process has a

double key role; US increase the disinfection performance not only by its disinfection power

but also by providing the constant cleaning of the UV lamps, guaranteeing constant

disinfection performances.

The advanced ultrasound disinfection (US–UV), applied under such conditions, may be

an effective technique in all WWTP where the wastewater reuse is an important

integrative/alternative resource for not drinking purposes. Nonetheless, further studies should


be performed to evaluate better the disinfection effectiveness on a different bacteria species

and in continuous operation, subsequently in terms of formation of unknown ultrasound

disinfection by-products (UDBPs).

Figure 4. Schematic longitudinal section of the US-UV reactor (Reactor 1).

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Chapter 8

USE OF ULTRASONICATION IN THE PRODUCTION

AND REACTION OF C60 AND C70 FULLERENES

Anne C. Gaquere-Parker1 and Cass D. Parker

2

1Chemistry Department, University of West Georgia, 2Chemistry Department, Clark Atlanta University, Atlanta, GA 30314

ABSTRACT

In this chapter, the use of ultrasounds on fullerenes (C60 and C70) and fullerene

derivatives is described. The focus is on the articles reporting the ultrasound-promoted

treatment of these nanoparticles written in English. The ultrasound-enhanced synthesis

and chemical modification of fullerenes are detailed. The improvement obtained by

sonicating the reaction mixtures while carrying out traditional organic reactions is

discussed. This includes many types of reactions, such as oxidation, cycloaddition,

reduction and amination. Also the ultrasound-enhanced crystallization of fullerenes,

producing fullerites, and the formation of colloids when the fullerenes are sonicated in

various solvent mixtures are detailed, providing the role of ultrasound in these processes.

INTRODUCTION

Since its discovery, fullerene represents a most unusual molecule to understand and has

fascinating potentials. The physical and electronic properties of the molecules have lead to a

number of potential applications in the biochemical and medicinal fields. Fullerenes have

biological activities that are inherent in the nature of the molecule. In addition, the physical

and chemical properties of fullerenes show promise in optics and nanodevice applications. To

achieve some of the potential biochemical and medicinal applications one must overcome the

low solubility of the fullerenes. To achieve increased solubility, fullerenes are derivatized to

methanofullerenes or fulleroids. To achieve functionalization of fullerenes sonochemistry has

shown extreme promise in producing sufficient quantities of the methanofullerenes and other

fullerene derivatives. The high temperature and pressure within the cavitation bubble provides

an unusual chemical environment that is much suited to fullerenes chemical modification. In

Anne C. Gaquere-Parker and Cass D. Parker 214

this chapter we discuss the application of ultrasounds (sonochemistry) in the chemical

modification of fullerenes.

SONOSYNTHESIS OF FULLERENES AND FULLERENE DERIVATIVES

Ultrasound has been used in the production of C60 on a very limited basis. Katoh et al.

[1] reported the production of C60 using ultrasounds. In their paper they reported the

production of C60 using benzene as the starting material (Figure 1).

They conducted ultrasonic irradiation using a MST, UH-600 ultrasonic homogenizer

equipped with a 20 mm in diameter titanium tip at a frequency of 20 kHz and 600 watts.

During irradiation argon gas was bubbled through the benzene starting material in an open

vessel. Production of C60 was noted to occur after 1 hour of irradiation. Irradiation continued

for 12 hours. The material was separated and verified using high performance liquid

chromatography and fast atom bombardment mass spectrometry via the 720 m/z nominal

mass.

One of the earliest applications of sonochemical modification of fullerene was in the

synthesis of C60H2 by Mandrus et al. [2]. They reported the production of C60H2 using a 20-

kHz titanium horn operating at 30 ± 5 watts (Figure 2).

)))), 0oC, 12 h.

Figure 1. Sonosynthesis of C60 from benzene.

)))), 25oC, 2-4 h.

HH

Hydrogens are placed on the [6,6]junction, although it is not specified in the article.

Figure 2. Sonosynthesis of C60H2 from C60.

The solvent chosen for the synthesis of C60H2 was decahydronaphthalene.

Decahydronaphthalene was chosen as a solvent for several reasons; sonochemistry of

decahydronaphthalene is well understood, fullerenes are soluble in it, and it produces highly

energetic cavitation bubbles due to its low vapor pressure. Ultrasonic irradiation of C60 in

decahydronaphthalene produced a brown precipitate that was analyzed using high resolution

mass spectrometry and HPLC. The material obtained produced was shown to contain the

Use Of Ultrasonication in the Production and Reaction of C60 and C70 … 215

C60H2 moiety conclusively based on the intensity of the 722 m/z peak that exceeded the

expected intensity of the C60 isotopic ratio for 13

C212

C58+ and subsequent deuterium

experiments. As an additional confirmation of the production of the C60H2, the sonochemical

reaction of C60 was conducted in deuterated decahydronaphthalene. The result provided

support for the production of C60H2 with the detection of the C60HD+ ion observed at 723 m/z.

This surprising result was characterized as arising from the rapid H/D exchange prior to mass

analysis. The source of the H was determined to be from moisture in the air or residual

sources that contacted surfaces of glassware or the mass spectrometer probe itself. The most

surprising result of their work was that only the C60H2 molecule was observed. It was

concluded however, that the results fit with several observations; 1) molecular H2 and smaller

alkenes are produced during sonolysis of alkanes, 2) such products are consistent with the

Rice radical chain mechanism of pyrolysis and 3) the intensity of cavitational collapse

increases with the lower vapor pressure of the solvent, which increases the sonochemical rate.

It still is not clear why only C60H2 was produced. The concluding factor is that it involves a

secondary reaction arising from the production of atomic hydrogen during sonolysis of the

solvent and reaction with the C60 fullerene.

In addition to the production of C60H2, the use of ultrasounds to produce

methanofullerenes derivatives have been reported, [3,4]. The methanofullerenes consist of a

bridging methyl group between two fullerene units using the [6,6]-[6,6] (Figure 3), or [6,6]-

[6,5] or [6,5]-[6,5] bonding scheme.

The methanofullerene dimer, C121 (C60CC60) and C122 (C60C2C60) were first synthesized

thermolytically and photolytically by Dragoe et al. [5]. The carbene adds across a [6,6] bond

of the C60 or the [6,5] which is across an annulene bond of the C60. Yinghuai and Yinghuai et

al. [3,4] report the synthesis of the methanofullerenes in ionic liquids (ILs) using ultrasonic

irradiation. The results of the synthesis were compared to thermolysis and photolysis

synthesis of the same compounds. In the sonochemical synthesis of methanofullerene, the

methanofullerene derivatives C60(CCl2), C60(CBr2), C60(CI2) and C121 were synthesized by

reacting the corresponding haloform, CHCl3, CHBr3 or CHI3, with fullerene in the presence

of base at 50 kHz for two or three days at 298K.

Figure 3. [6,6]-[6,6] Methanofullerene.


CHX3 / NaOH

IL, ))))X

X

X

X

Mg or Zn, C60

IL, ))))

X = Cl, Br or I

a.

b.

Figure 4. a: Synthesis of a dihalogeno methanofullerene. b. Synthesis of C121.

In each case the corresponding methanofullerene derivatives, C60(CCl2), C60(CBr2),

C60(CI2), were obtained with good yields in yields ranging from 55 – 84% in the ILs (Figure

4a). Once isolated the dihalomethanofullerenes were reacted with fullerene in the presence of

magnesium under similar conditions. Yields for the sonochemical synthesis of C121

methanofullerene dimer was on the order of 55 - 84% for using dichloro-, dibromo-, and

diiodo-methanofullerene. The yields of the methanofullerenes derivatives were compared

using different reaction media and solvents. Yields were significantly higher in the ionic

liquid media, 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4], 1-butyl-3-

ocylimidazolium tetrafluoroborate [OMIM][BF4] and 1-butyl-3-methylimidazolium

hexafluorophosphate [BMIM][PF6] than when conducted in the solvent THF. (Figure 4 b).

In comparison to the thermolytic and photolytic synthesis pathways, the sonochemical

synthesis was found to produce only one of the previously reported methanofullerene

derivatives. Sonochemical synthesis produced exclusively the [6,6] methanofullerene dimer.

This is in direct contrast to the photolytic or thermolytic synthetic routes that produces a

combination of the [6,6]-[6,6], or [6,6]-[6,5] or [6,5]-[6,5] methanofullerene derivative [5]. In

the closely related work, Yinghaui et al. reported the reaction of diiodomethane or 7,7-

dibromobicyclo [4,1,0] heptanes with fullerene in the presence of Zn or Mg respectively

under ultrasound in ILs to produce the corresponding methanofullerene as shown in the

following scheme with very good yields (Figure 5).


Zn, CH2I2

IL, ))))

Mg, IL, ))))

Br Br

Figure 5. Synthesis of methanofullerenes from C60 in ionic liquids.

Benzene, 5h, RT, ))))

Oxidation conditions

3-Chloroperoxybenzoic acid

4-Methyl morpholine-N-oxide

CrO3, CS2, acetone

KHSO5, H2O, 18-crown-6

O

C60OC60

or

or

or

Figure 6. Epoxidation of C60 in benzene.

Another area of significant interest is the sonochemical functionalization of fullerenes in

the synthesis of aromatic amines to produce monosugar derivatives. This work has been

conducted on a large part by the research work of Ko and is summarized here. The earliest

published report is the synthesis of fullerene oxides using various oxidants under ultrasound

irradiation conditions. They reported the synthesis of epoxylated fullerenes, C60(O)n, with n

ranging from 1-3 and C70(O)n, n from 1-2 [6,7,8]. Four different oxidizing agents were used,

3-chloroperoxy benzoic acid, 4-methyl morpholine N-oxide, chromium (VI) oxide and

oxone® monopersulfate KHSO5 (Figure 6).


Yields were not reported, however, 3-chloroperoxy benzoic acid and 4-methyl

morpholine N-oxide were able to produce the epoxide with n ranging from 1-3 under

ultrasound conditions. The latter two, chromium (VI) oxide and oxone® monopersulfate were

not successful for the production of n>1 of the fullerene epoxide. Ultrasound conditions for

the synthesis were not reported. In their follow up report on the epoxidation of fullerene C70

by the above oxidants, it is noted that the reactions were carried out using a horn type

sonicator operated at 20 kHz and at 750 watts [7]. The horn tip was 13 mm in diameter. The

reactions were carried out for 5 hours in air at room temperature. This work provides

evidence that reaction times are shortened using ultrasonic irradiation with much higher

yields and that mechanism involved for formation for the epoxides is non-thermal. In the case

of oxidation using 4-methyl morpholine N-oxide to oxidize C70 no reaction occurred in the

absence of ultrasound. The results strongly suggests that epoxidation under ultrasound is

viable and efficient for electron rich olefins and fullerenes. The proposed mechanism of

epoxidation of C70 fullerene under ultrasound is through a nucleophilic attack to a [6-6] bond

followed by heterolytic cleavage of the O-O bond. The epoxidation of C60 fullerene was

expanded to include several amine N-oxides. Under ultrasound irradiation fullerene C60 was

found to be oxidizable by 3-picoline N-oxide, isoquinoline N-oxide, pyridine N-oxide hydrate

or quinoline N-oxide to the fullerene epoxide, C60(O)n, where n = 1-2 or n=1, depending on

the amine N-oxide (Figure 7) [8].

In a separate report, Ko reported the synthesis of fullerene oxides using metal

hexacarbonyl complexes under ultrasounds [9]. In this method, fullerene C70, in hexane is

subjected to ultrasounds in the presence of a metal carbonyl complex (M(CO)6, M = Cr, Mo

or W) for 24 hours in air at 25-43°C. This method produced fullerene epoxides of the nature

C70On, with n = 1-2. The oxidation by ultrasonic irradiation may proceed by nucleophilic

attack on the [6-6] bond in the fullerenes as noted previously. These methods provide an easy

process to obtain fullerene epoxides that can be easily modified to produce a number of

interesting fullerene molecules. As an indicator, Ko utilized fullerene epoxides in the reaction

with aromatic amines under ultrasounds [10]. In this report, C70On was reacted with 3-

nitroaniline, 4-isopropylaniline, or 4-nitroaniline in the presence of FeCl3 in tetrahydrofuran

(THF) to produce aminofullerenols under ultrasonic irradiation (Figure 8). The

aminofullerenols were confirmed by MALDI-TOF-MS and UV-visible spectrophotometry.

THF, 24h, RT, ))))

Oxidation conditions

3-Picoline-N-oxide

Pyridine-N-oxide

Quinoline-N-oxide

Isoquinoline-N-oxide

O

C60OC60

or

or

or

Figure 7. Epoxidation of C60 in THF.


OH

NHR

O

FeCl3, THF, )))), 24 h

O2N

NO2

RNH2

R =

Figure 8. Amination of C70 epoxide with three aniline derivatives.

Ko has also undertaken the sonochemical synthesis of glycosyls fullerenes, C60 and C70

[11,12]. The interests in these materials are their potential medicinal applications due to the

solubility of such materials and possible biological activities. The glycosyls fullerenes are

prepared via a cycloaddition by reacting C60 or C70 with 2‘-azidoethyl per-O-acetylglycoside

of interest at a 1:1 ratio in benzene under ultrasounds for 2 days at room temperature, as

shown in the scheme below (Figure 9).

An ultrasound horn sonicator was used operating at a frequency of 20 kHz and 750 watts.

The resulting glycosyl fullerene derivatives were isolated using flash chromatography and

analyzed using FAB-MS, 1H and

13C NMR, FT-IR and UV-vis spectrophotometry. The

characterization results strongly support the production of the closed [5,6] and [6,6]

monoadducts. No evidence of the open [5,6] or open [6,6] isomer formation was observed. In

addition, no evidence was present for the formation of the bis-adduct formation under

ultrasounds. Although produced in very small yields, <5%, ultrasound provides a simple and

direct method for the production of the glycosyl mono-adducts.

SONOCHEMISTRY APPLIED TO FULLERENES AND CARBON

NANOTUBES MODIFICATION, SEPARATION AND PURIFICATION

Ultrasounds have potential applications in the production of fullerenes and carbon

nanotubes. In the following applications a careful distinction is made between the use of

―horn type‖ and ―bath‖ ultrasonic devices. Horn type typically operates at a higher power

level and induces a higher level of mechanical damage during processing. Bath type

ultrasonic devices operate a lower power level, but are very much capable of activating or

accelerating a reaction.


N sugar

sugarN3

benzene, ))))RT, 2 days

O

O

OAc

AcO

AcO

AcOO

O

OAc

AcO

AcO

AcO

OAc

O

O

OAc

AcO

AcO

OAc

AcOO

OAcO

OAc

O

O

OAc

AcO

AcO

O

O

OAc

AcO

OAc

2,3,4,6-tetra-O-Acetyl-alpha-D-mannopyranoside 2,3,4,6-tetra-O-Acetyl-beta-D-galactopyranoside

2,3,4,6-tetra-O-Acetyl-beta-D-glucopyranoside 2,3,4-tri-O-Acetyl-beta-D-xylopyranoside

octa-O-Acetyl-beta-D-maltopyranoside

Sugars derivatives are listed below:

Figure 9. Synthesis of glycosyls fullerenes.

One the earliest reports for the isolation and characterization of fullerenes using

ultrasounds was published by Diack et al. [13]. They reported the purification by the use of

ultrasonic bath at room temperature followed by a one step non-aqueous reversed phased

chromatography process using octadecyl silica as the stationary phase and n-hexane as the

mobile phase. Characterization was determined using mass spectrometry and UV-visible

spectroscopy. Diack et al. compared the use of THF and boiling toluene as solvents during the

ultrasound ―bath‖ treatment of the impure material. The extraction involves dispersion of the

soot samples in THF and submission to the ultrasonic bath for 20 minutes. The material is

then filtered and washed until the filtrate is clear. The washing and filtrate are combined and


placed into a rotary evaporator to remove the solvent. Benzene is added to aid in the removal

of residual moisture from the THF. The extract is recovered using methylene chloride and

dried under vacuum. For separation of the fullerenes from the extract a 25 cm, 4.6 mm id

preparative column was used. The column was packed with 8.9 µm IMPAQRG 2010

C18silica. The extract was dissolved in methylene chloride, injected on the column and eluted

with n-hexane at a flow rate of 1.0 mL per minute. Analysis of the collected fractions shows

clearly that C60 and C70 could easily be separated at a fairly high purity, along with traces of

higher mass fullerenes such as C76, C78, C82 and C84.

Using a very different approach, Gasgnier and Petit used ultrasound to increase the purity

of a fullerene extract mixture [14]. In their procedure a fullerene sample of an approximate

composition of 80/20 of C60/C70 and higher fullerenes and C60-solvates were dispersed in

dodecane (fullerenes are weakly soluble in this solvent). Using a double walled container for

water bath temperature control at 300-310 K, the solution was subjected to ultrasounds from a

horn type sonicator operating at 20 kHz and 180 W/cm2 for seven hours. After sonication, the

solvent was removed by drying and the resulting residue analyzed using XRD. The resulting

pattern for the material collected after sonication had the corresponding diffraction lines of

fcc C60. Based on the XRD results the conversion of the starting material to C60 fullerene was

90% or greater. The use of microwave was compared to the ultrasound method for the same

starting material with a conversion rate of greater than 95% within 2 minutes. Alternate

starting materials showed very little change when subjected to ultrasounds, however, those

same were convertible to a higher purity C60 when subjected to microwaves.

Minato et al. were able to reproducibly grow C60 fullerene nanotubes by the modification

of liquid-liquid interfacial precipitation method [15]. In their procedure they utilized

ultrasonic dispersion upon the addition of isopropyl alcohol to successfully grow C60

fullerene nanotubes from a C60 solution in pyridine. Solutions were maintained at 10°C

during the crystal growth process. Through the use of ultrasonic pulverization,

recrystallization and crystal growth they were able to produce C60 nanotubes of different

morphologies (Figure 10). They report the production of open and closed end C60 nanotubes

of various sizes ranging from several hundred nanometers to several micrometers (Figure 11).

The importance of their work is that the nanotubes grown can easily be redissolved such that

the C60 nanotubes could be used as a template or a reaction vessel for encapsulated materials.

Figure 10. (a) Photograph of C60 nanotubes grown by forming liquid–liquid interface between pyridine

saturated with C60 and isopropyl alcohol and (b) optical photomicrograph of C60 nanotubes. Reprinted

from [15], Copyright (2005), with permission from Elsevier.


Figure 11. Transmission electron microscope image of a C60 nanowhisker with partly dissolved

textures at the ends. Reprinted from [15], Copyright (2005), with permission from Elsevier.

Miller et al. used liquid-liquid interfacial precipitation to grow C60 nanotubes of

diameters reaching 300 nm[16]. The method employed is a modification of the Miyazawa‘s

technique, addition of a ninefold volume excess of isopropanol to a C60 solution prepared in

pyridine. Nucleation in their study was initiated at 2° C, the solution shaken after 24 hrs, and

crystal growth continued for several days at 2° C. In their study the sonochemical stability of

the tubes grown was studied by sonicating in a bath for up to 60 seconds. The tubes showed

no sonochemical stability. Damage was observed as early as after 5 seconds of sonication, as

observed by optical microscopy and AFM. After 30 seconds of sonication in a bath type

sonicator the tube structure was mostly compromised leaving very small fragments. As the

nanotubes are held together by weak van der Waals forces between the C60 fullerenes, the

nanotubes are easily broken by the force incurred during sonochemical processes.

GROWTH OF C60 FULLERITE CRYSTALS USING ULTRASOUNDS

The potential of growing macroscopic C60 crystals has been of considerable interest in

recent history. Most of the early approaches involved the vapor phase synthesis at high

temperature under high vacuum. This method produces high quality crystals, however it is an

expensive approach with limited availability of the specialized equipment. Gupta et al. [17]

reported on the ultrasound induced growth of C60 single crystals of mm size overnight. As

noted by Miller [16], they found that the crystals are bonded together entirely through van der

Waals forces. The crystals produced are extremely brittle. The method developed by Gupta et

al. is very straightforward; pure KBr powder is filled into a glass cylinder to a depth of ~5

mm, to this added C60, after which sufficient benzene is added to give a C60 concentration of 2

mg C60/ml benzene. The beaker is ultrasonically treated to dissolve the C60 into the benzene.

After dissolution of C60 sonication is continued for several more hours. The benzene is then

allowed to evaporate overnight at room temperature yielding partially or fully grown C60

single crystals. The crystals were characterized using X-ray diffraction analysis (Figure 12).

Peaks noted from the diffraction pattern were narrow, indicating good crystallinity, with a

calculated fcc lattice parameter of a=14.17±0.2 Å. SEM pictures show some KBr particles

(Figure 13 a and b) which can be washed with water. EDX spectrum shows one single peak,

confirming the high purity of the crystals (Fig 13.c). The C60 single crystals were reported to

be very brittle, probably due to structural defects or their van der Waals bonding, which both

prevent the motion of dislocations. Such fragility is not reported when the fullerites are

produced at high temperature, which can be explained by the incorporation of carbon in the

crystal. This novel technique allows the production of large size C60 fullerites in an easy and

very inexpensive way.


Figure 12. X-ray diffraction patterns of (a) original C60 powder and (b) C60 fullerites. Reprinted from

[17], Copyright (2006), with permission from Elsevier.

Figure 13. (a, b) SEM images of the C60 fullerites and (c) EDX spectra of the C60 fullerites. Reprinted

from [17], Copyright (2006), with permission from Elsevier.


Shul‘ga et al. [18] used ultrasound in the preparation of C60 fullerite by a precipitation

method using a different solvent than Gupta [17]. Shul‘ga was attempting to reduce the size

of the C60 crystallite and make an amorphous form of the C60 fullerite. In their procedure a

C60 solution is prepared, using toluene, chlorobenzene or 1,2-dichlorobenzene as the solvent,

sonicated by an ultrasound horn operating at 35 kHz and 200 watts for 5 minutes to

completely dissociate the C60. To the solution is added a five-fold excess of isopropanol and

sonicated for an additional 60 minutes. In their work they noted that ultrasounds produce very

little effect on the size of the crystallite in the fcc phase of the fullerite prepared regardless of

the solvent used. They did find that the parameter a of the fcc lattice was significantly smaller

for the samples subjected to sonication than those that were not subjected to ultrasound. They

concluded that sonication promotes the degasification of the solvent that can be captured

during the crystallization process, decreasing therefore the concentration of gases in the

crystal. As a result the fcc lattice of the fullerite is smaller.

COLLOID PRODUCTION

In a different approach to the use of ultrasounds, Todorovic-Markovic et al. used

ultrasound for the preparation of colloidal C60 that were characterized by atomic force

microscopy [19]. They prepared C60 or C84 colloidal suspension by ultrasound dispersion of

fullerenes in THF for 10 days. This solution was filtered through a .45 µm PTFE filter. The

solution is then purged using argon and to it is added an equal volume of MiliQ water at a rate

of 2 L/min under continuous ultrasounds. The THF is then evaporated using a rotary

evaporator at 45°C. Concentration of the fullerenes-based colloids was determined by drying

completely the suspension and reconstituting in xylene before submitting it to an additional

two hours of sonication. The absorption of the resulting solution was measured

spectrophotometrically and the concentration determined using from a calibration curve.

FTIR and atomic force microscopy were used to characterize the colloids. FTIR clearly shows

the presence of C60 and C84 fullerenes colloids. In addition, their results show that THF forms

a shell around the nanocrystals formed during sonication. AFM was used to characterize the

particle size of the fullerenes in THF. The results show a reduction in particle size with

increased sonication time. Particle size of the colloids before sonication was determined to be

between 80-90 nm for C60 and 95 nm for C84, with a solvent layer of approximately 31 nm

thick of THF (Figure 14 a). Particle size for the C60 was reduced from an average of 1.312 µm

to 90.33 nm when exposed to ultrasounds for four days and ten days respectively (Figure 15 a

and b). Similarly C84 particles size decreased from 240 to 170 nm under the same conditions.

The nanocrystal structure of C84 can be seen clearly (Figure 14 b and c).

Andrievsky et al. [20] prepared aqueous colloidal solutions of fullerenes in a totally

different manner. In their procedure a fullerene solution in toluene, .2 mg per mL, is mixed

with deionized water and subjected to ultrasounds for several hours until the toluene was

completely evaporated from the solution. This solution was then filtered through a .22 µm

filter resulting in a transparent, but brightly brownish orange solution. The solution was

opalescent suggesting a colloidal solution of ≤.2 µm in diameter. Loss of fullerene was

determined as the final solution was estimated to have a concentration of 5 µg/mL.


Figure 14. Top view AFM images of nC84 colloid recorded in (a) wave, (b) 2D phase and (c) 3D phase

modes. Mica was used as a substrate. Reprinted from [19], Copyright (2008), with permission from

Elsevier.

Figure 15. AFM images of C60 dissolved in THF within four (a) and (b) ten days. Mica was used as a

substrate. Reprinted from [19], Copyright (2008), with permission from Elsevier.

The mass spectrum of the final solution clearly shows the presence of the C60 and C70

fullerene with no evidence of fullerene modification chemically.

The goal of this study was to prepare soluble fullerene solutions to study their interaction

with water and the biological activity of fullerenes in such a media. Such a solution was


easily prepared by the use of ultrasounds to give a finely dispersed colloidal suspension of

high stability. Mass spectrometric analysis of the colloidal suspension indicates no

admixtures are formed during ultrasound irradiation. Stabilization of the fullerene colloids is

derived from fullerene-water interaction via the inclusion of fullerenes into the water structure

and formation of clathrate-like networks of water molecules around the fullerenes and

aggregates which is stabilized by the low mobility of the fullerenes. In addition, they

proposed that the electronic properties of the fullerenes may lead to donor-acceptor and

charge-transfer interactions that promote weak water-fullerene interactions. Such interactions

have been stated previously to explain the unusual properties of the fullerenes in other

solvents.

Andrievsky [21] continued their study of ultrasound produced colloidal solutions of

fullerenes in water (FWS) by electron microscopy to determine the structure of the colloidal

suspension. The FWS prepared by ultrasounds are stable for up to 18 months. Using a

suspension method for the preparation of samples for TEM analysis they were able to obtain

electron micrographs of the colloidal particles (Figure 16).

The results were compared to similarly prepared C60 solution prepared in benzene. The

results showed that the C60 FWS particles consists of sphere shaped aggregates of 7-72 nm in

size. This result is the proof of an ultramicroheterogeneous and polydispersed C60 hydrosol.

Electron diffraction results show the crystal-like character of primary aggregates of C60. The

results of their study show that the C60 FWS to be a molecular-colloid system containing

hydrated single fullerene molecules and fractal clusters.

Ko et al. utilized a different approach to preparing water soluble C60 [22] and C70 [23]

fullerenes under ultrasounds. They report the reaction of C60 or C70 under ultrasounds

with a mixture of sulfuric and nitric acid that give rise to a water soluble C60 or C70 fullerene.

Employing a horn type sonicator, operating at 20 kHz and 750 watts, 20 mg of the fullerene is

added to 10.0 mL of a concentrated sulfuric acid/nitric acid (3:1, v/v ratio) and reacted under

ultrasounds for 3 days in air at 25-43°C. The resulting solution is neutralized with NaOH

producing a brownish-orange solid and evaporated to retrieve the solid material. The resulting

solids were analyzed using MALDI-TOF MS and the C60 was also analyzed using 13

C-NMR.

Figure 16. TEM of fullerene C60 coagulates. Reprinted from [21], Copyright (1999), with permission

from Elsevier.


The MS results show the prerequisite molecular ion of C60 at m/z 720.3285 and C70 at

840.000 (Figure 17).

Figure 17. MALDI–TOF mass spectrum of a water-solubilized fullerene C70. Reprinted from [23],

Copyright (2006), with permission from Elsevier.

In contrast to Andrievsky et al. [20], Ko et al. [22,23] noted the degradation of the C60

and C70 moiety in their mass spectral results arising from a m/z of 24 mass units starting

with the parent peak ion. In their study a water soluble fullerene could be prepared under

similar chemical reaction conditions without ultrasound albeit the reaction time was longer

than three days. They proposed a water soluble species of the nature [C60@(H2O)n] [22] and

[C70@(H2O)n] [23] for the fullerenes. They reported a similar conclusion as Andrievsky [20]

based on published results of the study of fullerenes as to the properties of the colloidal

fullerenes.

CONCLUSION

The use of ultrasounds in the synthesis and production of fullerenes derivatives and

nanomaterials offers two major advantages; speed and specificity. The articles cited in this

chapter clearly show the advantages of using ultrasound for the synthesis of fullerene

derivatives that cannot be synthesized easily by conventional methods. In the case of

methanofullerenes not only was speed and specificity noted, solvent effects under ultrasounds

also led to a significant increase in yield paving the way for synthesizing additional fullerene

derivatives. Ultrasound was also noted be useful in the production of colloidal suspensions

and large crystals of fullerenes that will facilitate the study of fullerene interactions in the

environment and production of new materials.


REFERENCES

[1] Katoh, R.; Yanase, E.; Yokoi, H.; Usuba, S.; Kakudate, Y.; Fujiwara, S. Ultrason.

Sonochem. 1998, 5, 37-38. Possible new route for the production of C60 by ultrasound.

[2] Mandrus, D.; Kele, M.; Hettich, R. L.; Guiochon, G.; Sales, B. C.; Boatner, L.A. J.

Phys. Chem. B 1997, 101, 123-128. Sonochemical Synthesis of C60H2.

[3] Yinghuai, Z.; J. Phys. Chem. Solids 2004, 65, 349-353. Application of ultrasound

technique in the synthesis of methanofullerene derivatives.

[4] Yinghaui, Z.; Bahnmueller, S.; Chibun, C.; Carpenter, K.; Hosmane, N. S; Maguire, J.

A., Tetrahedron Lett. 2003, 44, 5473-5476. An effective system to synthesize

methanofullerenes: substrate-ionic liquid-ultrasonic radiation.

[5] Dragoe, N.; Tanibayashi, S.; Nakahara, K.; Nakao, S.; Shimotani, H.; Xiao, L.;

Kitazawa, K.; Achiba, Y.; Kikuchi, K.; Nojima, K. Chem. Commun., 1999, 85-86.

Carbon allotropes of dumbbell structure: C121 and C122.

[6] Ko, W.; Baek, K., Phys. Solid State 2002, 44, 424-426. The Oxidation of Fullerenes

(C60, C70) with Various Oxidants under Ultrasonication.

[7] Ko, W.; Baek, K., Ultrasonics 2002, 39, 729-733. The oxidation of fullerene [C70] with

various oxidants by ultrasonication.

[8] Ko W.; Nam J.; Hwang S., Ultrasonics 2004, 42, 611-615. The oxidation of fullerene

[C60] with various amine N-oxides under ultrasonic irradiation

[9] Ko, W.; Park, Y., Elastomer 2005, 40, 174-180. Sonochemical synthesis of fullerene

oxides [C70On] (n=1-2) using metal hexacarbonyl complexes M(CO)6 (M=Cr, Mo, W)

under air Atmosphere.

[10] Ko, W.; Park, B.; Lee, Y. Elastomer 2008, 43, 31-38. Sonochemical reaction of

fullerene oxides, [C70(O)n](n>1) with aromatic amines.

[11] Yoon, S.; Hwang, S.; Ko, W. J. Nanosci. Nanotechnol. 2008, 8, 3136-3141.

Sonochemical reaction of fullerene [C60] with several 2‘-azidoethyl per-O-acetyl

glycosides.

[12] Yoon, S.; Hwang, S.; Ko, W. Colloids and Surf., A 2008, , 313-314, 304-307. Synthesis

of glycosyls fullerene [C70] under ultrasonic irradiation.

[13] Diack, M.; Hettich, R. L.; Compton, R. N.; Guiochon, G. Anal. Chem. 1992, 64, 2143-

2148. Contribution to the isolation and characterization of buckminsterfullerenes.

[14] Gasgnier, M.; Petit, A. Mater. Res. Bull. 1998, 33, 1427-1432. Crystallographic data for

ultrasound-and microwave-treated fullerene C60.

[15] Minato, J.; Miyazawa, K.; Suga, T. Sci. Technol. Adv. Mater. 2005, 6, 272-277.

Morphology of C60 nanotubes fabricated by the liquid-liquid interfacial precipitation

method.

[16] Rauwerdink, K.; Liu, J.; Kintigh, J.; Miller, G. P. Microsc. Res. Tech. 2007, 70, 513-21.

Thermal, sonochemical, and mechanical behaviors of single crystal [60] fullerene

nanotubes.

[17] Gupta, V.; Scharff, P.; Miura, N. Mater. Lett. 2006, 60, 3156-3159. Ultrasound induced

growth of C60 fullerites over KBr.

[18] Shul‘ga, Y. M.; Baskakov, S.A.; Martynenko, V.M,; Petinov, V. I.; Razumov, V.F.;

Shchur, D.V. Russ. J. Phys. Chem. 2006, 80, 654-658. Effect of ultrasound treatment of

C60 solutions on the crystalline structure of precipitated fullerite.


[19] Todorovic-Markovic, B.; Jovanovic, S.; Jokanovic, V.; Nedic, Z.; Dramicanin, M.;

Markovic, Z. Appl. Surf. Sci. 2008, 255, 3283-3288. Atomic force microscopy study of

fullerene-based colloids.

[20] Andrievsky, G. V.; Kosevich, M. V.; Vovk, O. M.; Shelkovsky, V. S.; Vashchenko, L.

A. J. Am. Chem. Soc. 1995, 107, 1281-1282. On the production of an aqueous colloidal

solution of fullerenes.

[21] Andrievsky, G.V.; Klochkov, V.K.; Karyakina, E.L.; Mchedlov-Petrossyan. N.O.

Chem. Phys. Lett. 1999, 300, 392-396. Studies of aqueous colloidal solutions of

fullerene C60 by electron microscopy.

[22] Ko, W.; Heo, J.; Nam, J.; Lee, K. Ultrasonics 2004, 41, 727-730. Synthesis of a water-

soluble fullerene [C60] under ultrasonication.

[23] Ko W.; Park Y.; Jeong M. Ultrasonics 2006, 44, e367-9. Preparation of a water-soluble

fullerene [C70] under ultrasonic irradiation.



Chapter 9

APPLICATION OF ULTRASOUNDS TO

CARBON NANOTUBES

Anne C. Gaquere-Parker1 and Cass D. Parker

2

1Chemistry Department, University of West Georgia, 2Chemistry Department, Clark Atlanta University, Atlanta, GA 30314

ABSTRACT

In this chapter, the use of ultrasounds on carbon based nanotubes is reviewed with a focus

on the English written articles. The synthesis of carbon nanotubes and their surface

modification such as oxidation and covalent functionalization under ultrasounds are reported.

The synthesis of hybrid nanocomposite materials where carbon nanotubes are added as a

reinforcement agent via ultrasound-induced assembly is not described in this chapter. A

detailed survey of the literature concerning the purification and separation of carbon

nanotubes under ultrasounds is provided. The effect of sonication on carbon nanotubes

suspensions which covers aqueous and organic solutions in the presence of surfactants is

discussed with an emphasis being placed on the effect that ultrasounds have on non-covalent

interactions between the carbon nanotubes and the components of the suspensions. The effect

of ultrasounds on the physical properties of the carbon nanotubes, especially the introduction

of wall defects is analyzed. Finally the advantages and shortcomings of sonochemistry

described in this chapter are summarized, showing a possible trend in the direction of future

research in this field.

INTRODUCTION Ultrasounds can act in three ways on liquids: cavitation process, localized hot spots, and

radical formation. This chapter first reviews the use of high intensity ultrasound for the

synthesis of carbon nanotubes. The ultrasonic spray method is not described in this chapter

but can be found in the literature [1], [2], [3]. Single wall carbon nanotubes have a high


surface energy and in order to minimize that energy, carbon nanotubes form bundles and

multi wall carbon nanotubes can be entangled during the synthesis process, therefore

dispersion problems which cannot be solved by simple agitation are encountered for both

kinds of nanotubes. Carbon nanotubes can be mechanically separated during sonication, and

this has been used for purification and for the formation of dispersions as described in this

chapter. Functionalization of the nanotubes using ultrasounds is also an important feature,

since it affects their physical properties. The sonochemical preparation of polymer

nanocomposites through dispersions has been reviewed recently [4] and is not be described in

this chapter. Cavitation can cause defects in solids and the damage caused by ultrasounds to

carbon nanotubes is reported here. Finally, the applications combining the use of ultrasounds

and carbon nanotubes are presented.

SONOCHEMICAL PRODUCTION OF CARBON

NANOTUBES

Katoh et al. [5] reported results of the sonication of chlorobenzene and o-dichlorobenzene

in the presence of solid metallic particles that have a diameter under 200 m. ZnCl2, Zn,

ZnO, Ni, and NiCl2 were tested. Sonication of liquid chlorobenzene and benzene without

solid particles or in the presence of Ni, NiCl2 and ZnO yielded a polymer and graphitic

particles (Figure 1.a.). When ZnCl2 was added, carbon nanotubes were obtained (Figure 1.b.).

The authors suggest that the polymer obtained from the sonication of the halogenated

aromatic compound is annealed during the heterogeneous process, which is due to the high

rate of collisions of the relatively small particles in the mixture. Similar results were obtained

when o-dichlorobenzene was sonicated in the presence of ZnCl2 and Zn.

b.

Figure 1: a. TEM image of the product after ultrasound irradiation of dichlorobenzene with Ni. b. TEM

image of the product after ultrasound irradiation of clearly shows that annealing of the product proceeds

dichlorobenzene with ZnCl2. Reprinted from [5], Copyright (1999), with permission from Elsevier.

Jeong et al. [6], [7] reported results of the sonication of ferrocene and p-xylene in the

presence of silica powder under ambient conditions. The process provided single wall carbon

nanotubes without the production of multi wall carbon nanotubes. The presence of silica was

crucial in obtaining the carbon nanotubes, as it may serve as a nucleation site. Ferrocene is

Application of Ultrasounds to Carbon Nanotubes 233

decomposed by the ultrasounds into fine iron particles that are the catalytic site for the growth

of the single wall carbon nanotubes, as well. Since no multi wall carbon nanotubes were

observed, the formation of larger iron particles is unlikely to have occurred. The authors

suggest that the action of the ultrasounds not only decomposes the ferrocene and the p-xylene

to provide the iron and the carbon sources, but also to provide the energy necessary to

synthesize the carbon nanotubes. Under traditional conditions, high energy systems such as

high temperature, high vacuum, or arc discharge are required for the formation of carbon

nanotubes. With ultrasounds, ambient conditions are successfully used, potentially leading to

the large scale synthesis of carbon nanotubes.

In 2008, Manafi et al. [8] synthesized multi wall carbon nanotubes from a mixture of

dichloromethane, lithium and cobalt chloride in a basic aqueous solution at room temperature

in an ultrasonic bath, followed by 24 hours in a 160oC oven. The overall yield of nanotubes

was 70%, with amorphous carbon and carbon nanoparticles accounting for the remainder of

the carbon. Raman spectroscopy shows two peaks at 1586 and 1340 cm−1

(Figure 2),

corresponding to the vibration of sp2-bonded carbon atoms in a 2D hexagonal lattice and of

carbon atoms with dangling bonds in poorly ordered carbon, respectively. The ratio of these

peaks, in the range of G/D=65/35, indicates that the carbon nanotubes exhibit a rather

defective structure. The use of ultrasounds allows the formation of these multi wall carbon

nanotubes with this morphology whereas similar experiments conducted without ultrasonic

pre-treatment led to larger and shorter nanotubes and nanoparticles. It is believed that the

sonication step is an important step to generate the multi wall carbon nanotubes before the

heat treatment. The nanotubes were 2-5 m long with a 60±20 nm diameter with a ring shape

as seen by transmission electron microscopy (TEM) (Figure 3).

Figure 2. Raman spectrum Reprinted from [8], Copyright (2008), with permission from Elsevier.


Figure 3. High resolution TEM image of MWCNTs. Reprinted from [8], Copyright (2008), with

permission from Elsevier.

Ring formation has been described more recently for single wall carbon nanotubes by

Martel et al. [9]. They sonicated single wall carbon nanotubes in a mixture of sulfuric acid

and hydrogen peroxide for several hours at 40-50oC, filtered and resuspended the

nanoparticles in 1,2-dichloroethane using sonication. TEM and atomic force microscopy

(AFM) images revealed the formation of rings and ropes under these oxidative conditions.

The yield depends on the sonication duration as well as the concentration of hydrogen

peroxide. The oxidation step helps reduce the amount of metal catalyst as well amorphous

carbon particles. Single wall carbon nanotubes are cut during the oxidation process, leading to

carboxylated ends. Through van der Waals attraction, the tubes curl together to form rings.

The effect of the ultrasounds is to provide the energy activation necessary for the ring

formation, the carbon nanotubes being at the bubble-liquid interface would be bent

mechanically during bubble collapse. Komatsu et al. [10] also reported the formation of

toroidal aggregates of single wall carbon nanotubes after ultrasonic treatment in

tetrahydrofuran, chloroform and n-heptane, with n-heptane providing the highest purity in

toroidal nanotubes (Figure 4).

Figure 4. TEM images of toroidal aggregates of SWCNTs. Reprinted from [10], Copyright (2006), with


Li et al. [11], [12] sonicated chlorinated hydrocarbons (CH2Cl2, CHCl3 and CH3I) in the

presence of silicon nanowires to obtain multi wall carbon nanotubes and other carbon

nanoparticles such as onion-like carbon nanotubes. The silicon nanowires were pre-treated

with HF to ensure an active surface and the absence of oxide layers. Sonication ensured a


greater yield of nanotubes and nanoonions. The authors suggest a reaction between the Si-H

and the C-Cl bonds from the chloroform, forming C-H units able to polymerize forming

hydrogenated graphite sheets. These sheets wrap around the silicon template leading to the

formation of hydrogenated carbon nanotubes, which upon further sonication are removed

from the silicon templates and are dehydrogenated. Ultrasounds promote the heterogeneous

reaction between the silicon nanowire and the hydrocarbon material in solution as well as the

demolding of the products from the silicon nanowires.

Wang et al. [13] cut graphene oxide nanosheets into smaller hydroxylated analogues of

naphthalene, anthracene and pyrene after 10 minutes of sonication in nitric acid at room

temperature. Dehydration of the hydroxylated analogues under acidic condition and

subsequent recomposition eventually led to the formation of fullerenes and carbon nanotubes.

Upon increasing the temperature, different results were obtained. At 60oC only fullerene-like-

particles were obtained, and at 70oC, only polyaromatic amorphous carbon was detected.

Since cavitation efficiency is known to decrease at a higher temperature, the temperature

study showed the true sonochemical effect in the synthesis process. It is interesting to note

that Li et al. [14] reported sonicating carbon black in deionized water in an ultrasonic bath for

44 hours under ambient conditions produced carbon nanosheets but no carbon nanotubes.

This result shows the probable need for acidic and oxidative conditions as used by Wang [13].

PURIFICATION OF CARBON NANOTUBES

Very few articles deal with the purification of carbon nanotubes directly. Very often

articles refer to the dispersion of carbon nanotubes which upon centrifugation can lead to

purer samples. In this section articles dealing with the purification of the carbon nanotube

samples are described, whereas the latter part of this chapter will discuss dispersions.

In 1998, Dujardin et al. [15] synthesized what they called single shell nanotubes by the

laser-oven ablation method and purified them by sonicating the raw sample in nitric acid in a

sonication bath at room temperature, before refluxing the mixture for 4 hours. They obtained

purified nanotubes which do not show any distortion in the TEM images suggesting that little

oxidative damage had occurred on the walls as is usually observed with multi shell nanotubes.

Unfortunately no explanation is provided regarding the use of the ultrasounds in the first step.

The same year, Shelimov et al. [16] reported the ultrasonically assisted filtration of single

wall carbon nanotubes from soot obtained by a laser vaporization process. The sample is

dispersed in methanol and the ultrasounds generated by a horn inserted in the filtration funnel

help the filtration process by maintaining the particles in suspension, preventing the formation

of a ―cake‖ on the filter. Because the author is aware that sonication can lead to damage to the

walls, they refluxed without sonicating the purified single wall carbon nanotubes in nitric acid

for 18 hours. The TEM pictures show untangled bundles of the carbon nanotubes, with no

multi wall carbon nanotube or amorphous carbon. The authors suggest the first sonication

step created defects in the walls which were easily oxidized and cut during the nitric acid

treatment. However no experiment was carried out without the sonication to verify that

hypothesis. Also they report that the filtration process assisted by sonication is not enough to

cut the carbon nanotubes and that a longer sonication time with an acidic treatment is

necessary to seriously shorten the carbon nanotubes. Finally, only the outer nanotubes in the


single wall carbon nanotubes bundles are believed to incur damages, leaving inner carbon

nanotubes with little or no damage.

In 2002, Thien-Nga et al. [17] purified single wall carbon nanotubes by mixing the

impure particles with ZrO2 and CaCO3 in an ultrasonic bath. The ferromagnetic impurities

were removed from the nanoparticles and then trapped by magnets. In this case the

mechanical action of the ultrasounds provides an effective mixing of the slurry, while the

metal particles are being removed in what the authors called a ―snooker‖ process.

More recently in 2005, Li et al. [18] purified soot obtained from arc discharges samples,

which usually contain a significant amount of impurities along with single wall carbon

nanotubes (amorphous carbon, metallic catalyst and multi wall graphite nanoparticles). He

dispersed the sample by sonicating in an aqueous solution of sodium dodecyl sulfate (SDS)

for two hours. Further oxidation and acidic treatment were carried out. The overall process

led to a removal of 45% of the weight of the original sample. The ultrasounds helped detach

the impurities from the single wall carbon nanotubes, before the oxidative and acidic

processes removed the remaining impurities, as clearly seen in the scanning electron

microscopy (SEM) images, Figure 5a and 5b.

Figure 5: SEM images of (a) cloth-soot and (b) purified SWNT. Reprinted from [18], Copyright (2005),

with permission from Elsevier.

DISPERSIONS OF CARBON NANOTUBES UNDER

ULTRASOUNDS

Dispersion of carbon nanotubes obtained in neat organic solvents are first described, then

in aqueous solutions with surfactants and finally after acidic treatment.

Liu et al. [19] reported the stable dispersion of single wall carbon nanotubes by

sonicating them in N,N-dimethylformamide (DMF) or N-methyl pyrrolidinone (NMP) for 15

hours. AFM shows tube lengths of approximately 1m, evidence that the tubes were cut.

However, because the tubes underwent acidic treatment prior to sonication, it is hard to

determine when and how the cutting process occurred. AFM imaging also showed that the

tubes are not bundled any longer and are individualized, providing suspensions which

remained stable for several months. However, Ausman et al. [20] while reporting the effect of

various solvents on the stability of single wall carbon nanotubes disagree with the fact that

dispersions in DMF or NMP are stable suspensions for months but do aggregate within a few


days. Landi et al. [21] studied the effect of several alkyl amide solvents on the dispersion of

single wall carbon nanotubes using sonication as part of the dispersing process in N,N-

dimethylacetamide (DMA), N,N-dimethylpropanamide and N,N-diethylacetamide. UV-Vis

spectroscopy was used to assess the dispersion capability of each solvent, with DMA yielding

the largest absorbance and the most well resolved spectrum. They also reported that the

dispersions were stable only for days up to a week depending on the concentration. They

verified that DMA did not undergo any chemical change during the sonication process,

rationalizing the dispersion ability of the amides as a combination of stacking, high

polarity and appropriate geometry.

Niyogi et al. [22] dispersed single wall carbon nanotubes in o-dichlorobenzene and DMF

at ambient temperatures using a sonication bath. During sonication, the carbon nanotubes

undergo degradation and are coated with a polymer, resulting from the decomposition and

polymerization of o-dichlorobenzene. DMF did not undergo any chemical change during this

treatment and the carbon nanotubes show no sign of degradation in the absence of o-

dichlorobenzene. When o-dichlorobenzene is sonicated prior to the addition of the carbon

nanotubes an insoluble polymer is generated that cannot disperse the carbon nanotubes.

Similarly, if as little as 1% ethanol is used to inhibit the polymerization, no stable dispersion

is obtained. It is concluded that the process creates a polymer that adheres to the surface of

the nanotubes in an irreversible process. Although the exact mechanism is not known, it is

possible that the polymerization reaction is terminated by the presence of the carbon

nanotubes, with the polymers immobilized on them.

Ganter et al. [23] led another study on the dispersion of single wall carbon nanotubes in

various organic solvents: DMA, TMMA (N,N,N,N-tetramethylmalonamide), o-

dichlorobenzene and 1-chloronaphtalene. He monitored the effect of ultrasounds on the single

wall carbon nanotubes extinction coefficients using optical absorption spectroscopy. He

attributed a decrease of the single wall carbon nanotubes extinction coefficients with the

chlorinated solvents to the formation of a sonopolymer as Niyogi [22] described with o-

dichlorobenzene. The decrease observed with TMMA as a solvent was attributed to a

preferred orientation of the solvent molecules along the axial direction of the carbon

nanotubes, leading to a greater dielectric screening than with DMA. When samples were

sonicated for longer periods of time, the spectra obtained with the carbon nanotube dispersed

in alkyl amides did not show any change. However the formation of a sonopolymer from both

chlorinated solvents was shown. Sonication of the solvent without carbon nanotube led to

polymer formation as well, confirming Nioygi‘s results. Stirring the mixtures for 3 days in the

absence of sonication did not lead to a stable dispersion. This result provides further evidence

that the formation of the sonopolymer in situ is necessary to obtain a stable dispersion.

Finally Raman spectroscopy data showed that the single wall carbon nanotubes were only

minimally damaged during the dispersion process with the alkyl amides, making this family

of solvents appropriate for future work. Similar results were obtained when single wall carbon

nanotubes were dispersed in N-methylpyrrolidinone (NMP) using ultrasounds [24]. No

explanation is provided as to the efficacy of this solvent that led to the formation of stable

dispersions over several days. Raman spectroscopy was used to show that no damage was

detected on the tubes during the dispersion process. Kim et al. used ultrasounds when

dispersing single wall carbon nanotubes in NMP with the presence of a thiophene based

oligomer [25]. Although the study uses ultrasounds, their direct effect was not studied. N,N-

dimethylacetamide (DMAc) was used a solvent in the dispersion of single wall carbon


nanotubes with added polyimides as dispersants by Delozier et al. [26]. The sample was

sonicated before and after addition of the polyimides. Sonicating before the addition of the

polyimide led to suspensions where single wall carbon nanotubes chunks were seen as

suspended agglomerates. Only 3 of the polyimides tested would lead to a homogenous

suspension without any visible agglomerates, these were obtained from the polymerization of

2,7-diamino-9,9‘-dioctylfluorene with either 3,3‘,4,4‘-oxydiphtalic anhydride, or 3,3‘,4,4‘-

biphenyltetracarboxylic dianhydride or pyromellitic dianhydride. The results are explained by

the ability of these specific polyimides to adopt the right geometry and wrap around the single

wall carbon nanotubes to prevent their re-aggregation.

Chen et al. used ultrasounds for the synthesis of polymer grafted single wall carbon

nanotubes [27]. By sonicating methyl methacrylate (MMA) and single wall carbon nanotubes,

the authors acknowledge a sonochemical effect as follows: MMA monomers are thermally

decomposed during the cavitation into radicals while defects are created on the nanotubes,

leading after collision of the radicals and the nanotubes to the polymerization of MMA on the

tube surface. These PolyMMA-grafted single wall carbon nanotubes are soluble upon mild

sonication in traditional organic solvents such as THF, toluene, chloroform, dichloromethane.

Li et al. [28] studied the use of a horn sonication for the grafting of multi wall carbon

nanotubes using polyvinyl pyrrolidinone, a water-soluble polymer. IR spectra show the

grafted polymer on the tubes. Variation of the sonication time allows control over the

molecular weight of the polymer grafted onto the tubes. Molar mass of the polymer decreases

rapidly at first and levels off a minimum mass when sonication was performed from 5 to 80

minutes. It is believed that an increase in sonication time degrades the polymer grafted onto

the tube. The grafting mechanism is reported to be a radical one, where radicals are formed

during sonication and are trapped by the tubes. They also monitored the disentanglement of

the tubes as well as their shortening during the process by TEM. The length of the grafted

tubes is shorter than the ungrafted ones, showing the ability of the ultrasounds to cut the

tubes. Also seen in the TEM pictures, very few open tubes are found on the pristine nanotubes

whereas many open end tubes can be found on the grafted ones, again supporting the cutting

(Figure 6). The polymer is believed not to be wrapped around the tubes through van der

Waals forces but to be covalently linked, as dispersions of the pre-formed polymer with the

tubes did not lead to stable dispersions. This technique provided stable dispersions thanks to

the grafting of a water-soluble polymer and also to the shortening of the tubes.

Giordani et al. [24] sonicated single wall carbon nanotubes in NMP with a tip sonicator

for 2 minutes and then in a bath for 4 hours, and again with a tip for one minute. Dilutions

and centrifugations followed to reach a final concentration decreasing from 0.125 to 0.04

mg/L. The suspensions are stable for at least 17 days. AFM was used to monitor the size of

the bundle overtime showing no re-aggregation during this period in agreement with the lack

of sedimentation observed by absorbance at 650 nm. Raman spectroscopic data shows that no

damage was done to the nanotubes during the sonication process as well as the presence of

individual nanotubes. This paper reinforces the previously published data that amides are

good solvents to achieve stable suspensions without any surfactant.

Single wall carbon nanotubes were efficiently dispersed in ethylene glycol under horn

sonication by Amrollahi et al. [29]. The settling time was studied as a function of sonication

duration, showing a correlation of longer sonication time to the longer settling time. The

authors suggest that debundling of the nanotubes from closely packed to loosely packed to

individual particles as the explanation for this observation which was corroborated by the


TEM images. Similarly the thermal conductivity of the solution increased with longer

sonication time. The authors were able to correlate an increase in thermal conductivity to the

loosening of the carbon nanotubes clusters as well.

Figure 6. TEM images of (a), (b) original MWCNTs and (c), (d), (e), (f) PVP grafted MWCNTs.

Reprinted from [28], Copyright (2009), with permission from Elsevier.

The use of gum Arabic by Bandyopadhyaya et al. proved to be effective for the

dispersion of an aqueous solution of entangled ropes of single wall carbon nanotubes under

mild oxidation and sonication conditions [30]. The Arabic gum, a water soluble

polysaccharide used as a surfactant, was adsorbed onto the carbon nanotubes, which disrupted

the inter-tube interactions. The carbon nanotubes wrapped by the gum Arabic were

debundled, as their polymer chains tend to repulse each other under given solvent conditions.

Unfortunately the authors do not explain the effect of the ultrasounds in the process.

During a study of the fluorescence of single wall carbon nanotubes dispersions in

aqueous solution with sodium dodecyl sulfate (SDS) as an ionic surfactant in a flow-through

ultrasonication unit, Strano et al. [31] suggested that the ultrasonic process creates gaps at the

ends of the carbon nanotube bundles, which become available for interaction with the

surfactant. Surfactant adsorption and diffusion propagates these gaps in an ―unzippering‖

process. The equilibrium between the individualized nanotubes and the bundles is reversible

and stable and depends on the SDS concentration. Raman spectroscopy data showed no

increase in the D-band, indicating few or no defects were introduced in the carbon nanotubes.

This eliminates not only the possibility of damage from the sonication process but also of a

covalent interaction between the surfactant and the nanotubes. Grossiord et al. [32] also

studied the ―unzippering‖ process of single wall carbon nanotubes in an aqueous solution of

SDS. Using HiPCO nanotubes, 130 minutes of sonication with the horn are needed to

completely exfoliate the carbon nanotubes, leaving only individual carbon nanotubes as seen

by cryo-TEM. These results were also corroborated in the UV-Vis spectroscopy study, which

showed the maximum absorbance of the solution corresponds to the maximum exfoliation of

the carbon nanotubes observed by cryo-TEM. Finally the authors warned of the effect of the


mode of synthesis of carbon nanotubes. Because van der Waals interactions are lessened by

impurities, carbon nanotubes with less impurity take longer to debundle. Monitoring the

debundling process by UV-Vis spectroscopy allows one to stop the sonication when the

maximum absorbance possible has been reached as unnecessary damage could occur to the

carbon nanotubes beyond that point.

Paredes et al. [33] reported the combined use of tip and bath sonication of single wall

carbon nanotubes in an aqueous solution of sodium dodecyl benzene sulfonate (SDBS).

SDBS allows the dispersion of the tubes while minimizing the damage to the walls and their

shortening. Tip sonication was performed first and after centrifugation, bath sonication was

performed for a few minutes to several hours. After the tip sonication, AFM images showed

individual nanotubes (50% of all objects) and bundles. The bath sonication that followed led

to individualized tubes (80%). The tube lengths had a mean value of 820 (± 556) nm and then

770 (±572) nm after tip and bath sonication respectively. The mild sonication bath treatment

allows for the debundling without tube cutting because the suspension is already made of

well-dispersed bundles and not entangled bundles of tubes. UV-Vis data were also collected

to monitor the sedimentation rate and showed that little re-aggregation took place over the

course of several months.

Geckeler et al. [34] studied the effect of lysozyme as a surfactant on the debundling of

single wall carbon nanotubes. The dispersion obtained after sonication with a probe for 30

minutes was reported stable for 9 months. The ultrasounds debundled the carbon nanotubes

by overcoming van der Waals attractions, which provided the opportunity for the lysozyme to

adhere to the carbon nanotubes, making them well dispersed in the solution. The dispersion is

explained by the repulsive forces between the positive charges of the protein at that pH (6.5),

whereas increasing the pH from 8.5 to 11 provoked the aggregation of the carbon nanotubes.

However a higher pH (>11) produced a dispersion again due to the ionization of the

carboxylic acids into carboxylates, leading to electrostatic repulsive forces again.

Bottini et al. [35] used mercaptopropyltrimethoxysilane as a surfactant for the dispersion

of aqueous solutions of single wall carbon nanotubes obtained after 30 minutes of sonication

at room temperature. They concluded that mild sonication alone was not sufficient to disperse

the tubes in solution and did not introduce damage to the carbon nanotubes but was necessary

for the surfactant to adhere to the carbon nanotubes surfaces.

Gladchenko et al. [36] studied the sonication of single wall carbon nanotubes with DNA

buffered at pH 7 for 30 minutes using a tip sonicator. The dispersions were stable for months.

In order to estimate the damage done by the ultrasounds, they performed gel electrophoresis.

The DNA was fragmented down to 2.3 x 103 base pairs as early as within the first 4 minutes

of sonication. By the end of the 30 minutes, the DNA fragments length was less than 500 base

pairs. The DNA was more sensitive to degradation that the nanotubes, yielding to single and

double strand DNA fragments. The authors report the wrapping of untwisted single strand of

DNA onto the carbon nanotubes during the sonication process, which explains the stability of

the suspension over months.

Ciofani et al. [37] dispersed multi wall carbon nanotubes using Pluronic F127

(polyoxyethylene-polyoxypropylene co-polymer, Figure 7) as a non-ionic surfactant in a bath

sonicator.


O

O

O

OHH

a ab

Figure 7. Pluronic F127.

The process was monitored using UV-Vis spectroscopy in the 200-1200 nm region, since

van Hove singularities are observed for individual nanotubes but not for bundles. The authors

used the ―unzippering‖ model, although the surfactant did not participate in electrostatic

repulsions as previously described, but steric repulsions since it is not ionic. It took 2 hours of

sonication to achieve a plateau corresponding to the maximum absorbance and therefore

maximum degree of dispersion achievable under these conditions. Increasing the

concentration of Pluronic F127 does not increase the maximum degree of dispersion by much

(from 70 to 90 µg/mL ranging Pluronic concentration from 0.05% to 0.5%). In addition to the

shearing effect of the ultrasounds which breaks the bundles and allows the surfactant to

adhere to the walls, the higher temperatures also improved the dispersion due to increased

enthalpic energy. The suspensions were stable for many weeks after preparation. When the

tubes were heated at 70oC for up to 8 hours and then sonicated in the same conditions as

described above, a significant increase in the dispersion was observed. The heating and the

stirring pre-treatment did not really accelerate the reaction but were important for the

debundling of the agglomerates that could not be broken under the effects of ultrasounds

only. As a result, heating and sonicating doubled the concentration of the dispersion

compared to sonicating only.

Lopez-Pastor et al. [38] reported the sonication of single wall carbon nanotubes in an

ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate) and subsequent analysis by

capillary electrophoresis. When the nanotubes were mixed in the ionic liquids, aggregates

were observed under optical microscope, showing the need for ultrasounds for dispersing.

Under the conditions of the experiments, (power density varying from 46 to 460 W/cm2, tip

sonicator, used with a cycle for 2 to 40 minutes), Raman spectroscopy revealed an increase

G/D ratio, with the D band corresponding to sp3-hybridized carbons present as impurities and

single wall carbon nanotubes defects. This shows the carbon nanotubes have been purified

during the process, but when harsher conditions (higher power density) are used, single wall

carbon nanotubes degradation was observed. The authors concluded that a power density of

63 W/cm2 for 5 minutes was enough to obtain appropriate ionic liquid-single wall carbon

nanotubes dispersions. Further experiments involved adding an aqueous solution of SDS to

the dispersions, which dissolves the ionic liquid whereas sonication ensures the efficient

encapsulation of the nanotubes in micelles.

Kumar et al. [39] sonicated in a bath single wall carbon nanotubes in nitric acid for 2

hours at room temperature before refluxing the dispersion for an additional 2 hours. IR

spectroscopy analysis showed the presence of carboxylate groups due the oxidative action of

the nitric acid. Less degradation was observed in this study compared to their previous study

[40], although the oxidative conditions seem harsher in the second study. The most damage

observed in the first study could be explained by the amount of residual catalyst from the

carbon nanotubes synthesis (HiPco) being less, 2% wt versus 35% wt, using less nitric acid


and leaving more nitric acid to react with the nanotubes. The authors studied the effect of

sonication on the carbon nanotubes and found limited side-wall damage under these

conditions. The nitric acid treated nanotubes were dispersed easily in ethanol or butanol (or

their mixtures with toluene or xylene) due to the hydrogen bonding of the carboxylic acid

groups with the hydroxyl groups of the alcohols. The pristine nanotubes were soluble in

toluene and xylene but not in the alcohols or their mixtures. The presence of carboxylic acids

groups on acid-treated carbon nanotubes, as well as carbonyl and hydroxyl groups has been

confirmed by Yue-Feng et al. [41] when studying the influence of an electric field on the

dispersion of carbon nanotubes. An aqueous solution of original carbon nanotubes and acid

treated carbon nanotubes (with a mixture of sulfuric and nitric acids) was sonicated and

submitted to a 25V DC electric field. SEM images showed that the better dispersions were

obtained when both sonication and an electric current were applied on the acid treated

nanotubes. This effect was not observed on the original nanotubes. IR spectroscopy revealed

the presence of the oxygenated groups mentioned above which are necessary for an effective

dispersion regardless of the use of external devices such as ultrasounds and an electric field.

In his study on the analysis of nanocomposites of multi wall carbon nanotubes and

polycarbonate, Li et al. [42] boiled multi wall carbon nanotubes in a mixture of nitric and

sulfuric acid before dispersing them in water using an ultrasonic bath. The SEM images

showed that the nanotubes were fragmented and stacked loosely without any compact

agglomerates. The authors suggest that the behavior of the acid treated multi wall carbon

nanotubes is due to their many defects, making them easily broken.

Kim et al. [43] refluxed multi wall carbon nanotubes with sulfuric and nitric acid at 60oC

for 24 hours before sonicating for 8 hours in alcohols. The dispersions were monitored by IR

spectroscopy and using a turbiscan. IR data showed the presence of phenols groups on the

pristine nanotubes and the introduction of carboxylic acid groups after treatment. The length

of the tubes observed after treatment decreased by a factor of 4 when compared to the

untreated tubes. After 120 hours, no precipitate appeared in the vials, whereas the non acid-

treated carbon nanotubes precipitated rapidly in ethanol. The stability of the carbon nanotubes

came from the oxygen groups introduced on the tubes, forming interactions with methanol,

ethanol and iso-propanol.

Nadler et al. [44] compared the effect of bath ultrasounds for up to 16 hours on aqueous

solutions of pristine multi wall, single wall and hydroxylated multi wall carbon nanotubes and

SDS. TEM pictures were taken and the disc centrifuge method was used to monitor particle

sizes. Regardless of the nanotubes studied, the size analysis showed a bimodal distribution,

where the mass fraction above 0.1 µm decreased until a mono-modal distribution was reached

after 16 hours (Figure 8).

The density function maximum decreased from 1.8 µm for one minute of sonication to

300 nm at 4 hours. The maximum density function below 0.1 µm remained at an equivalent

spherical diameter of 30-40 nm. The fraction below 0.1 µm consisted of exfoliated nanotubes

whereas the fraction above represented the agglomerates. When the authors compared the

results of sonication with the results of ball milling, ultrasounds were the most efficient at

completely achieving the debundling of the nanotubes, although some individual breakage is

to be expected.


0.8

0.6

0.4

0.2

0.0

-0.210.10.01 d [µm]

q3(d)

[ - ]1min5min15min30min1h2h4h8h16h

Figure 8: q3(d) particle size density function by mass (area normalised) of Baytubes in aqueous

dispersion sonicated for different processing times. Reprinted from [44], Copyright (2008), with


Margrave et al. [45] reported the efficient dispersion of fluorinated single wall carbon

nanotubes in alcohol obtained by bath sonicating for 10 minutes. The solutions were reported

to be metastable for up to a week, depending on the solvents tested: methanol, ethanol, 2,2,2-

trifluoroethanol, 2-propanol, 2-butanol, n-pentanol, n-hexanol, cyclohexanol and n-heptanol,

with 2-propanol and 2-butanol as the best solvents tested. However sonication could not

achieve the solubilization of the fluorinated nanotubes in water, diethylamine, acetic acid or

chloroform, although chloroform somewhat solvated the tubes. Analysis of the fluorinated

tubes after 2 hours of sonication showed a loss of fluorine but they remained solvated. On the

contrary, when C4.4F(OCH3)0.25 were obtained after reaction of the fluorinated nanotubes with

sodium methoxide under ultrasounds, the tubes precipitated out of solution.

ULTRASOUND-INDUCED DAMAGE ON CARBON

NANOTUBES

The first article on possible damages done by ultrasounds on carbon nanotubes was

published by Lu et al. in 1996 [46]. Sonication with a horn in dichloromethane at 0oC for up

to 20 minutes introduced defects revealed by TEM imaging. They reported a thinning of the

outer graphitic layers similar to ones observed during oxidation processes. The extent of the

damages to the nanoparticles depended on the solvent with lesser effects observed in water or

ethanol. Raman spectroscopy was used to assess the damage by looking at the G band and the

D band, respectively 1585 cm-1

and 1286 cm-1

, with the ratio of the two bands corresponding

to the amount of defects observed by TEM. In addition, they monitored the broadening of the

peak g = 2000 in ESR as a tool to assess the increase in defects. The broadening could be

linked to the disturbance in the graphene layers and to the change in the conduction band


energy levels. The appearance of a peak at g = 1991, whose intensity increased with

sonication time, may come from dangling bonds. It is concluded that prolonged sonication

leads to the formation of amorphous carbon from the nanotubes and nanoparticles.

When Hilding et al. [47] monitored tube lengths with SEM during the sonication of multi

wall carbon nanotubes in toluene in an ultrasonic bath, a shortening by as much as 65%

during the first five minutes from 50 to 16 µm was observed. An additional 20 minutes were

needed to cut the tubes down to 6.5 µm, because shorter tubes are harder to break than longer

ones. Similar results were obtained when the same experiments were repeated with a

sonication horn.

Liu et al. [48] formed fullerene pipes by sonication of single wall carbon nanotubes in a

mixture of nitric and sulfuric acids at 40oC. The combination of the ultrasounds and oxidation

led to the cutting of the single wall carbon nanotubes ropes into thinned ropes and individual

nanotubes. Liu et al. in a later article [49] improved the oxidation conditions to cut and purify

single wall carbon nanotubes in three steps: nitric acid refluxing, followed by sonication in

the presence first of Triton at pH 10 and then of sulfuric acid and (NH4)2S2O8 (to form a very

strong oxidizer H2SO5), then a high temperature treatment with ammonia, known to recover

the single wall carbon nanotubes structure. The nitric acid treatment did not lead to any

noticeable length shortening but some wall damages. The ultrasounds in combination with the

in-situ generated H2SO5 led to significant damages through tube shortening, with a uniform

final length of 1 µm (the beginning length was several to several hundreds of µm). This

purification technique allows for the removal of undesirable carbon nanoparticles such as

multi wall carbon nanotubes which have more wall defects and therefore are more sensitive to

these harsh conditions and can be removed completely from the sample.

Monthioux et al. [50] reported the damaging effect of ultrasounds on acid pre-treated

single wall carbon nanotubes suspended in DMF using TEM. Single wall carbon nanotubes

were unexpectedly found to be sensitive to DMF. Defects in the tubes prior to sonication in

DMF create side-openings in the walls, enabling DMF to interact with the tubes and possibly

increase the amount of damage. However the authors were able to remove the wall defects by

thermal annealing at high temperature. Similarly, Furtado et al. [51] studied the effect of

ultrasounds on the single wall carbon nanotubes during their debundling in DMF. They

showed that the damage is due to the experimental conditions of acid treatment that the tubes

undergo prior to sonication in DMF. They found no evidence that ultrasounds created more

defects. The sonication was carried out for 4 hours with either DMF or NMP. When treating

the single wall carbon nanotubes with HCl or HNO3, the ones treated with HNO3 always

showed more defects in Raman spectroscopy, with carboxylic groups found on the wall

surface. Defects were also detected even with diluted nitric acid. The authors showed that the

acid treated single wall carbon nanotubes could form stable dispersions in DMF or NMP with

sonication for weeks, whereas skipping the acid treatment could not produce a stable

dispersion. This was explained by a weak charge transfer between the DMF and the

nanotubes and a reduction of van der Waals interactions within the bundles due to the acid

treatment. Smaller diameter bundles were obtained in the HNO3/DMF combination compared

to HCl/DMF or HCl/NMP. Tubes are functionalized by the nitric acid process with functional

groups such as COOH that are better able to interact with DMF and in turn lead to better

dispersions and smaller bundles. Raman spectroscopy also shows more defects in that case,

due to the introduction of the oxidized groups. When the tubes were annealed, they did not

disperse as well in the amide solvents. Finally the authors reported that the extent of the


length cutting depends on the oxidative process used in the acid treatment, which creates wall

defects prone to subsequent ultrasound cutting.

Koshio et al. [52], [53] observed the formation of ragged single wall carbon nanotubes

produced by sonication in a chlorobenzene solution of polymethylmethacrylate (Figure 9) in

an ultrasonic bath and then with a sonication tip.

C

C OCH3

O

CH3

CH2

n

n=5000

Figure 9: Polymethylmethacrylate.

After heating in a furnace to remove all carbon impurities like chlorobenzene, amorphous

carbon, fullerene and carbon nanocapsules, SEM images were taken (Figure 10).

Figure 10. High magnification TEM of r-SWCNT. Reprinted from [52], Copyright (2001), with


The single wall carbon nanotubes were short, appeared worm-eaten and were named

ragged single wall carbon nanotubes. They displayed an unusual thermal stability, with

degradation starting above 800oC. During the ultrasound treatment, the ragged tubes had

reactive dangling bonds ready to react with the fragments obtained from the degradation of

PMMA. This process started at defectives sites on the tubes and was propagated along the

walls during the process. Roughly 1 to 3 defect sites per 1 nm length were estimated. Later

on, Koshio et al. [54] used ragged single wall carbon nanotubes to synthesize fullerenes upon

pyrolysis. When the ragged single wall carbon nanotubes were heated in a sealed tube at

1200oC none remained, only leaving a deposit. Solubilization of the deposit in toluene gave a


purple solution, which after mass spectrometry analysis revealed the presence of fullerene C60

and C70 and traces of some higher fullerenes such as C76, C78 and C84. The amount of C70 was

about 30% of C60. When a similar experiment was performed without ultrasound treatment of

the nanotubes, fullerenes were not detected after heat treatment. The authors concluded that

defects in the ragged single wall carbon nanotubes make them susceptible to decomposition

and the material easily recomposes into C2 fragments during pyrolysis, eventually leading to

the formation of fullerenes.

The study of single wall carbon nanotubes in chlorobenzene by Koshio [52], [54] led

Zhang et al. [55] to perform TEM and thermogravimetric analysis (TGA) on samples coming

from the sonication of the acid-treated single wall carbon nanotubes in chlorobenzene. Three

kinds of single wall carbon nanotubes were detected: an amorphous-like material on both the

inside and outside of the walls, one with similar residue inside the walls and the third one did

not show any amorphous-like material. IR spectroscopy revealed the amorphous like material

consisted of hydrocarbons from the degradation of chlorobenzene. The exact nature of the

interaction between the single wall carbon nanotubes and the amorphous-like material was

not discussed.

Heller et al. [56] reported an effective method for the cutting of single wall carbon

nanotubes. Sonication of the tubes in a sodium cholate hydrate with a horn sonicator was

followed by centrifugation, additional sonication and gel electrophoresis. The samples

showed a systematic increase in the migration of the particles with the sonication duration.

Since gel electrophoreses migration is based on the particles length the shortest nanotubes

migrate faster than longer ones and the results showed that sonication cut the nanotubes in

proportion of the sonication time. Similar results were obtained when other ionic surfactants

were used (SDS and Triton X-100), showing that the cutting was independent of the

surfactant used. Raman spectroscopy was also used to determine the diameter of the

nanotubes. The results showed not only that the length was cut but also the diameter of the

bundles was diminished in the process. The cutting process is diameter selective and the

mechanism of the diameter-dependent cutting process is still under investigation. This work

could eventually lead to a process were nanotubes of specific length and diameter could be

prepared and isolated in a controlled way. Arnold et al. [57] suspended single wall carbon

nanotubes in sodium cholate as well with a sonicator tip and monitored the samples with

AFM, IR and UV-Vis spectroscopy. They postulated that the cavitation process cuts the tubes

into smaller parts and that there is a critical length below which tubes cannot be cut any

longer. They reached that point in their experiment after 40 minutes of sonication for a

median nanotubes length of 200 nm.

Wang et al. [58] used Pluronic P123 (Figure 11), an amphiphilic triblock copolymer:

poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) as a dispersant for the

preparation of stable single wall carbon nanotubes dispersions.

CH2 CH2 CH2 CH2CH2O O OCH

CH3

n np

Figure 11. Pluronic P123.


The authors treated the tubes under very oxidative conditions with sonication and

monitored the results using electron microscopy (Figure 12). They used a ―piranha‖ solution

which consists of concentrated sulfuric acid and 30% hydrogen peroxide. They attributed the

damages and cutting of the tubes to these harsh oxidative conditions, which are known to

attack defective sites of the nanotubes that are created by the ultrasounds.

Figure 12. TEM images of (a) the as-produced SWNTs dispersed in EtOH; (b) the piranha-treated

SWNTs dispersed in DMF; (c) the NH3-treated SWNTs dispersed in water. HRTEM image of (d) the

NH3-treated SWNTs dispersed in water. Reprinted from [58], Copyright (2006), with permission from

Elsevier.

Multi wall carbon nanotubes were shortened efficiently by sonication in DMF in a

sonication bath for various time lengths by Park et al. [59]. This process was compared to the

oxidative action of a mixture of sulfuric and nitric acids on nanotubes. The mean length of the

sonicated nanotubes was shorter than the acid treated ones, but the length distribution was

narrower in the previous case. Although more cutting was reported with the ultrasounds, less

wall damage was simultaneously observed.

The use of phosphomolybdic acid is reported by Warakulwit et al. [60] to cut multi wall

carbon nanotubes. TEM was used to monitor the length shortening which was dependent on

the sonication duration. The mild oxidative conditions allowed the discrimination between the

various carbon particles, oxidizing the ones with more defects at a faster rate than the ones

with less. This could not only limit damage to the nanotubes but also induce cutting in a

controlled manner. Also in search for mild oxidation conditions for multi wall carbon

nanotubes, Liu et al. [61] oxidized multi wall carbon nanotubes with a combination of

ultrasounds and ammonium persulfate. TEM indicated the shortening of the tubes into a

bimodal distribution: at 370 nm and 930 nm, respectively with thin and thick diameters,

making the tubes well dispersed in water. IR spectroscopy results indicated the introduction

of carboxylic acid groups on the surface as noted in previous reports, increasing the level of

dispersion. Luong et al. [62] tested the effect of another mild oxidant, ceric sulfate on single

and multi wall carbon nanotubes in the presence of ultrasounds. Although the treatment lasted


for only 2 to 5 hours with a 0.1 N of cerium sulfate, considerable structural damage was

observed on the single wall carbon nanotubes after only 15 minutes of sonication. Length

shortening of the nanotubes and the appearance of amorphous carbon was observed on the

TEM images. After 5 hours of treatment no tubes could be found. By comparison, nanotubes

sonicated for 5 hours in DMF showed no evidence of damage. Similar results were obtained

when multi wall carbon nanotubes were sonicated with ceric sulfate. The thinning of the tubes

exposed thin inner tubes to sonication and induced further damage. Tubes were degraded into

graphite or even amorphous carbon. Comparative sonication in DMF did not lead to any

damage on the multi wall carbon nanotubes, but sonication for 24 hours in ethanol resulted in

carbon fibers. XPS was used to assess the chemical oxidation. Carbonyl stretching was

observed and some mild defects were found on the tubes sonicated in DMF. Raman

spectroscopy also confirmed that no significant damage was introduced under these

conditions. However a complete disappearance of the radial breathing modes at 150-300 cm-1

,

a decrease of the tangential G-band at 1580 cm-1

and an increase in the disorder induced D-

band at 1325 cm-1

, corroborated the results of the TEM imaging. In conclusion single wall

carbon nanotubes were converted into amorphous non conducting carbon nanoparticles,

whereas multi wall carbon nanotubes were converted into graphitic or amorphous carbon

nanoparticles that retained some conductivity.

SONOCHEMICAL FUNCTIONALIZATION OF CARBON

NANOTUBES

Several articles reported the use of ultrasounds prior to the chemical functionalization.

Chen et al. [63] used ultrasounds to create an emulsion between water and chloroform

containing single wall carbon nanotubes and triethylbenzylammonium chloride as a phase

transfer catalyst, prior to adding the sodium hydroxide for the cyclopropanation reaction.

Although the reaction was successful, it could not be attributed to the ultrasounds which were

stopped when the base was added. Similarly, Saini et al. [64] reacted fluorinated single wall

carbon nanotubes with alkylithium with sonication as part of the procedure. However, even

though the substitution reaction was successful, there was no discussion of the role of the

ultrasounds in the study. Kovtyukhova et al. [65] performed the oxidation of single wall

carbon nanotubes with ultrasounds and an acidic treatment as well as the use of potassium

permanganate. TEM images showed the presence of nanotubes decorated with fragments of

oxidized and broken tubes. Raman and IR spectroscopy results were consistent with the

presence of carboxylic acid, carbonyl and hydroxyl functional groups, however the role of the

oxidative action of the ultrasounds was not assessed. Chen et al. [66] reacted a carboxylated

single wall carbon nanotubes obtained through acid treatment and ultrasounds with

octadecylamine, forming the corresponding octadecylammonium single wall carbon

nanotubes-carboxylate zwitterion. Once again, no effect or study on the use of ultrasounds

was indicated.

A number of articles reported the use of ultrasounds and their effect on the chemical

functionalization of the carbon nanotubes. Kaempgen et al. [67] studied the conductivity of

single wall carbon nanotubes as a function of the treatment with nitric acid and ultrasounds


using a tip sonicator. Carbonyl, hydroxyl, carboxylic acid groups were observed by IR

spectroscopy. A sonication time of 3 minutes provided maximum conductivity which then

dropped below the initial value. This was explained by a trade-off between doping of semi-

conducting single wall carbon nanotubes and scattering of the charge carriers when the

functional groups were introduced: doping took place during the first 3 minutes followed by a

breakdown of the crystal structure. Longer reaction times, up to 60 minutes, led only to

increased degradation and formation of amorphous carbon as monitored by AFM imaging.

Zhang et al. [68] functionalized single wall carbon nanotubes using ultrasounds. They

reported the need to use ultrasounds in order to obtain a good dispersion, leaving individual

nanotubes ready to react. Ultrasounds were also needed to initiate the reaction between the

nanotubes and SU-8. SU-8 is a polymeric epoxy resin dissolved in an organic solvent along

with a photoacid generator. The epoxy resin shown in Figure 13 consists of repeating novolac

glycidyl ether units. The oxidation process of the single wall carbon nanotubes with

potassium permanganate was not conducted using ultrasounds, however the initial mixture

was sonicated for mixing purposes only. The subsequent reaction between the hydroxylated

nanotubes and SU-8 was sonicated. The authors reported the need of acidic conditions and

sonication for the reaction to take place, without providing more explanation.

O

CH2

HC

O

H2C

CH2

CH3C CH3

O

CH2

HC

H2C

O

O

CH2

HC

O

H2C

CH2

CH3C CH3

O

CH2

HC

H2C

O

O

CH2

HC

O

H2C

CH2

CH3C CH3

O

CH2

HC

H2C

O

O

CH2

HC

O

H2C

CH3C CH3

O

CH2

HC

H2C

O

Figure 13. Epoxy resin SU-8, based novolac glycidyl ether groups.

Ren et al. [69] deposited metallic nanoparticles on multi wall carbon nanotubes by

sonicating and drying a suspension of carbon nanotubes and Pt, Pd or CoPt3. TEM pictures

showed clearly the metallic nanoparticles on the outer surface of the nanotubes. The

ultrasounds ensured a good dispersion of the carbon nanotubes and the nanoparticles, which

stayed in a well dispersed state during the evaporation process. The authors also noted that

any surfactant remaining from the synthesis of the carbon nanotubes should be removed


because they prevented the metallic nanoparticles from being uniformly deposited on the

nanotubes.

The deposition of metallic particles on multi wall carbon nanotubes has also been

realized by Qiu et al. [70]. Acid treated tubes were sonicated in the presence of tin (II)

chloride (SnCl2) using a tip sonicator. Nanosize particles of tin were obtained and most of the

carbon nanotubes were decorated with these crystalline tin particles, without any SnCl2 or

SnO present. Drying and sonicating the decorated carbon nanotubes did not change their

morphology as seen in the TEM micrographs, showing the strength of the bonds between the

tin particles and the carbon nanotubes. However there was no evidence of the presence of tin

inside the carbon nanotubes, either because the size of the tin and carbon nanotubes were not

compatible for such an interaction or the open end of the tubes was blocked by a tin particle.

The authors applied this technique using iron pentacarbonyl Fe(CO)5 instead of SnCl2 and

found multi wall carbon nanotubes decorated with iron nanoparticles, although amorphous in

nature. This evidence suggests a clear mechanistic difference between the formation of

crystalline nanoparticles of tin and iron.

Xing [71] used a sonochemical process for the acid treatment of carbon nanotubes before

platinum deposition. Multi wall carbon nanotubes were sonicated in a bath with sulfuric and

nitric acids at 60oC for 2 hours. The deposition of uniformly dispersed nanoparticles on

sonochemically treated nanotubes was due to the uniformity of the functionalization during

the sonication, since it prevents the tubes from forming aggregates. When the same

experiment was conducted without ultrasounds, the platinum nanoparticles were bigger in

size and not as well dispersed. Moreover ultrasounds allowed for a higher loading of Pt

nanoparticles on the tubes as it produced more functional groups able to interact with the Pt.

The authors did not describe the nature of these groups being formed. Xing et al. [72]

reinvestigated the process of sonochemical oxidation of multi wall carbon nanotubes and

identified the functional groups present on the oxidized nanotubes as a function of sonication

duration. Progressive appearance of carbonyl and C-O groups and then COO groups are

indicated by XPS results, with maximum oxidation reached in 4 hours. Pt nanoparticle

binding on functionalized multi wall carbon nanotubes was performed by Hull et al. [73] by

pre-treating the tubes with nitric and sulfuric acids and sonicating for 2 hours. Increasing the

sonication time to 4 hours did not result in better results as significant damages had occurred.

However one hour of sonication was found not to be enough because the carbon nanotubes

surfaces did not appear to be sufficiently functionalized for a stable platinum-tube interaction.

IR spectroscopy data showed the presence of the carbonyl band and a C-O group.

Sonochemical oxidation produced hydroxyl, carbonyl, C-O-C and –COO- groups with little or

no COOH present. Platinum will bind to these groups according to the two possible structures

shown below in Figure 14.

Worsley et al. [74] published a comparative study of stirring and sonicating during the

functionalization of carbon nanotubes using a Bingel reaction. The procedure consisted of:

sonicating/or stirring single wall carbon nanotubes in o-dichlorobenzene, 1,8-

diazabicyclo[5.4.0]undec-7-ene (DBU) and diethylbromomalonate for 19 hours. Scanning

tunneling microscopy (STM) images showed that the stirred reaction did not produce

debundled carbon nanotubes, leading to varying degrees of functionalization on the tubes

throughout the bundles. Sonicating for 19 hours produced a very high degree of

functionalization. This was explained by the fact that sonication increased the energy of the

reactants, hence increasing the probability of reaction between the tube and the malonate. In


addition, sonication can debundle the tubes leading to a more uniform functionalization. The

authors did not exclude the possible presence of a sonopolymer derived from o-

dichlorobenzene as previously reported [22], yet they did not detect any trace of the

sonopolymer. This was explained by the washing of the samples with dimethylsulfoxide,

probably washing away the sonopolymer as well.

C C

O

O

Pt

C

O O

Pt

a b

Figure 14. Pt-coordination to the functional groups.

Yang et al. [75] dispersed multi wall carbon nanotubes in deionized water using a

sonication bath. They reported an increase in dispersion as a function of sonication time.

Unsonicated nanotubes could not be dispersed in water, 2 minutes of sonication resulted in

some dispersion but 45 minutes formed a dispersion that was stable for at least 3 weeks. No

oxidizing agent was used during the sonication process. The nanotubes were deposited onto a

gold surface and SEM pictures taken. The nanotubes showed significant changes in

morphology. The interaction of the carbon nanotubes and gold are related to the wettability of

the nanotubes and increases with sonication time as early as 2 minutes. Such interaction is

stronger than inter-tube interaction as manifested by cracks in the carbon nanotubes layer

rather than a loss of carbon nanotubes adhesion to the gold surface. Unsonicated carbon

nanotubes did not interact with gold and could be easily washed from the surface of gold with

water, whereas sonicated ones cannot. No or little damage and no shortening were observed

by SEM, XPS or Raman spectroscopy analysis of the tubes when sonicated in water after 2

hours. IR spectroscopy did reveal some changes in the nanotubes upon sonication. The

presence of aliphatic C-H bonds was observed before sonication and disappeared during

sonication. Peaks corresponding to hydroxyl groups, carbonyl and carboxyl groups appeared

as sonication continues. This was most likely due to the oxidation of the already existing C-H

bonds on the outer walls. This oxidation could explain the increased wettability of the tubes

and their increased dispersion in water. This method seemed to be a promising technique as

little or no damage was reported to the tubes.

Goyanes et al. [76] studied the influence of the type of acid used and sonication time

when performing the carboxylation of multi wall carbon nanotubes. The acids used were

either nitric acid or a mixture of nitric and sulfuric acid. The sonication time was varied from

2, 4 and 6 hours at 30oC. The carbon nanotubes were used as received and it was determined

that carboxylic acid groups were present on the tubes prior to acid treatment. Treatment in

acidic mixture for 2 hours was reported to modify the open ends of the nanotubes without

altering the sidewalls. Therefore the new C-O groups were located at the open end of the

tubes (Figure 15). A longer sonication time shortened and eventually destroyed the walls of


the nanotubes as seen on the AFM images (Figure 16). Once bond symmetry is disrupted

within the nanotubes, adjacent carbons increase in reactivity resulting in the entire nanotubes

being oxidized.

Liu et al. [77] reported the functionalization and shortening of single wall carbon

nanotubes using peroxy-organic acids such as m-chloroperbenzoic acid (MCPBA), 2-bromo-

2-methylperpropionic acid (BMPPA), after 12 hours of sonication. In a previous study [78],

they reported the treatment of single wall carbon nanotubes with per-trifluoroacetic acid and

the covalent bonding of trifluoroacetic groups on the single wall carbon nanotubes. The tubes

were also shortened, small bundles and individual tubes were obtained, which provided stable

dispersions in water, DMF and ethanol. Treatment with MCPBA and BMPPA led to similar

results. Oxidation products were obtained with the presence of epoxide, carboxylic acid and

ester groups as confirmed by Raman and IR spectroscopy. After the epoxidation of the

nanotubes, the opening of the epoxide led to reaction with the organic acids producing

hydroxyl and ester groups. TGA was used to determine the degree of functionalization with

the following order: MCPBA > PTFAA > BMPPA. The milder strength of BMPPA was also

noted when AFM images were taken, showing that the first two peroxy organic acids

shortened the tubes length, whereas BMPPA introduced only very little or no damage.

Chang et al. [79] sonicated with a probe double wall carbon nanotubes in o-

chlorobenzene using an azobis-type radical initiator. The authors used ultrasounds to

debundle the tubes making them well separated and available for the subsequent radical

attack from the thermal decomposition of the radical initiators. The authors did not mention

the potential role of the ultrasounds on the radical pathway. The authors compared the

reaction with and without the use of ultrasounds and observed a great improvement in the

degree of functionalization when ultrasounds were used. The presence of carboxylic acids

groups was shown by XPS as the major functional group present, although some peaks were

assigned to carbonyl, nitrile and amide groups. It is important to consider the nature of the

radical initiators to understand the presence of some of the groups reported, i.e. 4,4‘-azobis(4-

cyanopentanoic acid); 2,2‘-azobis[2-methyl-N-(2-hydroxyethyl)propianamide], 2,2‘-azobis(2-

methylpropionamidine) hydrochloride, 2,2‘-azobis[2-(2-imidazolin-2-yl)propane] and 1,1‘-

azobis(cyclohexane-1-carbonitrile). Adhesion of PtRu nanoparticles also demonstrated the

functionalization of the surface by carboxylic acid groups. The authors expanded the work to

single wall carbon nanotubes and multi wall carbon nanotubes without providing details but

demonstrating the feasibility of this work to other types of nanotubes.

Figure 15. Schematic representation of the attack to the MWCNTs during the oxidation process.

Reprinted from [76], Copyright (2006), with permission from Elsevier.


Figure 16. Images of MWCNTs: (a) as-received; (b) treated with HNO3; treated with HNO3/H2SO4 for:

(c) 2 h; (d) 4 h and (e) 6 h (left/right: height/phase). Reprinted from [76], Copyright (2006), with



Figure 17. Settling behavior of SWCNTs suspension with US only treatment and O3/US treatment.

Experimental conditions: [SWCNTs] = 200 ppm, ozone mass rate = 0.14 g/min; pH=3.0; temperature =

23oC. Reprinted from [80], Copyright (2008), with permission from Elsevier.

Li et al. [80] carried out the oxidation reaction of single wall carbon nanotubes using

ozone in a sonication bath at room temperature. They compared the action of ultrasounds

only, ozone only and the two combined together and analyzed the results by XPS, light

scattering, zeta potential and turbidity measurements. When ultrasounds alone were used, no

stable dispersion was formed after sonication was stopped. Using ozone and ultrasounds

provided stable suspensions even after sonication had ceased (Figure 17).

The length of the tubes initially at 1200 nm was measured of ultrasonic treatment with or

without ozone, with a length after 24 hours of respectively 150 nm and 300 nm. The final

length for the combined technique was reached after only 7 hours of treatment. The turbidity

showed that sedimentation was slower to occur when the combination of techniques had been

used. However the zeta potential provided similar results for both cases, showing no

significant difference in terms of zeta potential between the ozone treatments and the ozone

and ultrasounds treatment. The two treatments seemed to share the same reaction pathways

(Figure 18) but with a greater reaction rate for the combined one. XPS results confirmed the

presence of carbonyl, hydroxyl and carboxylic acid groups. It is concluded that the

ultrasounds created defects on the walls which led to further oxidation and also the high

temperatures resulting from cavitation enhanced the overall reaction rate.

C C

O3

OH.

O.

/

C C

OHHO

O

OH

O

O3

OH.

O.

/

Figure 18. Reaction pathway.

You et al. [81] used ultrasounds in a sonication bath as a way to assemble multi wall

carbon nanotubes based gels. The ultrasounds dispersed the nanotubes bundles in DMF

before facilitating the assembly of poly(amidoamine) functionalized nanotubes via hydrogen

bonds (Figure 19). Hyper branched poly(amidoamine) (HPAA) are grafted onto multi wall

carbon nanotubes during this process. HPAA has a great ability to assemble via hydrogen

bonds and to form gel under an external stimulus. Ultrasounds were found to accelerate the


gelling process of multi wall carbon nanotubes and HPPA dispersed in a solution of

poly(amidoamine) in DMF. The ultrasound induced the assembly of hyper branched

poly(amidoamine) functionalized multi wall carbon nanotubes with linear poly(amidoamine)

into a gel, because the ultrasounds disrupted the carbon nanotubes interactions, dispersing the

bundles and stretching the polymer functional groups on the surface of the tubes, thus

increasing the potential for interaction between the polymers and the nanotubes. Vigorous

stirring and microwave could not initiate the gel formation but could revert it back to its

original state, which could in turn be sonicated to lead to a gel again. The gels were sensitive

to acids hydrolysis under ultrasounds, probably because of acid catalyzed breaking of the

hydrogen bonds after the protonation of the amine groups.

MWCNTN

O

N N

NH

O

NH

O

NH

NH

O

O

MWCNTNH

O

NH

NH

O

O

N

N

MWCNT1. HNO3

2. AEPZ, EDC

AEPZ: 1-(2-aminoethyl) piperazine

MBA: N,N'-methylene bisacrylamide

EDC: 1-Ethyl-3-[3-dimethylaminopropyl] carbodiimide hydrochloride

MWCNTN

O

N NH2

MWCNTNH

O

N

NH

+

+

Figure 19. Amidoamine functionalized nanotubes.


COMBINED USE OF CARBON NANOTUBES AND

ULTRASOUNDS FOR ENVIRONMENTAL APPLICATIONS

Very few applications where carbon nanotubes and ultrasounds are combined have been

reported and interestingly all investigate the use of multi wall carbon nanotubes and

ultrasounds for environmental remediation purposes.

Zhao et al. [82] used carbon nanotubes as a trap for pyridine in water. Multi wall carbon

nanotubes were treated with nitric acid, heat, ultrasounds and polyvinyl alcohol and used as

adsorbents for the removal of pyridine. The adsorption capacity increased in going from the

pristine multi wall carbon nanotubes to the multi wall carbon nanotubes sonicated in nitric

acid, whereas the multi wall carbon nanotubes heated in nitric acid showed an intermediate

lower value, respectively 3.8 mg g-1

, 4.36 mg g-1

, and 4.28 mg g-1

, tested with a pyridine

concentration of 80 mg l-1

. As expected the surface area and the amount of suspended

nanotubes increased during the acid treatment. Data also showed that further treatment of the

acid treated multi wall carbon nanotubes with polyvinyl alcohol decreased the adsorption

capability of the tubes. The authors attributed the increase in pyridine adsorption to an

increase in surface area and pore volume, due to the fracture where the defects were located

on the carbon nanotubes.

The field of sonophotocatalysis has been widely investigated in the past few years [83,

84, 85], but only one article reports the use of carbon nanotubes as part of the process.

Traditionally the photodegradation of a pollutant dissolved in an aqueous solution is enhanced

through the use of UV light and ultrasounds using titanium dioxide TiO2 as a catalyst, with a

great synergistic effect. Wang et al. [86] reported for the first time the use of a multi wall

carbon nanotube titanium dioxide composite (CNT-TiO2) as the catalyst in the

sonophotodegradation of methyl orange. They compared its catalytic activity to P25, a

titanium dioxide catalyst traditionally used for such experiments. P25 is a combination of two

crystalline phases of TiO2, anatase and rutile in a 3:1 proportion. X-Ray diffraction on CNT-

TiO2 identified the crystalline nature of the titanium dioxide to be anatase. A TEM picture of

the catalyst CNT-TiO2 is shown below in Figure 20.

Figure 20. TEM image of CNT-TiO2 composite material. Reprinted from [86], Copyright (2008), with



The degradation of methyl orange for 60 minutes showed a 66% removal rate in the case

of sonophotocatalysis, 44% for photocatalysis and 9% for sonocatalysis. Also the kinetic

results obtained with CNT-TiO2 as a catalyst were twice as high as P25 under

sonophotocatalysis (Figure 21). Unfortunately the authors did not provide any explanation for

these results and it would have been interesting to compare the specific surface area of the

two catalysts.

Figure 21. Reaction kinetic plots of MO degradation. Reprinted from [86], Copyright (2008), with


Yu et al. [87] studied the adsorption of benzoic acid from an aqueous solution using multi

wall carbon nanotubes, treated with nitric acid and ultrasounds. This treatment raised the

oxygen contents of the multi wall carbon nanotubes surface and increased the specific surface

area. A high adsorption of benzoic acid was observed, which was explained via electrostatic

interaction between the nanotubes and the benzoic acid. The authors claimed the surface of

the nanotubes had no functional groups, but only a considerable amount of chemisorbed

oxygen. However no IR or Raman spectroscopy was provided in this article in support of this

statement which is in direct contrast of other researchers conclusions as discussed above in

this chapter.

CONCLUSION

Ultrasounds have been widely used with carbon nanotubes. Synthesis, purification,

chemical functionalization and applications of sonochemistry applied to carbon nanotubes

have been reviewed in this chapter with a large part reserved to the formation of stable carbon

nanotubes dispersions. Ultrasounds have been shown to promote the chemistry of carbon

nanotubes by debundling them, allowing them to react with the components in solution either

in a covalent way or through van der Waals interactions. It is widely accepted that ultrasounds


are necessary to individualize the nanotubes. Acid treatment of carbon nanotubes is the most

common reaction performed. It introduces oxygenated groups on the nanotubes which can

interact with the solvent or surfactants forming stable dispersions or with metallic

nanoparticles yielding decorated nanotubes. Structural defects on the carbon nanotubes walls

are reported as a result of the chemical modification under harsh conditions and can hinder

the use of ultrasounds with carbon nanotubes. However mild experimental conditions

combined with careful monitoring of the progress of the damages can be combined to allow a

greater development of the sonochemistry of carbon nanotubes.

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Acid.

INDEX

A

absorption, 11, 20, 21, 82, 83, 224, 237

absorption spectra, 11

absorption spectroscopy, 237

acetate, 12, 17, 21, 22, 24, 25, 46, 52, 56, 131,

132, 133, 135, 137, 138, 141, 144, 147, 148,

151, 163, 167

acetic acid, 167, 174, 243

acetone, 18, 114, 131, 138, 176

acetonitrile, 132

acetophenone, 112, 167, 168

acetylation, 175, 185

acetylene, 33, 57

acidic, 4, 111, 112, 235, 236, 248, 249, 251

acidity, 111

acoustic, vii, viii, 1, 2, 36, 49, 63, 64, 65, 66, 67,

68, 69, 70, 72, 73, 74, 75, 76, 77, 78, 79, 80,

81, 82, 83, 84, 85, 86, 87, 88, 91, 94, 96, 97,

98, 99, 100, 101, 102, 108, 130, 136, 140, 145,

150, 160, 161, 175, 179, 191, 195, 196

acoustic waves, 66, 68, 83

acoustical, 66, 101

acrylate, 35, 171, 175

ACS, 156

activated carbon, 168, 206

activation, 130, 137, 140, 147, 149, 150, 168,

172, 234

activation energy, 140, 147, 149, 150

activation entropy, 149

activation parameters, 130, 149

acute ischemic stroke, 200

acylation, ix, 157, 170, 178, 183, 187

additives, 12, 27, 133

adriamycin, 192, 193, 194, 198, 199, 200

adsorption, 9, 60, 239, 256, 257, 260, 263

AFM, 222, 224, 225, 234, 236, 238, 240, 246,

249, 252

Ag, 8, 10, 11, 16, 25, 26, 27, 28, 31, 36, 50, 51,

55, 56, 57

agent, ix, x, 4, 5, 7, 20, 22, 23, 31, 46, 47, 48, 60,

176, 189, 191, 193, 195, 196, 198, 231, 251

agents, vii, ix, 3, 12, 21, 48, 167, 180, 181, 189,

190, 191, 192, 195, 198, 217

aggregates, 18, 21, 22, 23, 28, 30, 140, 226, 234,

241, 250, 258

AIBN, 31

air, 9, 14, 15, 16, 17, 18, 20, 25, 33, 36, 41, 42,

46, 48, 79, 84, 119, 172, 183, 203, 204, 215,

218, 226, 228

alcohol, 10, 35, 109, 112, 114, 116, 132, 134,

135, 137, 144, 145, 151, 175, 178, 221, 243,

256, 261

alcohols, viii, 105, 110, 113, 130, 133, 137, 145,

146, 150, 172, 174, 175, 177, 185, 242, 260

Alcohols, 185

aldehydes, ix, 106, 110, 111, 112, 113, 114, 115,

116, 117, 118, 119, 164, 165, 167, 169, 171,

172, 182, 183

aliphatic amines, 173, 184

alkali, 20, 107, 113, 184, 187

alkaline, 6, 7, 9, 21, 38, 47, 132, 167, 181

alkaline hydrolysis, 132

alkanes, 215

alkenes, 32, 106, 116, 185, 215

alkylation, 106, 169, 173, 178, 184, 187

alkylation reactions, 187

alloys, 83, 96, 110

alpha, 36, 55, 58, 60

alternative, ix, 157, 195, 201, 204, 208

ambient air, 14, 20, 41, 46

ambient pressure, 9, 160

amide, 18, 53, 237, 244, 252, 259, 261

amine, 38, 163, 164, 180, 218, 228, 255

amines, 171, 173, 178, 184, 186, 217, 218, 228

Index 266

amino, 27, 56, 60, 115, 168, 170, 178, 182, 183,

186

amino acid, 27, 56, 170, 183

amino acids, 27, 56, 170, 183

amino groups, 115

ammonia, 12, 29, 38, 244

ammonium, 17, 19, 22, 30, 32, 33, 45, 112, 165,

167, 169, 172, 180, 247, 262

ammonium chloride, 112, 165

ammonium salts, 167

amorphous, 10, 25, 31, 33, 36, 51, 57, 60, 224,

233, 234, 235, 236, 244, 245, 246, 248, 249,

250

amorphous carbon, 233, 234, 235, 236, 244, 245,

248, 249

amphoteric, 111

amplitude, viii, 19, 26, 63, 67, 70, 71, 75, 84, 85,

86, 87, 94, 96, 97, 98, 100, 101, 158, 161, 194,

200, 205

anaerobic, 15

anatase, 18, 256

angiogenesis, 196, 200

aniline, 32, 33, 43, 219

anionic surfactant, 45

annealing, 16, 36, 39, 40, 232, 244

antagonism, 197

antagonist, 106

anthracene, 235

antibacterial, 29, 31, 56

antibody, 196

anticancer, vii, ix, 189, 190, 191, 192, 193, 195,

196, 197, 198

anti-cancer, 199

anticancer drug, 190, 191, 192, 193, 195, 196,

197

antihypertensive agents, 180

antioxidant, 165

antitumor, 190, 192, 195, 200

apoptosis, 192, 196, 199, 200

apoptosis pathways, 196

application, vii, ix, 2, 25, 49, 52, 53, 56, 57, 121,

134, 135, 137, 153, 158, 162, 163, 173, 184,

198, 201, 214, 260, 263

aqueous solution, ix, 3, 4, 6, 7, 8, 10, 11, 14, 16,

17, 18, 20, 21, 22, 24, 25, 27, 28, 32, 33, 34,

35, 40, 42, 45, 50, 51, 110, 113, 129, 130, 131,

132, 134, 137, 143, 202, 210, 211, 233, 236,

239, 240, 241, 242, 256, 257, 260, 263

aqueous solutions, ix, 3, 8, 11, 20, 21, 27, 33,

129, 130, 134, 137, 202, 210, 211, 236, 240,

242, 260, 263

aqueous suspension, 19, 205

argon, 3, 4, 11, 15, 26, 29, 36, 38, 140, 175, 176,

203, 214, 224

aromatic compounds, 177, 186

aromatics, 169, 178, 186

Arrhenius equation, 139, 149

aspect ratio, 4, 41, 44

assumptions, 69, 78

atmosphere, 11, 17, 24, 25, 26, 27, 30, 33, 36,

175

atomic force, 224, 234

atomic force microscopy, 224, 234

atomic force microscopy (AFM), 234

atoms, 6, 119, 146, 210, 233

ATP, 192, 199

Au nanoparticles, 3

availability, 84, 110, 179, 222

B

Bacillus, 207, 210

Bacillus subtilis, 207

bacteria, 29, 204, 205, 208, 209, 210

bacterial, 204, 205, 206, 210

bacterial cells, 205

band gap, 12, 20, 21, 30, 44

behavior, 17, 26, 38, 72, 78, 134, 191, 242, 254

Belgium, 154

benzene, 35, 111, 112, 113, 114, 115, 116, 141,

142, 166, 214, 217, 219, 222, 226, 232, 240

binding, 12, 35, 134, 199, 250, 262

binding energies, 35

binding energy, 12

biochemistry, 134

biodiesel, 103

biological activity, 225

biological coating, 18

biotechnology, 36, 153

bismuth, 19, 20, 55, 61, 171, 183

bladder, 194, 200

bladder cancer, 195, 200

bonding, 146, 147, 215, 222, 242, 262

bonds, 20, 106, 134, 141, 160, 233, 235, 244,

245, 250, 251, 254

boundary conditions, 89, 91

bromination, 178, 186

bubble, viii, 2, 31, 49, 63, 65, 66, 67, 68, 69, 70,

71, 72, 73, 75, 76, 78, 82, 83, 102, 108, 130,

131, 138, 139, 160, 161, 162, 202, 203, 213,

234

bubbles, vii, ix, 1, 2, 49, 66, 67, 68, 72, 73, 74,

75, 76, 77, 78, 81, 83, 102, 108, 130, 131, 140,

157, 160, 161, 174, 191, 198, 202, 204, 208,

214

Index 267

Bubbles, 2, 179

bulk crystal, 23

by-products, 142, 204, 209

C

calcination temperature, 22, 24

calcium, 30, 40, 180

calcium channel blocker, 180

calibration, 94, 224

calorimetry, 73, 136

Canada, 208, 260

cancer, ix, 167, 181, 189, 190, 191, 192, 193,

194, 195, 196, 197, 198, 199, 200

cancer cells, 190, 191, 192, 194, 195, 196, 197,

199

cancer treatment, 196

capillary, 241, 260

Carbon, viii, 33, 43, 58, 59, 105, 106, 219, 228,

231, 232, 258, 260, 261, 263

carbon atoms, 146, 233

carbon dioxide, 203

carbon materials, 258

carbon nanotubes, x, 34, 35, 43, 59, 219, 231,

232, 233, 234, 235, 236, 237, 238, 239, 240,

241, 242, 243, 244, 245, 246, 247, 248, 249,

250, 251, 252, 254, 256, 257, 258, 259, 260,

261, 262, 263

Carbon nanotubes, 232

carbonates, 24

Carbonyl, 105, 248, 249

carboxyl, 251

carboxyl groups, 251

carboxylates, 240

carboxylic, 22, 38, 116, 132, 240, 242, 244, 247,

248, 249, 251, 252, 254

carboxylic acids, 132, 240, 242, 252

carboxylic groups, 244

carburization, 34

carcinoma, 192, 195, 198, 199

catalysis, vii, ix, 2, 3, 22, 42, 44, 157, 184, 196,

203

catalyst, 9, 10, 18, 28, 35, 36, 43, 51, 55, 56, 106,

107, 108, 109, 111, 141, 163, 165, 166, 171,

172, 175, 176, 177, 178, 183, 184, 186, 234,

236, 241, 248, 256, 257

catalytic activity, 9, 28, 35, 110, 256

catalytic effect, 132

catalytic properties, 61

cathode, 34, 53

cavities, 75, 108, 160, 161, 202, 203

C-C, 185, 235

cell, 6, 64, 191, 192, 193, 194, 195, 196, 197,

198, 199, 200, 204, 205, 206, 211

cell death, 192, 195, 197

cell killing, 199

cell line, 194, 198, 199, 200

cell lines, 200

cellulose, 46

ceramic, 206

ceramics, 2, 49

cerium, 58, 248

cervix, 193

chalcogenides, vii, 2, 44, 47

channels, 16

chemical bonds, 161

chemical interaction, 26

chemical oxidation, 248

chemical properties, 213

chemical reactions, ix, 2, 106, 129, 130, 131, 133,

141, 158, 159, 160, 174, 196, 202

chemical reactivity, 129, 134, 158

chemicals, 109, 110, 111, 190, 195, 196, 202

chemiluminescence, 61

chemopreventive agents, 167, 181

chemoresistance, 189, 192, 193, 199

chemoresistant, ix, 189, 192, 193, 194, 196

chemotherapy, 189, 190, 191, 192, 196, 197, 198,

199

China, 105, 126, 189, 198

chiral, viii, 105, 106, 107, 120

chiral center, viii, 105, 107

chloride, ix, 7, 11, 129, 130, 132, 135, 137, 146,

147, 148, 149, 150, 151, 152, 167, 171, 172,

173, 176, 183, 185, 221, 233, 248, 250

Chloride, 146

chlorinated hydrocarbons, 234

chlorination, 204, 206, 208

chlorobenzene, 175, 224, 232, 245, 246, 252

chloroform, 9, 234, 235, 238, 243, 248

chromatography, 219, 220

chromium, 217, 218

circulation, 3, 194

cis, 16, 176, 177, 185

cisplatin, 193, 195, 197, 199, 200

Claisen, ix, 107, 157, 167, 168, 182

classical, 57, 67, 78, 106, 109, 110, 114

cleaning, 45, 46, 84, 106, 129, 158, 161, 165,

168, 203, 208, 209

cleavage, 9, 130, 139, 178, 186, 218

clouds, 102

cluster model, 134, 141

clusters, 11, 12, 39, 60, 134, 141, 142, 144, 145,

150, 204, 226, 239

CNTs, 34, 35

Index 268

Co, 10, 24, 51, 103

CO2, 9, 28

coatings, 18

cobalt, 233

coil, 87, 94

coliforms, 207

collisions, 232

colloidal particles, 226

colloids, x, 213, 224, 226, 229

colon, 194, 195

colon cancer, 194, 195

combustion, 15, 28, 56

components, x, 64, 65, 83, 137, 203, 231, 257

composites, 28, 29, 30, 56, 57, 58

composition, 16, 19, 36, 132, 133, 135, 136, 137,

147, 150, 151, 152, 221

compounds, vii, viii, 47, 61, 105, 106, 107, 109,

114, 133, 140, 145, 163, 164, 165, 167, 169,

171, 173, 176, 180, 181, 182, 184, 200, 203,

204, 209, 215

concentration, 5, 8, 9, 12, 14, 19, 20, 22, 23, 27,

36, 43, 67, 68, 69, 79, 113, 139, 145, 176, 177,

195, 222, 224, 234, 237, 238, 239, 241, 256,

260

condensation, ix, 107, 129, 130, 132, 134, 141,

142, 143, 157, 164, 166, 167, 168, 171, 181,

182, 183

condensed media, 147

conducting polymers, 43

conductivity, 32, 35, 57, 73, 239, 248, 262

configuration, 103

Congress, 102, 199

construction, viii, 73, 76, 78, 85, 88, 105, 106

control, viii, ix, 2, 37, 51, 56, 97, 105, 107, 120,

153, 189, 195, 198, 221, 238

conversion, 9, 14, 31, 40, 76, 110, 112, 113, 170,

178, 186, 221

conversion rate, 221

cooling, ix, 2, 87, 108, 157

copolymer, 21, 23, 246

copper, 22, 24, 29, 54, 55, 60, 163, 164, 166, 180

copper oxide, 29

core-shell, 10, 11, 36, 38, 39, 40, 51, 58

correlation, 130, 133, 134, 143, 153, 174, 238

correlation analysis, 130, 143

corrosion, 8, 113, 170

coupling, vii, viii, ix, 105, 106, 107, 109, 110,

111, 112, 113, 114, 115, 116, 117, 118, 119,

120, 121, 157, 162, 163, 164, 166, 175, 178,

179, 180, 181, 185

covalent, x, 231, 239, 252, 257

covalent bond, 252

covalent bonding, 252

critical value, 76

cross-sectional, 84, 88

crystal growth, 221, 222

crystal lattice, 140

crystal structure, 249

crystalline, 4, 6, 7, 15, 24, 25, 36, 41, 45, 48, 50,

59, 60, 228, 250, 256

crystallinity, 3, 17, 222

crystallites, 28

crystallization, x, 16, 18, 106, 140, 213, 224

crystals, 5, 8, 23, 36, 40, 222, 227

CTAB, 3, 4, 17, 22, 40, 45, 46

cyanide, 173, 184

cycles, 48, 108, 158, 160, 194

cyclodextrin, 5, 48, 60

cyclohexane, 252

cyclohexanol, 243

cyclohexanone, 176

cyclophosphamide, 196

cysteine, 30

cytochrome, 196

cytokine, 196

cytometry, 211

cytosol, 196

cytotoxic, ix, 189, 191, 192, 195, 197, 198

cytotoxic agents, 195

cytotoxicity, 192, 194, 198, 199, 200

D

decomposition, 15, 16, 18, 20, 34, 36, 40, 42,

141, 158, 202, 237, 246, 252

deduction, 143

defects, x, 109, 231, 232, 235, 238, 239, 241,

242, 243, 244, 246, 247, 254, 256, 258

definition, 140

deformation, 85, 203, 262

degradation, 10, 18, 22, 24, 31, 66, 138, 139, 141,

153, 205, 227, 237, 240, 241, 245, 246, 249,

257, 260, 261, 263

Degussa, 18

density, 33, 68, 72, 73, 75, 76, 77, 78, 79, 86,

100, 139, 205, 241, 242, 243

depolymerization, 203

deposition, 55, 56, 84, 100, 250, 259

derivatives, x, 35, 164, 166, 175, 176, 181, 185,

213, 215, 216, 217, 219, 227, 228

destruction, 71, 143, 194, 210, 262

diagnostic ultrasound, 196

diamines, 106, 178, 186

Diamond, 258, 262

dichloroethane, 234

dienes, 180

Index 269

diffraction, 17, 61, 221, 222, 226, 256

diffusion, 2, 67, 68, 72, 239

dimer, 215, 216

dimethylformamide, 42, 236

dimethylsulfoxide, 251

dipole, 131

dipole moment, 131

dipole moments, 131

discontinuity, 67, 71, 73

discrimination, 247

disinfection, 204, 205, 206, 208, 209, 210

dispersion, 14, 28, 31, 57, 72, 81, 82, 91, 98, 220,

221, 224, 232, 235, 236, 237, 239, 240, 241,

243, 244, 247, 249, 251, 254, 259, 260, 261

dissociation, 23, 131, 203

distilled water, 14, 46

distribution, vii, 1, 5, 7, 9, 10, 12, 18, 21, 29, 31,

34, 36, 38, 66, 68, 69, 71, 72, 77, 91, 92, 93,

95, 242, 247, 261

divergence, 81

DMF, 42, 43, 166, 169, 236, 237, 244, 247, 248,

252, 254

DNA, 193, 199, 211, 240, 260

DNA damage, 199

DNA repair, 193

donor, 22, 226

dopant, 43

doped, 17, 18, 20, 22, 29, 41, 42, 43, 44, 53, 54,

56, 181, 187

doping, 2, 18, 53, 249

drug delivery, 24, 190, 199

drugs, 106, 192, 194, 195, 196, 199

drying, 18, 221, 224, 249

durability, 27

duration, 19, 108, 191, 194, 234, 238, 246, 247,

250

E

E. coli, 205, 208

earth, 17, 24, 42, 54, 55, 107

Education, 210

electric conductivity, 35

electric current, 242

electric energy, 83

electric field, 242, 260

electrical power, 88

electrocatalyst, 51

electrochemistry, 44, 106, 107

electromagnetic, 20, 159

electron, ix, 106, 109, 111, 112, 114, 116, 117,

165, 170, 171, 218, 222, 226, 229, 247, 261

electron microscopy, 226, 229, 233, 236, 247,

261

electrophoresis, 193, 240, 241, 246, 260

emission, 17, 20, 22, 40, 41, 110, 208

employment, 173

emulsification, 55, 72, 98, 109, 203

emulsion polymerization, 58

emulsions, 109

encapsulated, 30, 31, 32, 193, 194, 195, 199, 221

encapsulation, 31, 241

energy density, 33

energy parameters, viii, 63, 64

England, 210

entanglement, 5

entropy, 137, 149

entropy of activation, 137

environment, 17, 49, 73, 75, 110, 161, 213, 227

environmental protection, 210

Epoxidation, 172, 183, 217, 218

epoxides, 106, 178, 186, 218

epoxy, 98, 249

equilibrium, 68, 70, 71, 77, 79, 137, 239

equilibrium state, 70, 79

erosion, 72, 82, 203

Escherichia coli, 29, 205, 207, 208, 209, 210

ESR, 243

ester, ix, 30, 129, 134, 137, 139, 140, 141, 143,

144, 145, 152, 252

esters, 130, 132, 134, 135, 137, 139, 141, 144,

145, 165, 168, 172, 178, 180, 183, 186

Estonia, 129

ET, 109

etching, 39, 40

ethanol, 8, 9, 16, 17, 24, 31, 34, 38, 40, 46, 50,

52, 113, 132, 134, 135, 136, 137, 138, 141,

142, 143, 144, 146, 147, 148, 149, 150, 151,

152, 172, 237, 242, 243, 248, 252

Ethanol, 52, 142, 149

ethers, 106, 166, 173, 178, 181, 186

ethyl acetate, 56, 131, 135, 137, 138, 151

ethylene, 4, 6, 7, 10, 11, 24, 38, 46, 47, 48, 238,

246

ethylene glycol, 4, 6, 7, 10, 11, 24, 38, 46, 47, 48,

238

ethylene oxide, 246

ethylenediamine, 47

Euro, 181

excitation, 20, 22, 44, 160

exfoliation, 239, 260, 262

experimental condition, 19, 41, 131, 136, 244,

258

exposure, 100, 191, 194, 199

external magnetic fields, 27

Index 270

extinction, 237

extraction, 106, 220

extrapolation, 146

F

fabrication, 2, 3, 49, 51, 55, 57, 60

family, 42, 85, 237

fatigue, 64, 85, 86, 91, 94

ferromagnetic, 16, 17, 38, 58, 236

fibers, 29, 56, 248

film, 36, 205

films, 35, 60, 260

filtration, 106, 165, 235, 259

flow, viii, 15, 38, 63, 64, 66, 70, 71, 74, 75, 83,

100, 101, 160, 204, 206, 211, 221, 239

flow rate, 221

fluid, 102, 160

fluid mechanics, 102

fluorescence, 239

fluoride, 14, 113, 168

fluorinated, 243, 248, 261

focusing, viii, 63, 71, 72, 82, 190

formaldehyde, 18, 28

formamide, 169

fouling, 208

fractal cluster, 226

fracture, 256

fragility, 222

fragmentation, 109

France, 102

free energy, 137, 143

free radical, 129, 191, 202, 211

free radicals, 129, 191, 202, 211

free-radical, 211

FTIR, 30, 40, 224

FT-IR, 219

fuel, 9, 10, 49

fuel cell, 9, 10, 49

fullerene, x, 213, 214, 215, 216, 217, 218, 219,

221, 224, 225, 226, 227, 228, 229, 235, 244,

245, 246

fullerenes, vii, x, 213, 214, 217, 218, 219, 220,

221, 222, 224, 225, 226, 227, 229, 235, 245

Fullerenes, 213, 214, 219, 228, 260

functional changes, 190

functionalization, x, 213, 217, 231, 248, 250,

252, 257, 259, 262, 263

G

gadolinium, 20

gamma-ray, 210

gas, vii, 2, 9, 14, 38, 49, 52, 66, 67, 68, 69, 70,

71, 72, 73, 75, 76, 77, 78, 81, 82, 83, 97, 108,

109, 131, 139, 160, 177, 191, 202, 203, 204,

214

gas diffusion, 72

gas phase, vii, 2, 49, 131, 139

gases, 71, 158, 203, 206, 224

gel, 22, 240, 246, 254, 263

gel formation, 255

gene, 192, 193, 198

gene expression, 192, 193

generalization, 142

generation, viii, 2, 63, 119, 129, 136, 160

Georgia, 213, 231

glass, 14, 18, 222

glucose, 4, 5, 6

glycerin, 98, 99

glycerol, 47

glycol, 4, 6, 10, 11, 20, 21, 26, 27, 46, 48, 173

glycosaminoglycans, 40, 58

glycosides, 228

gold, 3, 4, 5, 10, 12, 26, 27, 32, 50, 51, 57, 251

gold nanoparticles, 4, 5, 10, 50, 57

graphite, 81, 82, 235, 236, 248

groups, 22, 30, 34, 38, 106, 111, 115, 119, 131,

144, 163, 171, 241, 242, 244, 247, 248, 249,

250, 251, 252, 254, 255, 257, 258, 262

growth, vii, ix, 1, 2, 3, 6, 12, 17, 27, 37, 41, 43,

47, 49, 50, 51, 52, 59, 68, 74, 75, 76, 108, 130,

157, 160, 161, 194, 196, 200, 221, 222, 228,

233

growth factor, 196, 200

growth mechanism, 12, 59

growth temperature, 17

H

H2, 9, 174, 203, 215, 253

HA, 30, 56, 199

halogenated, 232, 259

HDPE, 56

health, 194, 204

hearing, ix, 158, 201

heat, 15, 16, 18, 24, 25, 34, 86, 97, 136, 139, 190,

206, 233, 246, 256

heating, ix, 17, 22, 24, 54, 66, 72, 108, 157, 241,

245

heavy metal, 107

heavy metals, 107

hematite, 15, 16, 52

heptane, 234

Index 271

heterogeneous, 129, 130, 131, 134, 138, 159,

161, 162, 163, 172, 173, 177, 206, 232, 235

heterogeneous systems, 161

hexafluorophosphate, 216

hexagonal lattice, 233

hexane, 218, 220

high intensity ultrasound, 182, 231

high pressure, ix, 2, 157, 160, 191

high resolution, 214

high temperature, 2, 34, 49, 82, 130, 161, 191,

213, 222, 233, 244, 254

high-speed, 66, 102, 131, 162

HIV, viii, 105, 106

homogeneity, 28, 64, 100

hot spots, 2, 49, 130, 140, 231

House, 156

HPLC, 214

HPM, 165

HR, 29, 199

HRTEM, 13, 26, 39, 41, 45, 247

human, ix, 109, 158, 192, 193, 194, 195, 197,

198, 199, 200, 201, 204

hybrid, x, 32, 57, 231

hydrate, 7, 47, 218, 246

hydro, 35, 43, 178, 187, 234, 246

hydrocarbon, 43, 235, 258

hydrocarbons, 43, 246

hydrodynamic, 81, 160, 206

hydrogels, 262

hydrogen, 2, 5, 18, 34, 40, 43, 53, 57, 132, 133,

141, 146, 147, 172, 177, 183, 202, 203, 205,

210, 215, 234, 242, 247, 254, 258

hydrogen atoms, 210

hydrogen bonds, 132, 133, 141, 254

hydrogen gas, 177, 203

hydrogen peroxide, 5, 18, 53, 172, 183, 203, 205,

234, 247

hydrogenation, 10, 32, 35, 57, 177, 185, 186

hydrolysis, ix, 18, 21, 38, 52, 129, 130, 131, 132,

133, 134, 135, 137, 138, 139, 140, 141, 144,

145, 146, 147, 148, 150, 151, 255

hydrophilicity, 31

hydrophobic, vii, ix, 129, 130, 132, 133, 134,

135, 137, 139, 141, 142, 143, 144, 145, 146,

147, 150, 152, 153

Hydrophobic, 129, 133, 134, 156

hydrophobic interactions, vii, 132, 133, 134, 137,

141, 144, 145, 152, 153

hydrophobicity, 139, 141, 143, 144, 145, 150,

153

hydrothermal, 258

hydroxide, 12, 30, 61, 168, 172, 248

hydroxides, 12, 22

hydroxyapatite, 30, 40, 58, 59

hydroxyl, 2, 19, 202, 203, 210, 242, 248, 249,

250, 251, 252, 254

hydroxyl groups, 242, 251

hydroxylation, 106

hydroxypropyl, 18, 53, 60

hyperthermia, 190

hypothesis, 193, 235

I

id, 141, 221, 241, 251

illumination, 24

images, 4, 5, 11, 13, 15, 20, 23, 26, 27, 28, 29,

35, 38, 39, 40, 41, 42, 45, 196, 223, 225, 234,

235, 236, 239, 240, 242, 245, 247, 248, 250,

252

imaging, 24, 159, 196, 236, 243, 248, 249

immersion, 132, 168

immobilization, 27, 56, 57

implementation, viii, 63, 64, 100

impurities, 109, 236, 240, 241, 245

in situ, 30, 32, 58, 60, 237

in vitro, 191, 198, 199

in vivo, 191, 194, 195, 196, 198, 199, 200

inactivation, 202, 204, 205, 207, 208, 211

Indian, 49, 120, 124, 125, 126, 183

indication, 81

indigenous, 139

indium, 17, 53, 60, 110, 182

indole, 164, 169, 173

induction, 199

industrial, viii, 18, 63, 64, 83, 84, 91, 94, 98, 100,

101, 120, 133

industrial application, 91, 94, 120

inflammatory, 167, 181

inhibitors, viii, 105, 106

inorganic, 2, 25, 29, 40, 57, 111

insertion, 2, 56

insulation, 97

integration, 69

interaction, 10, 11, 30, 66, 102, 119, 120, 133,

141, 143, 197, 198, 199, 225, 239, 246, 250,

251, 255, 257

interactions, vii, x, 30, 132, 133, 134, 135, 137,

138, 141, 143, 144, 145, 146, 152, 153, 197,

226, 227, 231, 239, 240, 242, 244, 255, 257

interface, 108, 139, 202, 203, 221, 234

intrinsic, 3, 25, 138, 139, 150, 174, 192

Investigations, 194

iodine, 165, 168, 171, 183, 203

Iodine, 180

Index 272

ionic, ix, 5, 7, 14, 18, 50, 52, 53, 57, 60, 129,

130, 131, 132, 138, 139, 143, 152, 153, 157,

161, 162, 165, 174, 175, 184, 185, 215, 216,

217, 228, 239, 240, 241, 246, 260

ionic liquids, ix, 157, 174, 175, 184, 185, 215,

217, 241

ionization, 113, 240

ions, 3, 6, 10, 11, 17, 21, 22, 23, 27, 45, 47, 48,

131

IR, 200, 238, 241, 242, 246, 247, 248, 249, 250,

251, 252, 257

IR spectra, 18, 238

IR spectroscopy, 241, 242, 246, 247, 248, 249,

250, 251, 252

Iran, 1

iron, 15, 16, 25, 36, 37, 38, 39, 52, 108, 112, 233,

250

isolation, 77, 170, 220, 228, 258

isothermal, 77

Italy, 201, 208

J

Japan, 199

Japanese, 21, 34

Jun, 55

Jung, 12, 51, 52

K

KBr, 222, 228

ketones, ix, 106, 109, 110, 112, 113, 114, 115,

116, 167, 169, 171, 172, 174, 183, 185

kinetics, ix, 2, 75, 129, 130, 131, 138, 141, 143,

203

KOH, 20, 113, 173

Korean, 126

L

laboratory studies, vii, 63, 88, 100

lactams, 168, 182

lamellae, 59

lamellar, 111

Langmuir, 50, 51, 56, 60, 260, 262

lanthanide, 21, 22, 40, 41, 59

lanthanum, 61

lattice, 17, 20, 222, 224, 233

Legionella, 209

Legionella pneumophila, 209

lesions, ix, 189, 192, 196, 197, 198

lifetime, 108, 139

ligand, 22, 61, 163, 164, 180

ligands, viii, 27, 48, 105, 106, 119, 175

light scattering, 14, 254

limitation, 84

limitations, vii, viii, 1, 63, 87, 92, 94, 100, 116,

182, 190

linear, ix, 44, 66, 129, 130, 136, 139, 141, 143,

152, 195, 255

linear dependence, 136

lipid, 165

lipid peroxidation, 165

liposome, 194

liquid chromatography, 214

liquid interfaces, 109

liquid phase, 32, 75, 76, 109, 130, 140, 173, 206

liquids, viii, ix, 63, 64, 72, 83, 97, 98, 102, 103,

108, 140, 157, 160, 172, 174, 184, 185, 215,

217, 231, 241, 260, 261

lithium, 34, 57, 233

London, 102, 120, 121, 155, 209, 210

low-intensity, 66, 195, 199

low-level, 194, 198, 199

low-temperature, 35, 59

luminescence, 17, 23, 59

lymphoma, 199

lysozyme, 240, 260

M

maghemite, 27, 56

magnesium, 19, 22, 112, 216

magnetic, 3, 8, 17, 24, 26, 27, 31, 32, 36, 38, 56,

57, 94

magnetic field, 26, 27, 31, 38

magnetic properties, 17, 24, 26, 31, 32, 36

magnetic resonance, 24

magnetic resonance imaging, 24

magnetite, 16, 25, 26, 27, 38, 52, 55

magnetization, 25, 26, 36, 38, 55

manganese, 17, 33, 34, 53, 57, 114

Manganese, 16, 114

mass spectrometry, 214, 220, 246

mass transfer, 162, 172

matrix, 27, 31, 32, 51, 206

MDR, 194

measurement, 38, 49, 82, 94, 158, 260

measures, 96

mechanical behavior, 228

mechanical energy, 83

mechanical stress, 64, 83, 85, 204

media, 9, 44, 67, 107, 108, 110, 116, 134, 147,

149, 153, 201, 216, 225, 262

Index 273

medical diagnostics, 129

medicine, 159

melanoma, 194, 200

melting, 26, 31, 140, 204

membrane permeability, 211

mercury, 19, 47, 53, 60

mesoporous materials, 2

metal carbides, vii, 2

metal hydroxides, 22

metal ions, 11, 23, 27

metal nanoparticles, 3, 27, 56, 57

metal oxide, vii, 2, 32, 35

metal oxides, vii, 2

metals, vii, 2, 3, 8, 27, 56, 107, 110, 159, 168

metastasis, 194, 200

metastatic, 194, 196, 199

methanol, 9, 10, 26, 27, 32, 134, 146, 150, 151,

165, 175, 235, 242, 243

methyl group, 215

methyl methacrylate, 35, 238

methylene, 18, 167, 169, 172, 181, 183, 221

methylene chloride, 221

mice, 194, 195, 199, 200

micelles, 194, 195, 199, 200, 241

microbial, 202

microcirculation, 196

microemulsion, 55

microorganisms, 31, 204, 205, 206, 207

microscope, 94, 222, 241

microscopy, 222, 224, 226, 229, 234, 247, 250,

261, 263

microspheres, 12, 38, 51

microstructures, 14

microwave, 54, 56, 60, 107, 162, 173, 175, 178,

184, 185, 186, 221, 228, 255

microwaves, 221

migration, 194, 199, 246

mirror, 137

mitochondria, 196

mitochondrial, 196, 200

mixing, 137, 160, 236, 249

MMA, 35, 238

mobility, 226

modalities, 196

modality, ix, 189, 190, 192, 195, 196, 198

models, 66, 87, 134, 199, 200

modulus, 36, 85

moieties, 30

moisture, 109, 215, 221

molar ratio, 5, 6, 8, 9, 20, 21, 32, 112, 113

molar volume, 134

mole, 9, 134, 137

molecular structure, ix, 189, 196, 198

molecular weight, 238

molecules, vii, 1, 2, 5, 17, 108, 130, 134, 139,

140, 142, 150, 160, 161, 190, 192, 194, 196,

201, 213, 218, 226, 237

molybdenum, 19

monomer, 6, 31, 32, 43

morphological, 45, 46, 56

morphology, vii, 1, 2, 3, 7, 8, 10, 11, 12, 14, 15,

16, 19, 21, 36, 37, 41, 42, 43, 44, 52, 55, 56,

233, 250, 251, 261

movement, 66, 69, 70, 73, 75, 76, 78, 81

MS, 27, 218, 219, 226, 227

multidrug resistance, 199

multiwalled carbon nanotubes, 34, 260, 262, 263

N

nanobelts, 4, 5, 48, 50, 60, 61

nanocapsules, 245

nanocatalyst, 35

nanocomposites, vii, 2, 10, 25, 27, 29, 31, 32, 34,

35, 54, 56, 57, 242

Nanocomposites, 25, 27, 30

nanocrystal, 42, 224

nanocrystalline, 18, 24, 36, 50, 52, 53, 54, 55, 58,

60, 61, 262

nanocrystals, 6, 7, 12, 16, 18, 21, 22, 23, 25, 31,

40, 42, 47, 55, 58, 60, 61, 224

Nanocrystals, 54

nanocubes, 17, 46, 60

nanofibers, 43, 44, 59

nanomaterials, 2, 49, 60, 129, 227

nanometer, 24, 28, 51

nanometers, 48, 221

nanoribbons, 14

nanorods, 3, 4, 6, 8, 12, 13, 18, 20, 21, 23, 24, 28,

36, 40, 41, 42, 46, 47, 48, 50, 51, 52, 53, 55,

58, 59, 60, 61

nanosheets, 40, 235, 259

nanostructured materials, 2

nanostructures, 3, 7, 8, 10, 12, 13, 14, 22, 41, 42,

43, 44, 46, 48, 50, 51, 53, 59, 60, 61, 258

Nanostructures, 46, 59

nanotechnology, vii, 1

nanotube, 7, 34, 35, 43, 58, 235, 237, 239, 256,

258, 259, 260, 261, 262, 263

nanotube films, 260

nanowires, 7, 12, 14, 43, 46, 50, 60, 234, 258

Nanowires, 52, 59

naphthalene, 178, 235

natural, viii, 29, 105, 106, 164, 210

Nd, 21, 40, 41, 54

neck, 99, 100

Index 274

necrosis, 190, 192, 196, 197

New York, iii, iv, 63, 102, 103, 120, 121, 126,

153, 154, 156, 209

Ni, 24, 52, 59, 107, 155, 177, 232

nickel, 112, 175

nitrate, 6, 7, 10, 18, 22, 25, 29, 48, 50, 165, 169,

180

nitric acid, 168, 176, 185, 226, 235, 241, 242,

244, 247, 248, 250, 251, 256, 257, 260

nitrobenzene, 27

nitrogen, 24, 48, 79, 96, 135, 203

NMR, 112, 113, 115, 117, 118, 140, 219, 226

NO, 14, 35, 52

noble metals, 3, 27, 56

non-destructive, 159

non-thermal, 218

non-uniform, 72, 160

normal, 17, 78, 79, 80, 81, 94, 140, 183, 190, 191

normal conditions, 81

novel materials, 49

nucleation, 17, 21, 23, 27, 38, 43, 45, 109, 130,

232

nuclei, 2, 6, 27, 68, 72, 74, 75, 76, 191, 194

nucleus, 16, 26, 31, 70, 169

numerical analysis, 66

O

octane, 169

o-dichlorobenzene, 232, 237, 250

oil, 9, 24, 103, 157

oils, 98, 173

olefins, 106, 174, 183, 218

oligomer, 237

olive, 18

onion, 234

optical, 3, 23, 44, 60, 159, 221, 222, 237, 241

optical microscopy, 222

optical properties, 23, 60

optical storage, 44

optics, 3, 42, 213

optimization, viii, 64, 114, 115, 262

optoelectronic, 3, 44, 49

optoelectronic devices, 44

optoelectronics, 42

organic matter, 204

organic solvent, 43, 108, 110, 130, 132, 133, 134,

152, 173, 185, 236, 237, 238, 249, 258, 259,

260

organic solvents, 43, 110, 132, 133, 134, 152,

236, 237, 238, 258, 260

organometallic, 109

Organometallic, 184

oscillation, 70, 72, 76, 94, 96, 97, 98

ovarian cancer, 192, 193, 194, 197, 198, 199, 200

ovarian cancers, 193, 199

overtime, 238

oxidants, 217, 218, 228

oxidation, ix, x, 9, 10, 15, 18, 19, 34, 36, 51, 57,

119, 135, 138, 157, 160, 173, 176, 184, 185,

204, 208, 213, 218, 228, 231, 234, 236, 239,

243, 244, 247, 248, 249, 250, 251, 252, 254,

262, 263

oxidation products, 135

oxidative, 234, 235, 236, 241, 245, 247, 248, 262

oxidative damage, 235

oxide, 14, 15, 16, 19, 20, 29, 33, 34, 37, 52, 53,

57, 58, 61, 217, 218, 234, 235, 246, 259

oxide nanoparticles, 16, 29, 52, 53

oxides, 2, 9, 12, 15, 19, 20, 217, 218, 228

oxygen, 9, 33, 50, 110, 119, 172, 176, 183, 203,

242, 257

P

paclitaxel, 194, 197

PAHs, 35

palladium, 6, 27, 34, 50, 57, 163, 171, 175

palliative, ix, 189, 197, 198

PANI, 32, 33, 35, 43, 44

parameter, 66, 143, 144, 222, 224

particles, vii, 1, 3, 4, 5, 7, 10, 11, 12, 15, 17, 18,

19, 20, 21, 22, 23, 24, 27, 28, 30, 31, 32, 33,

36, 37, 38, 43, 47, 48, 51, 54, 57, 58, 70, 81,

160, 195, 203, 204, 208, 222, 224, 226, 232,

233, 234, 235, 236, 238, 246, 247, 250, 259

pathways, 196, 216, 254

patients, 194, 200

Pb, 19, 21, 22, 23, 24, 45, 46, 47, 48

PbS, 44, 45, 46, 60

PCT, 101

periodicity, 262

permeability, 211

permeabilization, 196

permit, viii, 64, 66, 83, 85, 98, 107, 140

peroxide, 18, 205, 234

perturbation, 135, 138, 140, 141, 146, 150

perturbations, 69

pesticides, 106

P-glycoprotein, 192

pH, 3, 4, 17, 20, 21, 23, 27, 34, 35, 41, 46, 48, 50,

107, 116, 132, 135, 206, 240, 244, 254, 260

pH values, 23

pharmaceuticals, 164

pharmacogenetics, 199

pharmacokinetics, 199

Index 275

phase-transfer catalysis, 184

phenol, 171, 176

phenotype, 194

phenyl esters, 178, 186

phosphate, 30, 31, 40, 56

phosphonates, 178, 186

phosphors, 54

photocatalysis, 18, 209, 257

photocatalysts, 53

Photocatalytic, 18

photochemical, 22, 176, 185

photodegradation, 256

photographs, 66, 98, 99

photoluminescence, 17, 23, 40, 42, 51, 53, 57

Photoluminescence, 22

photolysis, 215

photonic, 36

photonic crystals, 36

photons, 160

photooxidation, 176

photovoltaic, 44

physical environment, 130

physical properties, vii, x, 1, 231, 232, 260

physicochemical, 136

physicochemical properties, 136

physics, 120

piezoelectric, 83, 84, 100, 159

planar, 4, 67, 85, 88

plastic, 38, 42, 158

plastics, 84, 106

platinum, 10, 11, 28, 29, 32, 38, 51, 57, 250

PMMA, 30, 56, 245

polarity, 139, 146, 147, 237

pollutant, 20, 256

pollutants, 263

pollution, 171

poly(vinylpyrrolidone), 4, 6

polyacrylamide, 31, 56

polyaniline, 32, 43, 44, 57, 59

polycarbonate, 242

polycrystalline, 23, 35, 41

polycyclic aromatic hydrocarbon, 35

polyethylene, 6, 11, 21, 46, 48, 173

polyimide, 238

polyimides, 238

polymer, 11, 23, 31, 32, 44, 46, 47, 48, 57, 61,

98, 232, 237, 238, 239, 240, 255, 258, 261,

263

polymer chains, 31, 239

polymer melts, 98

polymer nanocomposites, 232, 258

polymerase, 193

polymerase chain reaction, 193

polymeric materials, 2, 14

polymerization, 31, 32, 44, 56, 57, 237, 238

polymerization temperature, 44

polymers, 2, 11, 43, 44, 47, 48, 49, 53, 237, 255

polymethylmethacrylate, 30, 245

polypropylene, 57

polysaccharide, 239

polystyrene, 31

polyvinyl alcohol, 256

polyvinylpyrrolidone, 60

poor, 133, 165, 172, 194

poor health, 194

population, 65, 101

pore, 16, 28, 191, 256

pores, 16, 28, 29, 75

porosity, 56

porous, 14, 28, 29, 31, 42, 51, 52, 57, 75, 97, 103,

170, 191

potassium, 10, 163, 166, 168, 172, 173, 177, 179,

180, 184, 203, 248, 249

powder, viii, 9, 18, 19, 24, 25, 26, 43, 47, 48, 53,

55, 60, 61, 82, 105, 109, 110, 111, 112, 166,

222, 223, 232

powders, 18, 21, 24, 47, 53, 54, 55, 112, 258

power, viii, ix, 2, 3, 5, 7, 9, 10, 12, 14, 25, 31, 33,

34, 38, 42, 44, 46, 57, 64, 83, 84, 85, 86, 87,

88, 94, 97, 98, 100, 101, 102, 106, 108, 116,

129, 134, 136, 141, 145, 158, 160, 161, 201,

202, 205, 208, 210, 219, 241

PPO, 23

precipitation, 22, 40, 59, 221, 222, 224, 228

pressure, 2, 9, 17, 49, 68, 69, 70, 71, 72, 73, 75,

76, 77, 78, 79, 80, 81, 86, 87, 94, 96, 98, 101,

108, 140, 147, 160, 161, 174, 191, 201, 203,

204, 206, 208, 213, 214

pristine, 29, 31, 238, 242, 256, 259, 260

probability, 131, 250

probe, 9, 24, 42, 43, 48, 153, 161, 176, 205, 206,

215, 240, 252, 263

production, vii, 1, 12, 18, 100, 106, 108, 110,

159, 201, 214, 215, 218, 219, 221, 222, 227,

228, 229, 232, 258

propagation, 67, 69, 158, 160, 161

propane, 252

property, iv, 14, 57, 60, 191

Propranolol, 106

propylene, 246

protease inhibitors, viii, 105, 106

protection, 210

protein, 192, 240

protocol, 24, 168, 171, 172, 176

pseudo, 138, 205

Pseudomonas, 207

Index 276

Pseudomonas aeruginosa, 207

PT, 173

PTCs, 108, 173

PTFE, 224

pure water, 18, 43, 112, 136, 137, 139, 141, 142,

146, 147, 151

purification, x, 174, 220, 231, 232, 235, 244, 257,

259, 261, 262

PVP, 4, 6, 10, 31, 46, 47, 179, 239

pyrene, 235

pyrolysis, 3, 215, 245, 258, 261

pyromellitic dianhydride, 238

Q

QSAR, 156

quantum, 40, 44, 48, 61

quantum dot, 61

quantum yields, 40

quaternary ammonium, 172

quaternary ammonium salts, 172

R

radiation, 17, 30, 35, 49, 64, 66, 67, 77, 79, 81,

83, 85, 91, 92, 94, 97, 98, 169, 192, 199, 203,

208, 211, 228

radical formation, 2, 138, 211, 231

radical mechanism, 165

radical reactions, 107, 114, 203

radius, 2, 44, 68, 69, 70, 73

Raman, 31, 233, 237, 238, 239, 241, 243, 244,

246, 248, 251, 252, 257

Raman spectra, 31

Raman spectroscopy, 233, 237, 239, 241, 243,

244, 246, 248, 251, 257

range, ix, 3, 7, 8, 9, 10, 14, 17, 18, 19, 20, 23, 25,

29, 31, 36, 42, 69, 70, 79, 88, 129, 132, 133,

134, 137, 141, 147, 151, 152, 157, 158, 159,

160, 195, 206, 233, 262

rare earth, 17, 24, 55, 107

Rayleigh, 68, 69, 72, 78, 82, 103

reactant, 22, 174

reactants, 12, 47, 133, 137, 141, 152, 169, 172,

250

reaction mechanism, 47, 130, 144, 152, 153, 170

reaction medium, 138, 152

reaction rate, vii, 1, 19, 47, 49, 131, 134, 135,

136, 141, 142, 146, 147, 152, 153, 159, 162,

169, 172, 176, 177, 178, 254

reaction temperature, 17, 34, 44, 140, 162

reaction time, viii, 105, 107, 110, 112, 113, 114,

116, 158, 165, 166, 168, 170, 171, 173, 174,

175, 218, 227, 249

reactivity, viii, 105, 106, 107, 111, 112, 114, 117,

120, 129, 132, 133, 134, 145, 151, 158, 166,

252

reagent, viii, 22, 23, 105, 107, 109, 110, 111,

114, 115, 138, 141, 142, 146, 150, 158

reagents, viii, ix, 8, 105, 107, 108, 109, 111, 119,

120, 129, 130, 134, 137, 142, 143, 147, 150,

151, 152, 153

recrystallization, 221

recycling, 111, 175

refractory, 192, 198

regioselectivity, 171, 175

relationship, 73, 80, 88, 144, 147, 152

relationships, ix, 80, 129, 143

relaxation, 55, 140, 158

relaxation process, 140

relaxation processes, 140

relaxation time, 140

relaxation times, 140

reliability, 85, 130, 132

remediation, 204, 256, 263

resistance, 192, 193, 198, 199

resolution, 33, 120, 214, 234

restructuring, ix, 189

retardation, 71, 130, 141, 142, 153

rhodium, 34

rings, 141, 142, 234

RNA, 211

room temperature, 6, 9, 17, 20, 21, 35, 38, 46, 61,

79, 112, 113, 116, 117, 161, 166, 171, 172,

173, 175, 177, 185, 205, 218, 219, 220, 222,

233, 235, 240, 241, 254

room-temperature, 14

Royal Society, 120

ruthenium, 7, 10, 11, 50, 51, 184

rutile, 18, 53, 256

S

Saccharomyces cerevisiae, 207, 211

safety, vii, ix, 1, 133, 189, 195, 198

salt, 7, 40, 172, 180

salts, 114, 163, 172, 174, 179, 184

sample, 17, 21, 24, 36, 38, 42, 77, 140, 158, 221,

235, 236, 238, 244

sand, 157, 169

saturation, 25, 26, 35, 36, 85

scanning electron microscopy, 236

scattering, 14, 249, 254

SCW, 139

Index 277

SDBS, 240

SDS, 5, 7, 10, 11, 31, 32, 42, 236, 239, 241, 242,

246

SDT, 191

search, 110, 247

sedimentation, 238, 240, 254

selected area electron diffraction, 8, 35

selecting, 88, 98

selectivity, ix, 9, 10, 28, 107, 111, 157, 158, 170,

175, 176, 177

SEM, 5, 13, 15, 19, 29, 37, 41, 222, 223, 236,

242, 244, 245, 251

semiconductor, 18, 44

semiconductors, 2, 12, 44

sensors, 9, 20, 49

separation, x, 56, 111, 221, 231, 261

series, ii, 25, 42, 79, 91, 98, 149

serum, 14, 52, 196

serum albumin, 14, 52

shape, vii, 1, 2, 3, 4, 10, 12, 21, 36, 40, 46, 51,

66, 72, 77, 84, 87, 100, 198, 233

shear, 108, 191, 205

Shell, 10, 36, 40

shock, vii, viii, 2, 49, 63, 67, 71, 72, 73, 75, 78,

81, 83, 108, 131, 140, 160, 161, 202, 203, 204,

205

shock waves, 67, 72, 73, 131, 140, 160, 161, 202,

204

silica, 38, 40, 43, 51, 220, 232

silicon, 43, 234, 258

silver, 7, 8, 10, 25, 26, 29, 31, 32, 50, 51, 55, 56,

57, 58, 60

similarity, 146, 152

simulation, 66, 102

simulations, 102

single crystals, 40, 222

single walled carbon nanotubes, 258

single-crystalline, 41, 50, 59

single-wall carbon nanotubes, 258, 259, 261

SiO2, 38, 39, 40, 58

sites, 2, 9, 31, 34, 38, 138, 139, 140, 142, 245,

247

sludge, 153, 204

Sm, 21, 40, 41, 54, 107

snooker, 236

sodium, 5, 10, 14, 22, 24, 32, 33, 42, 45, 172,

236, 239, 240, 243, 246, 248

sodium dodecyl sulfate (SDS), 32, 236, 239

sodium hydroxide, 172, 248

solar, 18, 44, 208

solar cell, 18, 44

solar cells, 18, 44

solubility, 133, 213, 219

solvation, ix, 129, 130, 132, 133, 134, 135, 138,

139, 145, 146, 147, 149, 150, 151, 152, 153

solvent molecules, 134, 237

solvents, ix, 7, 47, 110, 129, 130, 133, 134, 135,

137, 138, 140, 143, 144, 145, 146, 150, 152,

174, 177, 216, 220, 226, 236, 237, 238, 243,

244, 259, 261

sonodynamic therapy, 192

soot, 220, 235, 236

sound speed, 67, 78

spatial, 66, 67, 72, 73, 77, 139

species, 3, 15, 23, 29, 119, 130, 131, 133, 134,

136, 139, 140, 147, 153, 161, 172, 191, 209,

210, 227

specific surface, 18, 20, 28, 34, 257

specificity, 227

spectrophotometric, 132

spectrophotometry, 218, 219

spectroscopy, 16, 220, 237, 239, 241, 242, 244,

246

spectrum, 11, 20, 35, 40, 222, 225, 227, 233, 237

speed, 26, 38, 66, 67, 78, 86, 102, 116, 131, 162,

176, 177, 203, 204, 210, 227

spheres, 39, 40, 45, 58

spin, 140, 203, 210, 211

spindle, 14, 15, 21, 22, 23, 43, 52

stability, 21, 35, 48, 67, 222, 226, 236, 240, 242,

245, 260

stabilization, ix, 6, 129, 130, 141, 142, 144, 146,

147, 153

stabilize, 134, 146

Staphylococcus, 29

Staphylococcus aureus, 29

steric, 111, 113, 119, 241

storage, vii, 2, 24, 44, 49

storageError! Bookmark not defined. media, 44

strain, 17, 91, 92, 93, 95

strains, 89, 91

strategies, 192, 198, 199

streams, ix, 157

strength, 36, 64, 66, 67, 68, 76, 83, 85, 86, 91, 94,

250, 252

stress, 83, 85, 86, 88, 204

stretching, 248, 255

stroke, 200

strontium, 25, 55

structural defect, 222

structural defects, 222

substitution, 169, 171, 172, 248

substitution reaction, 172, 248

substrates, 13, 111, 112, 113, 114, 131, 144, 172

sulfate, 10, 19, 32, 42, 236, 239, 247, 262

sulfonamides, 178, 186

Index 278

sulfur, 27, 30, 40, 56

sulfuric acid, 177, 186, 226, 234, 242, 244, 247,

250, 251

Sun, 52, 53, 54, 58, 59, 60, 121, 122, 123, 126,

184, 258, 261

supercritical, 138, 139

superoxide, 173, 184

supramolecular, 48, 61, 260

surface area, vii, 1, 18, 20, 22, 27, 34, 54, 97,

109, 111, 203, 256

surface energy, 232

surface modification, x, 231

surfactant, 4, 10, 17, 21, 31, 41, 42, 45, 54, 238,

239, 240, 241, 246, 249, 260

surfactants, x, 10, 231, 236, 246, 258

surgical, 196, 200

survival, 192, 199

survival rate, 192

surviving, 192

susceptibility, 151, 152

suspensions, x, 210, 227, 231, 236, 238, 241, 254

swelling, 111

SWNTs, 247, 260

symmetry, 76, 252

synergistic, 199, 256

synergistic effect, 256

systems, vii, viii, ix, 2, 63, 64, 84, 85, 91, 98,

100, 101, 105, 107, 112, 114, 116, 117, 120,

132, 134, 135, 136, 137, 138, 142, 144, 150,

151, 152, 153, 157, 161, 162, 174, 179, 204,

205, 206, 207, 233

T

tar, 198

TCC, 195

technology, 24, 81, 97, 106, 107, 108, 129, 153,

179

Tehran, 1

tellurium, 6, 47, 50, 107

TEM, 4, 6, 8, 10, 11, 13, 15, 16, 20, 23, 25, 26,

27, 28, 30, 32, 33, 34, 35, 36, 37, 38, 39, 40,

41, 42, 44, 45, 226, 232, 233, 234, 235, 238,

239, 242, 243, 244, 245, 246, 247, 248, 249,

250, 256

temperature dependence, 25

temperature gradient, 158

tensile, 66, 67, 68, 76

tensile strength, 66, 67, 68, 76

TEOS, 38

tetrahydrofuran, 7, 9, 218, 234

TGA, 246, 252

therapeutic goal, 190

therapy, ix, 129, 189, 190, 191, 192, 194, 196,

197, 198, 199

thermal decomposition, 36, 252

thermal degradation, 31

thermal energy, 36, 157

thermal load, 83

thermal stability, 35, 48, 245

thermal treatment, 42

thermodynamic, 107, 130, 147, 149

thermodynamic parameters, 147

thermogravimetric, 246

thermogravimetric analysis, 246

thermolysis, 215

thin film, 17, 30

three-dimensional, 134

threshold, ix, 67, 75, 76, 77, 80, 81, 87, 191, 194,

195, 196, 197, 201

time periods, 24

tin, 6, 19, 34, 50, 57, 112, 183, 250, 262

tin oxide, 19, 34, 57

TiO2, 18, 53, 209, 256, 257, 263

tissue, 109, 189, 190, 191, 195, 197

titanium, 18, 25, 36, 43, 46, 48, 53, 94, 96, 107,

115, 116, 117, 119, 132, 206, 214, 256

Titanium, 36, 58, 115

titanium dioxide, 18, 53, 206, 256

Tokyo, 103

tolerance, 192, 194

toluene, 18, 36, 220, 224, 238, 242, 244, 245

total energy, 67

toxic, viii, 105, 107, 110, 171, 202

toxicities, 189, 190

toxicity, 190, 191

trans, 16, 175, 176, 177

transcription, 193

transducer, viii, 63, 64, 65, 83, 84, 85, 86, 87, 88,

94, 97, 98, 100, 101, 159, 161, 197, 208

transesterification, 103

transfer, viii, ix, 20, 63, 64, 98, 106, 108, 109,

157, 160, 161, 165, 172, 173, 176, 183, 184,

226, 244, 248

transference, viii, 64, 85, 86

transition, 31, 72, 74, 75, 81, 83, 107, 134, 141,

146, 149

translational, 140, 160

transmission electron microscopy, 233

transparency, 82

transparent, 7, 14, 224

transport, 160, 192

transport processes, 160

transportation, 190

treatment methods, 261

trifluoroacetic acid, 165, 252

Index 279

tumor, 193, 194, 197, 200

tumor growth, 194

tumors, 189, 190, 198, 199

tumour, 199

tumours, 198

tungstates, 24

tungsten, 34, 57

tungsten carbide, 34, 57

tunneling, 250

Turbulent, 204

two-dimensional, 47, 48, 56, 59

U

UK, 156

ultrasonic vibrations, 83

ultrasonic waves, 10, 14, 19, 43, 140, 158, 165

ultrasonography, 195

ultraviolet, 206, 208, 210, 263

ultraviolet irradiation, 206, 210

uniform, vii, 1, 15, 29, 35, 38, 40, 44, 47, 49, 51,

72, 85, 160, 162, 244, 251

urea, 22, 25, 42, 164, 165

UV, 18, 20, 24, 54, 208, 209, 211, 218, 219, 220,

237, 239, 240, 241, 246, 256, 260

UV light, 24, 256

UV-radiation, 211

UV-Visible spectroscopy, 260

V

vacuum, 38, 221, 222, 233

values, 23, 64, 71, 72, 73, 75, 76, 78, 79, 80, 82,

83, 87, 88, 94, 96, 97, 98, 139, 147, 149, 150,

152

van der Waals, 222, 234, 238, 240, 244, 257

van der Waals forces, 222, 238

vanadium, 14, 15, 52, 114

vanadium oxides, 15

vapor, viii, 2, 63, 147, 214, 222

variation, 91, 147, 160

vascular endothelial growth factor, 200

vegetable oil, 173, 184

velocity, 67, 68, 69, 70, 72, 73, 75, 76, 77, 78, 79,

80, 81, 82, 85, 86, 91, 92, 93, 95, 96

vibration, 64, 67, 84, 85, 87, 89, 91, 94, 96, 161,

205, 233, 260

violent, 110, 202

viscosity, 98, 137

visible, 20, 21, 53, 54, 65, 238

VOCl3, 114

volatility, 77, 161

W

wastewater, 103, 204, 208, 209, 210, 263

wastewater treatment, 103, 204, 208, 263

water clusters, 145

water vapour, 203

water-soluble, 229, 238

wave number, 89

weak interaction, 138, 153

workers, 47, 165, 168, 176, 177, 203, 205, 208

X

xenografts, 195

XPS, 31, 35, 248, 250, 251, 252, 254

X-ray analysis, 11

X-ray diffraction, 48, 134, 137, 222, 223

X-Ray diffraction, 256

XRD, 14, 18, 19, 27, 30, 40, 55, 221

xylene, 43, 224, 232, 242

Y

yield, ix, 2, 36, 57, 110, 111, 112, 113, 114, 115,

116, 117, 118, 119, 157, 164, 165, 168, 169,

171, 172, 173, 174, 175, 182, 196, 227, 233,

234, 235

yttria-stabilized zirconia, 28

Z

zeolites, 168, 182

zeta potential, 254

zinc, 12, 24, 55, 110, 111, 112, 168, 174, 176,

184, 185, 206

Zinc, 22, 55, 110, 111, 174

zirconia, 28, 29, 56

zirconium, 31, 56

Zn, 12, 13, 22, 24, 107, 110, 111, 114, 115, 116,

117, 118, 174, 216, 232

ZnO, 12, 13, 14, 28, 29, 51, 52, 56, 232

ZnO nanorods, 12, 13, 28, 51, 52

ZnO nanostructures, 12, 13, 14, 51

73811957-sonochemistry-1617286524

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