Chapter I
General Introduction and Literature Review
GENERAL INTRODUCTION AND LITERATURE REVIEW
Liquid mixtures are indispensable in almost all industries and all biological
sciences. There exists an imperative need to understand these systems and to be
able to predict their behavior from the molecular point of view. The use of
measurable macroscopic or global quantities to probe into the microscopic, or the
local properties of the system has become an essential area of research. The types
of properties probed by these tools are local composition, local change of order, or
structure etc. The traditional characterization and study of the properties of liquid
mixtures by means of the global excess thermodynamic functions has become
handy as it provides richer and more detailed information on the immediate
environment of molecules in the mixture.
1.1 Molecular Interaction Studies
Molecular studies lie in the ability to assess the information stored in the
structure of molecule as a function of their physical and chemical properties. There
had been many developments in Chemistry and Physics during the 19th and 20th
centuries particularly in the fields of statistical mechanics, thermodynamics and the
nature of chemical bond. The ensuing atomistic view shall be presented and
discussed in the context of molecular interactions. Molecular interactions are
generally electrostatic in nature. The strength of these interactions and the forces
among atoms, can be analyzed according to their thermodynamic and kinetic
behaviour1.
1.1.1 Types of Molecular Interactions
There exist a wide variety of physical interactions relevant to the structure
and function viz., attractive or repulsive electrostatic interactions like ion-ion, ion-
dipole, dipole-dipole, dipole-induced dipole, induced dipole-induced dipole,
hydrogen bonded and hydrophobic interactions. The strength of intermolecular
interaction determines the physical properties of a substance2. These weak forces
are responsible for the secondary structure of biomolecules like carbohydrates,
proteins, nucleic acids and lipids and these forces are also responsible for the
specific biological functions3. Physical interactions can be either long range non-
specific or short range specific interactions. The long range interactions are the
2
first to control the repulsion or attraction between the molecules thereby
determining whether short range biophysical interactions come into play4.
Ion-ion interaction exists between point charges whereas ion-dipole
interaction is a force of attraction between a point charge and dipolar molecule.
The strength of these interactions determines the solubility of ionic and polar
compounds. Van der Waals forces between atoms and molecules are responsible
for the condensation or freezing even when there is no covalent or ionic bond
present between the atoms. Van der Waals interactions are categorized as weak
forces and they are relatively short range forces. The strength of interaction varies
inversely as the sixth power of distance between the molecules. Van der Waals
interaction represents three types of interactions namely orientation interaction
between two permanent dipoles, induction interaction between permanent and
induced dipoles and London dispersion interactions. The relative contribution of
each of these interactions to the total forces depends on the type of molecule. The
energy associated with van der Waals interaction is less than 1 kcal mol-1.
Hydrogen bond is a bond formed between two electronegative atoms with
hydrogen atom acting as a bridge. A hydrogen atom has only one ‘s’ orbital, which
becomes saturated after the formation of one covalent bond thus making it
incapable of forming a second covalent bond. However, the positive hydrogen can
polarize the lone pair orbital of another atom, delocalize the lone pair of electrons
and there by form a weak long covalent bond. Thus hydrogen bond is a simple
case of dipole-dipole (or ion-dipole) attraction. Hydrogen bonds have also been
found to exist between a carbon atom and an electronegative atom provided there
are some electronegative atoms attached to the carbon atom to activate the
hydrogen atom.
H - bonds have the following special features:
1. These are weak in nature, for example, they have energy of only ~ 2-
10 kcal mol-1 where as an ordinary covalent bond has energy of
about 80-100 kcal mol-1.
2. Increase in electronegativity of an atom increases its power of
forming hydrogen bonds.
3
3. Hydrogen ion in a hydrogen bond is not centrally situated but is
nearer to one of the two adjacent electro negative atoms5.
Although the hydrogen bond is not a particularly strong bond it has great
significance in determining the properties of substances. Because of its small bond
energy and the small activation energy involved in its formation and dissociation,
the hydrogen bond is expected to play an important part in reactions occurring at
normal temperatures. For example, the hydrogen bond is believed to be responsible
for the retention of the native configuration of protein molecules. Since it plays
such an important role in a number of different phenomena much work has been
carried out in recent years with the hope of the eventual elucidation of the nature of
the hydrogen bond6.
A variety of methods have been employed in the investigation of the
formation, strength and structure of hydrogen bonds. Among various methods,
physico-chemical and spectroscopic methods are widely used to study the
hydrogen bonds in organic liquids. Since the hydrogen bond is largely responsible
for the interaction between molecules of associated liquids, a comparison of the
properties displayed by substances capable of forming hydrogen bonds with those
of closely related compounds in which hydrogen bonding takes place unlikely
requires a qualitative understanding of the bonding. Hydrogen bonds and van der
Waals interactions are categorized as weak forces. Substances in which
intermolecular hydrogen bonds exist are usually associated and it influences heat
of vaporization, melting and boiling points and to some extent the dielectric
constants7, acidic strength basic strength reactivity of organic compounds8. The
hydrogen bonding is weaker than covalent bond but stronger than van der Waals
interactions9.
1.1.2 Molecular Interactions in Liquid Mixtures
The phenomenon of preferential interaction between molecules, between
unlike molecules, solute-solvent and similar ones, solute-solute is observed
depending on the degree of affinity between components. When solute dissolves in
the solvent, the structure of both the components may change due to reorganization
of the component molecules and the phenomenon of solvation and association are
4
observed depending on the nature of solvent. The molecular interaction studies in
solution provide information regarding the internal structure, molecular
association, complex formation and internal pressure.
1.2 Different Investigating Techniques & Literature Survey
The molecular interactions are investigated by employing different
techniques by many researchers. The nuclear magnetic resonance10,11,
microwave12,13, ultraviolet14,15, infrared spectroscopy16,17, X-ray18, viscometric19,20,
refractive index measurement21 and neutron scattering studies22 are frequently
employed techniques to identify the molecular interactions; the NMR technique
reflects the effect on the proton bearing molecule, the Microwave absorption and
refractive index measurement provide information through the dielectric constant,
the X-ray and neutron scattering help in the study of molecular motion, the
spectroscopic techniques provide information on molecular interactions. Weak
molecular interactions cannot be resolved from the observed spectra. Thus, the
molecular interactions cannot be identified thoroughly by a single technique.
However, the ultrasonic technique is a simple, non-destructive and low cost
technique that reveals successfully the inter-molecular interactions.
1.2.1 Ultrasonic Investigation of Molecular Interactions
Among various experimental techniques developed to obtain information on
the nature and strength of intermolecular interactions in dilute solutions, ultrasonic
technique has established itself as a promising tool. The propagation, dispersion
and attenuation of ultrasonic wave in a medium are intimately connected with the
structural aspect of the medium. The successful applications of acoustical methods
to physico-chemical investigation of the solutions become possible after the
development of adequate theoretical approaches and methods for precise ultrasonic
velocity measurements in small volume of liquids. In the year 1945 Lagemann23
successfully used the sound velocity approach for qualitative estimation of the
interactions in liquid mixtures. The measurement of the ultrasonic speed enables
the accurate determination of some useful acoustic and thermodynamic parameters
which are highly sensitive to molecular interactions thus provide physical nature
and strength of intermolecular interactions in liquid mixtures.
5
1.2.1.1 Literature Review on Ultrasonic Investigation of Molecular Interactions
in Liquids and Liquid Mixtures
Ultrasonic technology finds many applications in the fields of Chemistry,
Physics, Biology and Medicine. Ultrasonic measurements are very useful in
chemical and food processing, material testing, under water ranging and cleaning.
Ultrasonic vibrations are commonly employed in mechanical machinery of
materials24, preparation of colloids or emulsions, the pre-germination of seeds,
imaging of biological tissues and non-destructive testing (NDT). Ultrasonic AFM
(Atomic force microscope) can improve fabrication technologies on nanometer
scale25. Ultrasound eliminates friction at a nanometer scale26.
A study of ultrasonic propagation provides an important means of probing
the liquid state, as different molecular processes taking place in liquids can
influence directly or indirectly, the propagation constant of sound wave in the
medium. A wealth of data accumulated on this subject has appeared in excellent
reviews by Hertzfeld and Litovitz27, Nozdrev28, and also in the volumes edited by
Mason29, Flugge30, Bergmann31, Beyer and Letcher32. Ultrasonic investigations
find extensive application in characterizing physicochemical behaviour of liquid
mixtures. Tumikoski and Nurmi33 Fort and Moore34,35, Flory and co-worker36,37
have studied the non-ideal behaviour of binary liquid mixtures.
Ultrasonic methods are important to study the molecular association,
dissociation and complex formations. The intermolecular forces of liquids in a
liquid mixture show considerable effect on the physical and chemical effects. From
the knowledge of ultrasonic velocity, density and viscosity of a liquid, various
acoustic parameters such as intermolecular free length, isentropic compressibility,
free volume and internal pressure etc., can be obtained. In an ideal mixture in
which the components are non-interacting, the variation of density, viscosity and
velocity with the concentration is expected to be linear. Addition of another liquid
changes the values depending upon the nature of components. The ultrasonic
velocity and isentropic compressibility reflect the degrees of deviation from
ideality. Deviations from ideal behaviour have been widely used for the study of
structural variations and molecular interactions of the mixtures. These studies also
6
yield information about changes with composition in the packing efficiencies that
take place in solution during the mixing process38-41.
Schaaff16 introduced sound velocity in van der Waal’s equation of state and
obtained a formula for molecular radius from sound velocity and density values.
He compared the molecular radius calculated from sound velocity and density with
the value evaluated from molecular refraction. It was found that the molecular
radius obtained from sound velocity and density represents a very useful measure
for size of molecules. In another paper Schaaff17 derived an improved formula for
molecular radius by replacing the isothermal sound velocity with adiabatic sound
velocity.
Ultrasonic studies have been carried out to find out ion-solvent interactions
in electrolytic solutions containing different electrolytes and found that ultrasound
speed in these solutions depend on the size of the ion, polarity of the solvent and to
some extent on the electronic configuration of the transition metal ions. Ultrasound
has been extensively used to determine the ion-solvent interactions in aqueous and
non-aqueous solutions containing electrolytes42-48.
Number of studies has been focused on the ultrasonic properties of polymer
solutions49-51. The degree of rubber blends52 and other polymer blends53,54 by
ultrasonic technique is reported. The glass transition temperature, an important
parameter of polymers, is reported by Shanthi for some synthetic polymers55.
Soap–solvent interactions in soap solution are investigated in view to
determine critical micelle concentration (CMC) and action of mixed surfactants by
several authors using acoustical investigation56,57. In recent years, the ultrasound
technique is used to detect the charge transfer complexes in binary and ternary
liquid mixtures by nonlinear variation in the ultrasonic speed and other acoustical
parameters as a function of composition58,59.
1.2.2 Excess Thermodynamic and Viscometric Investigation of Molecular
Interactions in Binary Liquid Mixtures
Fundamental thermodynamic and viscometric properties are essential
sources of information necessary for a better understanding of the non-ideal
behaviour of complex systems because of physical, chemical and geometrical
7
effects, which are caused by molecular interactions, intermolecular forces, etc., of
unlike molecules. From a practical point of view, these properties are necessary for
the development of thermodynamic models required in adequate and optimized
processes of the chemical, petrochemical, pharmaceutical, and other industries. In
addition, extensive information about structural phenomena of mixtures is essential
for the development of theories of the liquid state and predictive methods.
The excess thermodynamic functions introduced by Scatchard in the year
1931, provided a way to represent directly the deviation of solution from ideal
behaviour. The excess functions are very useful in understanding molecular
interactions between components of liquid mixtures. By definition, the excess
function E idY Y Y represents the excess of a given quantity Y of a real
mixture over its value for an ideal mixture idY at the same conditions of pressure,
temperature, and composition60.
Various theories of solutions were developed to predict the properties of
liquid mixtures composition and independently observable properties of pure
components. These theories were formulated to account for the departure of a real
solution from the ideal behaviour. Theories concerning excess volumes of binary
liquid mixtures were thoroughly reviewed and discussed by Rowlingson61-63,
Flory64, Scott and Fenby65, Hijmens and Holleman66, Baattino67, Kehiaian68, Handa
and Benson69.
Van der Waals70 and van Laar71 proposed the initial theories of binary liquid
mixtures. These theories successfully explained certain excess properties in critical
region of liquid mixtures. In an attempt to improve van Laar’s theory, Hildebrand
and Scott72 and Scatchard73-76 used Hildebrand’s77 concept of regular solutions to
formulate a relation for excess volume. Priogogine et al78 have used the cell model
to extend the theory of corresponding states to chain molecules. In this approach
the chain molecules were considered as a series of quasi-spherical segments.
Eyring79 developed the cell model, which was extensively used by Lennard-Jones
and Devonshire80,81 and Prigogine et al82. This model relates the thermodynamic
properties of liquid mixtures to the intermolecular energy parameters.
8
Redlich and Kister83 proposed an empirical equation to predict EY values
for binary mixtures.
1
12121 )(
j
j
E
cal xxAxxY
where Aj-1 are adjustable parameters.
In general, the excess thermodynamic functions such as excess molar
volume, excess compressibility, excess internal pressure, excess free volume and
excess Gibb’s free energy are much useful in understanding the nature of
molecular interaction in binary and ternary liquid systems. Extensive studies of the
ultrasonic velocity in liquids and liquid mixtures and their interpretation in the
light of molecular structure were also made by several investigators84-89. The study
of the ultrasonic velocity and their interpretation in the molecular structure was
made by several workers90-95. From the study of ultrasonic velocity measurements
in a large number of organic liquids, the following general conclusions were drawn
about the dependence on molecular structure:
1. Liquids having higher density give lower ultrasonic velocity but not
necessarily in proportion.
2. Long molecules generally give rise to higher velocity even though their
density is higher.
3. Aromatic compounds have usually higher velocities than the aliphatic
compounds even though the density of aromatic compounds is higher.
4. A double bond of unsaturation is found to be resulting in low velocity.
5. Polar molecules have higher velocities in alcohols, ketones, acetic
anhydride, nitrobenzene, aniline, acetophenone, cyclo hexanol and
water.
6. A decrease in velocity is observed by substitution of a heavier atom in
place of lighter atom.
7. In non-polar groups, the viscosity of amines and alcohols will not differ
much and increase with increase in chain length.
8. In isomeric amines, the branched amines exhibit greater viscosity than
those of the straight chains.
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1.2.2.1 Previous Work on Excess Thermodynamic, Thermo-Acoustic and
Viscometric Investigation of Molecular Interactions in Binary Liquid Mixtures
Positive excess molar volumes and negative deviations in isentropic
compressibilities are reported by Syamala et al96 in the binary mixtures of dimethyl
sulphoxide with chloro and nitro substituted aromatic hydrocarbons. The viscosity
data is analyzed on the basis of corresponding states approach and the positive
deviations in viscosity explain the intermolecular interactions between the unlike
molecules in these mixtures.
Gadzuric et al97 calculated the isobaric thermal expansion coefficients,
partial molar volumes, apparent molar volumes, partial molar excess volumes and
excess thermal expansions for the binary liquid mixtures of N-ethylformamide
with tetrahydrofuran, 2-butanone and ethyl acetate over the entire composition in
the temperature range 293.15 to 313.15 K.
Sastry et al98,99 analyzed the effect of hydrophilic additives on volumetric
and viscosity properties of amino acids viz., glycine, l-valine, l-phenylalanine,
l-leucine, and l-aspargine in aqueous solutions of sucrose, urea and 1,3-butanediol
from 283.15 to 333.15 K and suggested that the solute–co-solute interactions are
more favored at elevated temperatures and in presence of high concentration of
sucrose. Otherwise the hydrophobic side chains facilitate the solute–solute
interactions and also time induced hydrophobic hydration in the bulk water.
To discover the nature and type of bulk state interactions present in the
binary mixtures of Alkyl (Methyl, Ethyl, Butyl, and Isoamyl) Acetates + Glycols
Satry et al99 determined densities, excess molar volumes, viscosities, speeds of
sound, excess isentropic compressibilities, and relative permitivities at different
temperatures. Further, they have correlated the experimental viscosities and
ultrasonic speeds with several theoretical models viz., Grunberg-Nissan,
McAllister, Auslander and collision factor theory etc.
Isabel et al100 reported the ultrasound speeds and molar isentropic
compressions of aqueous 1-propoxypropan-2-ol mixtures from 283.15 to 303.15 K
probing the role of branching and chain length on mixing and especially on
aggregation patterns. Kondaiah et al101 determined the ultrasonic velocities,
10
densities, and excess molar volumes of binary mixtures of N,N-dimethyl
formamide with methyl acrylate, or ethyl acrylate, or butyl acrylate, or 2-ethyl
hexyl acrylate at 308.15 K and published the positive values of VmE indicating the
presence of dispersion forces between the DMF and acrylic ester molecules.
Narendra et al102 investigated the excess parameters of binary mixtures of
anisaldehyde with o-cresol, m-cresol and p-cresol at 303.15, 308.15, 313.15 and
318.15 K. The negative and positive values of deviation or excess thermo-acoustic
parameters observed have been explained on the basis of the intermolecular
interactions present in these mixtures. Sangeeta et al103 measured experimental
densities, speeds of sound, and refractive indices of the binary mixtures {1-butyl-
3-methylimidazolium methylsulphate and methanol, or 1-propanol, or 2-propanol,
or 1-butanol} over the whole range of composition at T = 298.15, 303.15, 308.15,
and 313.15 K. From the experimental data, excess molar volumes, excess
isentropic compressibilities, deviation in refractive indices and molar refractions
were calculated. The Lorentz–Lorenz equation was applied to correlate the
volumetric properties and predict the density or the refractive index of the binary
mixtures. For all the systems studied, the excess molar volume and excess
isentropic compressibility are negative, while the change in refractive index on
mixing is always positive over the entire composition range and at all
temperatures.
The short-range dipolar interactions which lead to structural changes in
water - methyl, ethyl, n-propyl alcohol binary systems at 298.15 K are reported by
Hulya et al104. A comparative study of excess thermodynamic properties and
deviations in the thermodynamic properties in the binary mixtures using different
approximations is carried out by several researchers105-106. Anjali Awasthi et al107
studied the acoustic, volumetric, and spectroscopic properties of formamide with
2-alkoxyethanols at different temperature and reported the presence of extensive
hydrogen bonding between oxygen atom of CO group of formamide and hydrogen
atoms of the H–O group of 2-alkoxyethanol molecules in these binary liquid
mixtures.
11
The density and speed of sound of the binary mixtures of ethylenediamine
(EDA) with alcohols (1-hexanol, 1-octanol, and 1-decanol) were measured by
Dubey et al108 from 293.15 to 313.15 K and viscosity from 298.15 to 308.15 K
over the entire composition range and at atmospheric pressure. Using the
experimental values of density, viscosity and speed of sound, the excess molar
volume, viscosity deviation, deviation in speed of sound, deviation in isentropic
compressibility were calculated and fitted to the Redlich–Kister type polynomial
equation. Further, the viscosity data was analyzed in terms of Heric−Brewer and
McAllister models. Gonzalez et al109 enlightened the thermodynamics of (ketone +
amine) mixtures considering volumetric and speed of sound data at 293.15, 298.15,
and 303.15 K for (2-heptanone + dipropylamine or dibutylamine or triethylamine)
systems and indicated that structural effects increase with the ketone size in
mixtures with a fixed amine. They concluded that (i) interactions between unlike
molecules are more easily created in solutions containing shorter amines and (ii)
this effect predominates over the disruption of the amine–amine interactions.
These general trends were confirmed by the treatment of the mixtures using the
PFP theory, and the internal pressure concept.
Specific interactions between unlike molecules through hydrogen bonding
and dipole-dipole interactions between unlike molecules in the mixtures of 2-
chloroaniline or 3-chloroaniline with diisopropyl ether or oxolane are reported by
Pandiyan et al110 at different temperatures. Further, they have estimated the speed
of sound in these mixtures using several empirical and theoretical models to
determine their relative predicting ability in terms of pure component properties.
The values of ultrasonic velocity, density and viscosity measured by
Parveen et al111 in binary mixtures of tetrahydrofuran with methanol and o-cresol
at different temperatures along with estimated values of various thermo-acoustic
parameters suggest the existence of hydrogen bonding between unlike molecules
in all the mixtures studied and dispersive forces in therhydrofuran and methanol
mixture. An estimation of physical properties made by different theoretical
procedures, due to the strong dependence of the adequate industrial design on
12
computation and simulation, are found to be in good agreement with experimental
values.
Density, speed of sound, viscosity, and surface tension of the binary
mixtures N-ethyl-2-pyrrolidone + ethanolamine or diethanolamine or
triethanolamine were measured at different temperatures from 293.15 to 323.15 K
over the entire range of concentrations by Antonio et al112. The excess molar
volumes and isentropic compressibility deviations were calculated. Moreover, the
excess molar volumes were fitted using a Redlich−Kister equation, and the surface
tension data were fitted by the Connors−Wright model. They have concluded that
an increase in the substitution degree in the nitrogen atom causes an increase in the
magnitude of each property.
Slobodan Gadzuric et al113 calculated the isobaric thermal expansion
coefficients, partial molar volumes, apparent molar volumes, partial molar excess
volumes and excess thermal expansions for the binary liquid mixtures of
N-ethylformamide with tetrahydrofuran, 2-butanone, and ethyl acetate over the
entire composition in the temperature range 293.15 to 313.15 K.
Different physical properties (density, speed of sound, viscosity, and
refractive index) were measured for the system NMP + water + ethanol over the
entire composition range by Blanco et al114. They have reported that density was
found to increase with NMP concentration and excess molar volumes shows
negative values with large deviations that indicate the important interaction
Satyanarayana et al115 computed the excess isentropic compressibilities by
measuring the density and speed of sound of binary mixtures of N-methyl
acetamide with ethyl acetate, ethyl chloroacetate, and ethyl cyanoacetate over the
entire range of volume fraction in the temperature range of 303.15 to 318.15 K.
The results showed that the volume reduction factors are dominating in all the
systems over the entire range of temperatures studied.
Fan-Li et al116 measured densities as a function of composition for a
ternary–pseudo binary mixture of {(styrene + ethyl acetate or benzene) + (N,N-
dimethylformamide + ethyl acetate or benzene)} at atmospheric pressure and the
temperature 298.15 K and showed that the third component, ethyl acetate or
13
benzene, have a significant influence on the interaction between styrene and
N,N-dimethyl formamide.
Excess volumes, speeds of sound and viscosities were measured for binary
mixtures of methyl acetate, ethyl acetate, butyl acetate, isoamyl acetate with n-
butyl amine and tert-butyl amine at 303.15 K over the entire range of composition
by Sankara Reddy et al117,118. Isentropic compressibilities were computed from
speed of sound and density data. Speeds of sound were evaluated using Jacobson
free length theory and Schaaff’s collision factor theory. The viscosity data were
analyzed on the basis of corresponding states approach and Grunberg and Nissan
treatment. The experimental results on excess volume, deviation in isentropic
compressibility and in viscosity were discussed in terms of molecular interactions
between unlike molecules.
Nikam et al119,120 measured the densities and viscosities at 298.15, 303.15,
and 308.15 K for binary mixtures of ethyl acetate with pentan-1-ol, hexan-1-ol,
3,5,5-trimethylhexan-1-ol, heptan-1-ol, octan-1-ol, decan-1-ol, linear and branched
alkanols (C1-C4). Molar excess volume and deviation in viscosity were computed
from the measured data and fitted to the Redlich-Kister polynomial. From the
results, it was observed that VE values of all binary systems were found to increase
with an increase of temperature due to decreased ester-ester and alkanol-alkanol
contacts and to the structural contributions arising from the geometrical fitting of
one component (methanol) into the other (ethyl acetate) owing to differences in the
molar volumes between components. Further, they reported that even though the
geometrical contribution seems to be negligible with alkanols (C2-C4) but it was
found to be increasingly significant in the case of higher alkanols and deviations in
viscosity were negative in all the systems and become more negative with an
increase in chain length and branching of alkanols.
Aminabhavi et al121,122 investigated the thermo-physical properties of the
binary mixtures of Methyl Acetate, Ethyl Acetate, n-Propyl Acetate, and n-Butyl
Acetate with 2-Chloroethanol and acetonitrile. They observed that the relative
Lorenta-Lorentz molar refraction values decrease systematically with increasing
14
size of the ester molecules i.e., form methyl acetate to butyl acetate in these binary
mixtures.
Palaiologou et al123,124 and Lee et al125 established specific interactions
between component molecules in the binary mixtures of Dimethyl Sulfoxide +
butyl acetate , 4-chloro toluenes + alkyl ester and nitro methane + alkyl ester by
investigating densities, viscosities, refractive indices, and surface tensions of these
binary mixtures at 293.15, 298.15, 303.15, and 313.15 K. Negative excess molar
volumes are reported by several researchers126-129 in the binary mixtures of NMP
indicating strong interactions between unlike molecules owing to dipole-dipole,
charge-transfer or hydrogen bonded interactions.
Awwada et al130 reported the negative excess molar volumes and difference
in the partial molar volumes in the binary mixtures of NMP with some aromatic
hydrocarbons, which follow the order: toluene > p-xylene > benzene > o-xylene >
m-xylene. Further, they suggested that the contraction in the volume of the mixture
on mixing, is due to the charge transfer complex formation and the partially
interstitial accommodation of aromatic molecules in the empty spaces in 2-
pyrrolidone structure. Partial molar volumes and isentropic compressibilities of
polycyclic aromatic hydrocarbons in 1-methyl-2-pyrrolidone at 298.15 K are
reported by Takuya et al131. The results are attributed to the formation of
intermolecular interactions in mixture, like dipole–induced dipole intermolecular
interactions between the polycyclic aromatic hydrocarbon and NMP. A shift to the
long wavelength was observed in the peak of UV spectra of benzene and
naphthalene in the polar solvents suggesting the existence of an induced effect in
the intermolecular interactions.
The excess molar Gibb’s energies for the binary mixtures of NMP with
several chloroalkanes are reported by Gimenez and co-workers132 through the VLE
studies in these mixtures. The theoretical DISQUAC group model used by them to
examine the results, has reproduced the experimental data quite well.
1.2.3 Spectroscopic Investigation of Molecular Interactions
Spectroscopic techniques like infrared (IR) and nuclear magnetic resonance
(NMR) have been extensively used to investigate solution structure and provide
15
physical information about intermolecular interaction133. The infrared spectrum
which gives significant information about the functional groups can be
substantially influenced by the surrounding condensed medium. The resultant
effects in IR spectra of liquid mixtures, due to intermolecular association (between
unlike molecules) are mainly peak position shifting (solvatochromic shift), change
in the intensity of the spectral line and change in the shape and width of the
spectral band etc.
1.2.3.1 Introduction to Infrared Spectra
An invaluable tool in organic structure determination and verification
involves the class of electromagnetic radiation with frequencies between 4000 and
400 cm-1. Chemical bonds in different environments will absorb varying intensities
and at varying frequencies. Thus IR spectroscopy involves collecting absorption
information and analyzing it in the form of a spectrum. The frequencies at which
there are absorptions of infrared radiation ("peaks" or "signals") can be correlated
directly to bonds within the compound under study134.
A molecule absorbs only selected frequency (frequencies) of infrared
radiation and is excited to a higher energy state which occurs when frequency of
infrared radiation exactly matches a natural vibrational frequency of the molecule.
A net transfer of energy takes place during energy absorption. The energy
absorbed serves to increase the amplitude of the vibrational motions of the bonds
in the molecule and changes the bond angles. Not all bonds in a molecule are
capable of absorbing infrared energy even if frequency of the radiation exactly
matches that of the bond motion. Only those bonds having a dipole moment absorb
infrared energy135.
Hydrogen bonding is formed between a function group and an atom or
group of atoms in the same or different molecules. It can exist in the solid and
liquid phases or in solutions. Hydrogen bonding can be detected in many ways, but
the most important way is by the infrared (IR) spectroscopy.
Because each inter atomic bond vibrates in a different way (stretching or
bending) individual bonds may absorb more than one infrared frequency.
Stretching absorptions usually produce stronger peaks than bending, however the
16
weaker bending absorptions can be useful in differentiating similar types of bonds
(e.g. aromatic substitution). It is also important to note that symmetrical vibrations
do not cause absorption of infrared radiation. A molecule consisting of n atoms has
a total of 3n degrees of freedom, corresponding to the Cartesian coordinates of
each atom in the molecule. In a nonlinear molecule, 3 of these degrees are
rotational and 3 are translational and the remaining corresponds to fundamental
vibrations; in a linear molecule, 2 degrees are rotational and 3 are translational.
The net number of fundamental vibrations for nonlinear and linear molecules is
therefore136 3n-6 and 3n-5 respectively.
Symmetric stretching
vibrations
In-plane bending
vibrations (Scissoring)
Out-of-plane bending
vibrations (Wagging)
Asymmetric Stretching
vibrations
In-plane bending
vibrations (Rocking)
Out-of-plane bending
vibrations (Twisting)
Fig. 1.1 Different types of stretching and bending vibrational modes.
The stretching frequency of a bond can be approximated by Hooke’s Law.
In this approximation, two atoms and the connecting bond are treated as a simple
harmonic oscillator composed of 2 masses (atoms) joined by a spring.
17
Figure 1.2 Energy curve for a simple harmonic oscillator (left) and energy
constrained to quantum mechanical model (right).
The energy curve for a simple harmonic oscillator is illustrated in Figure
1.2. According to Hooke’s law, the frequency of the vibration of the spring () is
related to the mass and the force constant of the spring by the following formula:
1
2
k
m
where k is the force constant, m is the mass.
A molecule is not just two atoms joined on a spring, of course. A bond can
come apart, and it cannot be compressed beyond a certain point. A molecule is
actually an anharmonic oscillator. As inter atomic distance increases, the energy
reaches a maximum, as seen in Figure 1.3. Note how the energy levels become
more closely spaced with increasing inter atomic distance in the anharmonic
oscillator. The allowed transitions, hν, become smaller in energy. Therefore,
overtones can be lower in energy than predicted by the harmonic oscillator theory.
Figure 1.3 Energy curve for an anharmonic oscillator.
18
The following formula has been derived from Hooke’s law. For the case of
a diatomic molecule, ( has been substituted for ν, recall that ν = c ):
1 2
1 2
( )1
2
f m m
c m m
where is the vibrational frequency, m1 and m2 are the mass
of atoms 1 and 2, respectively, ‘c’ is the velocity of light, ‘f’ is the force constant
of the bond. This equation shows the relationship of bond strength and atomic
mass to the wave number at which a molecule will absorb IR radiation. As the
force constant increases, the vibrational frequency (wave number) also increases.
The force constants for bonds are: single bond 5 x 105 dyne cm-1; double bond 10 x
105 dyne cm-1 and triple bond 15 x 105 dyne cm-1
As the mass of the atoms increases, the vibration frequency decreases. The
regions of an IR spectrum where bond stretching vibrations are seen depends
primarily on whether the bonds are single, double, or triple or bonds to hydrogen.
Although a useful approximation, the motion of two atoms in a large molecule
cannot be isolated from the motion of the rest of the atoms in the molecule. In a
molecule, two oscillating bonds can share a common atom. When this happens, the
vibrations of the two bonds are coupled. As one bond contracts, the other bond can
either contract or expand, as in asymmetrical and symmetrical stretching. In
general, when coupling occurs, bands at different frequencies are observed, instead
of superimposed (or degenerate) bands as you might expect from two identical
atoms in a bond vibrating with an identical force constant.
If hydrogen bonding is possible between solute and solvent, this greatly
increases solubility and often results in large or even infinite solubility. There is
evidence that double and triple bonds, aromatic rings can form hydrogen bonds
with polar functional groups, but these bonds are very weak. In many cases, there
is partial hydrogen bonding in dilute solution, that is, some functional groups are
free and some are hydrogen bonded. In such cases two IR bands appear. IR
spectroscopy can also distinguish between inter- and intra-molecular hydrogen
bonding, since intermolecular bands are intensified by an increase in concentration
while intra-molecular bands are unaffected.
19
Table 1.1 Characteristic IR Absorptions
Frequency (cm-1) Bond Functional Group
20
1.2.3.2 Introduction to Nuclear Magnetic Resonance
NMR spectroscopy is one of the principal techniques used to obtain
physical, chemical, electronic and structural information about molecules due to
either the chemical shift, Zeeman effect, or the Knight shift effect, or a
combination of both, on the resonant frequencies of the nuclei present in the
sample. It is a powerful technique that can provide detailed information on the
topology, dynamics and three-dimensional structure of molecules in solution and
the solid state. Thus, the structural and dynamic information is obtainable from
NMR studies of quadrupolar nuclei even in the presence of magnetic "dipole-
dipole" interaction broadening (or simply, dipolar broadening) which is always
much smaller than the quadrupolar interaction strength because it is a magnetic
versus an electric interaction effect.
Table 1.2 Characteristic 1H1 NMR chemical shifts for different classes of
compounds
1.2.3.3 Literature Review on spectral analysis of molecular interactions in
binary liquid mixtures
Study on intermolecular interactions in liquid acetonitrile - propan-1-ol
mixtures is carried out by Kinart et al137 measuring the 1H NMR spectra and
analyzing their physicochemical properties. It is concluded that active electron
donor sites of acetonitrile lead to the formation of H-bonds with OH proton of
alcohols in these mixtures.
21
Mehta et al138 reported the positioning of maxima/minima at about the same
mole fraction of DMF in excess chemical shifts of 1H and 13C NMR values and
excess molar volume for the binary mixtures of DMF with butane diols. It is
concluded that carbonyl group of DMF and hydroxyl groups of butanediol are the
two active sites of interactions in these mixtures whereas in butanediol and
pyrrolidone systems139, hydroxyl groups of butanediol is the only active site of
interactions
Bricknell et al140 found some relationships between the infrared
spectroscopic properties of water molecules hydrogen bonded to a number of bases
viz., amines, ethers, nitriles etc., in binary liquid mixtures, and the partial molar
excess enthalpies at infinite. The results suggest new approach to the use of the
well-known Badger-Bauer relationship. The general adoption of the use of the
partial molar excess enthalpy at infinite dilution of a solute in a solvent as the
property of choice for correlating infrared spectroscopic and thermodynamic data
for such binary liquid systems is proposed.
Solvation properties of aliphatic alcohol–water and fluorinated alcohol–
water solutions are probed by Takamuku et al141 taking amide molecules as solutes
using infrared (IR) and 1H and 13C NMR techniques. They suggested that
hydrophilic moieties of both carbonyl and amino groups of amide molecules are
hydrogen-bonded with water molecules in aqueous amide solutions and gradual
weakening of hydrogen bonds with the increase of alcohol content is observed in
both aliphatic alcohol–water and fluorinated alcohol–water solutions.
Aashees Aswathi et al reported the presence of hydrogen bonding between
unlike molecules in the mixtures of pyridine and quinoline with phenol and in the
mixtures of DMSO with phenol and o-Cresol142,143 based on the FT-IR spectral
analysis of their mixtures. The C H--O hydrogen bond is of increasing interest
because of the significant role in protein conformation, recognized lately144-146.
Dependence of 1:1 intermolecular complex existing in the whole range of the
mixture composition on the environment is established by Stangret et al147 using
IR spectral analysis of DMF + methanol binary mixtures.
22
1H-NMR spectra in binary mixtures of sulfolane with ethylene glycol,
diethylene glycol, triethylene glycol and tetraethylene glycol have been recorded
over the whole composition range at 303 K under atmospheric pressure by Kinnart
et al148. They have suggested that the specific interaction between sulfolane and
glycols increases as the glycol carbon chain length increases and it is also as a
result of molecular size differences.
Based on the concept of local composition, the 1H NMR chemical shift data
of the OH proton over the whole concentration for alcohol + hydrocarbon systems
are correlated by Yingjie et al149. They have concluded that using 1H NMR
spectroscopy coupled with only one activity coefficient at infinite dilution, the
viscosity for alcohol + hydrocarbon systems can be accurately predicted.
The application of IR spectroscopy in the development of thermodynamic
models for the solutions in which association and solvation occur simultaneously
is presented by Asprion et al150. Extensive IR spectroscopic studies of binary and
ternary solutions where solvation either occurs between an alcohol and a solvent or
between two alcohols were performed at around 283–313 K. The results showed
that a quantitative interpretation of the monomer-band is possible in the systems
studied. They reported that the IR spectrum of the monomer band splits into two
bands, one resulting from free, the other from solvated monomers.
The existence of hydrogen bond between of –C=O group of
N,N-dimethylacetamide with –OH group of propan-1-ol and propan-2-ol is
confirmed through FT-IR spectra by Lakshminadh et al151. They have concluded
that the theoretical FT-IR values determined using Hartree-Fock and Density
Functional Theory methods (with 6-31+G* and 6-311+G** basis sets) are in
reasonable agreement with the experimental values.
1.3 Nature and Scope of the Present Work
Ultrasonic velocity in a medium is fundamentally related to the binding
forces between the molecules. Ultrasonic velocities of the liquid mixtures are of
considerable importance in understanding intermolecular interaction between
component molecules and find applications in several industrial and technological
processes152-154. Ultrasonic velocity measurements have been employed
23
extensively to detect and assess weak and strong molecular interactions in binary
mixtures, because mixed solvents find practical applications in many chemical and
industrial processes. Increasing use of N-methyl-2-pyrrolidone (NMP) in many
industrial processes have greatly stimulated the need for extensive information on
the acoustic and transport properties of it and its mixtures.
Detailed literature survey shows that work has been carried out by different
researchers on excess molar volumes of binary mixtures containing NMP + an
alkanols or hydrocarbon156-158, + an ether159, + ketone160,161, + aromatic
hydrocarbons162,163, + water164, + methanol165 etc. However, no effort appears to
have been made to investigate the physico-chemical properties, for the binary
mixtures of NMP with substituted benzenes or aliphatic esters or alkyl amines.
The literature survey on the ultrasonic studies indicates that enormous work
has been carried out in binary and ternary liquid mixtures of weak and strong
interacting systems and very few studies are reported with NMP as main
component. Moreover, thermodynamic properties of these liquid mixtures are of
interest for different branches of science and engineering and also play significant
role in technological processes, biological process of living organisms and in
nature. This fact has encouraged the author to carry out a series of systematic
investigations on the solvent properties of these liquid mixtures.
N-methyl-2-pyrrolidone (NMP) is an important solvent as it is water-
miscible, hygroscopic, colorless, and strongly polar liquid. NMP has the potential
for use in, solvent extraction process as strong solubilizing agent166, purification
and crystallization of drugs167. It is also used in the manufacture of various
compounds, including pigments, cosmetics, insecticides, herbicides, and
fungicides. Its non-hazardous and ecological properties account for the reality that
it is increasingly being used as an alternative for chlorinated hydrocarbons.
In the chemical industry, there exists a continuing need of reliable
thermodynamic data of binary liquid systems. This is particularly true for systems
involved in industrial process. With this objective in mind and the lack of
availability of the correlation studies between spectroscopic and thermo acoustic
parameters in the NMP and substituted benzenes or aliphatic esters or alkyl amines
24
systems in literature has encouraged the author to carry out a series of systematic
investigations on the properties of the binary mixtures of NMP with substituted
benzenes, aliphatic esters and alkyl amines.
The present work is focused on the study of acoustic, volumetric and
viscometric properties, by measuring the ultrasonic velocities, densities and
viscosities of the binary mixtures of NMP viz.,
I N-methyl-2-pyrrolidone (NMP) + Aminobenzene (AB)
N-methyl-2-pyrrolidone (NMP) + Chlorobenzene (CB)
N-methyl-2-pyrrolidone (NMP) + Bromobenzene (BB)
II N-methyl-2-pyrrolidone (NMP) + Methylacetate (MA)
N-methyl-2-pyrrolidone (NMP) + Ethylacetate (EA)
N-methyl-2-pyrrolidone (NMP) + Butylacetate (BA)
III N-methyl-2-pyrrolidone (NMP) + Propylamine (PA)
N-methyl-2-pyrrolidone (NMP) + Butylamine (BAM)
N-methyl-2-pyrrolidone (NMP) + Dipropylamine (DPA)
over the complete composition range at temperatures of 303.15, 308.15, 313.15
and 318.15 K and atmospheric pressure. It is also aimed to study the molecular
interactions between the unlike molecules of the binary liquid systems under
investigation, with special reference to dipole–dipole and hydrogen bonding
interactions with the help of spectral (Fourier Transform Infrared and 1H NMR
Spectra) data of these mixtures. Further, the experimental data of these binary
mixtures is used to test the applicability of empirical relations of, Nomoto, Van
Dael and Vangeel, Schaaff’s collision factor theory, Rao’s specific velocity,
impedance relation and Junjie’s equation for ultrasonic velocity and Grunberg–
Nissan, Katti–Chaudhri, Heric–Brewer, McAllister (4-body model) and Hind et al
for viscosity. Also, the effect of temperature on the shapes of the interacting
molecules is analyzed using Scaled Particle Theory (SPT).
25
The following thermodynamic and thermo-acoustic parameters evaluated
from the measured values are used for understanding the nature of the interactions
in the binary mixtures under study:
molar volumes ( mV ) in m3 mol-1,
partial molar volumes ( imV , ) in m3 mol-1,
apparent molar volumes ( iV , ) in m3 mol-1,
excess molar volumes ( E
mV ) in m3 mol-1,
excess partial molar volumes ( E
imV , ) in m3 mol-1,
isentropic compressibility (s) in Pa-1,
excess isentropic compressibility ( E
s ) in m2 N-1,
isobaric thermal expansion coefficient (p) in K-1
excess isobaric thermal expansion coefficient ( E
P ) in K-1
inter molecular free length (Lf) in m,
excess intermolecular free length ( E
fL ) in m,
deviation in viscosity () in m Pa.s,
acoustic impedance (Z) kg m-2 s-1,
excess acoustic impedance (ZE) in kg m-2 s-1,
excess velocity (UE) m s-1,
free volume ( fV ) in m3 mol-1,
excess free volume ( E
fV ) in m3 mol-1,
internal pressure ( i ) in N m-2,
excess internal pressure ( E
i ) in N m-2,
viscous synergy (Is)
viscous relaxation () in s,
classical absorption coefficient (/f2)
cohesive energy (H) in J mol-1,
Gibb’s free energy of activation of viscous flow ( EG* ) in J mol-1.
26
The data obtained can be used to understand intermolecular interactions
between the unlike molecules and to test the theories of solutions. The departure of
these real mixtures from ideal behaviour can be explained in terms of effect of
hydrogen bond breaking, loss of dipolar association, differences in size and shapes,
dipole-dipole interactions between different component molecules etc.
As far as possible, S.I. units are used but there are some circumstances
where this system has no particular advantage. Duplication of some mathematical
symbols has been unavoidable but confusion should not arise because their
meanings are clearly explained in the appropriate parts of the text.
27
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