analytical investigation of the chemical reactivity and stability. mathieu etienne, alain walcarius

7/27/2019 Analytical Investigation of the Chemical Reactivity and Stability. Mathieu Etienne, Alain Walcarius

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Analytical investigation of the chemical reactivity and stability

of aminopropyl-grafted silica in aqueous medium

Mathieu Etienne, Alain Walcarius *

Laboratoire de Chimie Physique et Microbiologie pour lEnvironnement, Unite Mixte de Recherche UMR 7564, CNRS-Universite H.

Poincare Nancy I, 405 Rue de Vandoeuvre, F-54600 Villers-les-Nancy, France

Received 9 September 2002; received in revised form 12 December 2002; accepted 20 December 2002

Abstract

Various samples of aminopropyl-functionalized silica (APS) have been prepared by grafting an organosilane

precursor 3-aminopropyl-triethoxysilane (APTES) onto the surface of silica gel. The amine group content of the

materials has been adjusted by varying the amount of APTES in the reaction medium (toluene). The grafted APS solids

have been characterized with using several analytical techniques (N2 adsorption, X-ray photoelectron spectroscopy,

infrared spectrometry) to determine their physico-chemical properties. Their reactivity in aqueous solutions was studied

by acid-base titration, via protonation of the amine groups, and by way of complexation of these groups by HgII

species. APS stability in aqueous medium was investigated at various pH and as a function of time, by the quantitative

analysis of soluble Si- or amine-containing species that have been leached in solution upon degradation of APS. The

chemical stability was found to increase when decreasing pH below the pKa value corresponding to the RNH3'/RNH2

couple, but very low pH values were necessary to get long-term stability because of the high local concentration of the

amine groups in the APS materials. Adsorption of mercury(II) ions on APS was also performed to confirm the long-

term stability of the grafted solid in acidic medium. Relationship between solution pH and APS stability was discussed.

For sake of comparison, the stability of APS in ethanol and that of mercaptopropyl-grafted silica (MPS) in water have

been briefly considered and discussed with respect to practical applications of silica-based organic/inorganic hybrids,

e.g., in separation science or in the field of electrochemical sensors.

# 2003 Elsevier Science B.V. All rights reserved.

Keywords: Aminopropyl-grafted silica; Acid-base reactivity; Chemical stability; Dissolution kinetics; Analytical investigation

1. Introduction

The application of organic/inorganic hybrid

materials in various fields of chemistry, and

especially in analytical sciences, is a current area

of research [1/5]. Indeed, these solids have the

advantage to combine in a single material the

properties of both components: the rigid three-

dimensional inorganic skeleton imparts mechan-

ical stability, while adequately chosen organic

functions bring a specific chemical reactivity. In

this variety of materials, the organically modified* Corresponding author. Fax: '/33-3-83-27-54-44.

E-mail address: [email protected] (A. Walcarius).

Talanta 59 (2003) 1173/1188

www.elsevier.com/locate/talanta

0039-9140/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0039-9140(03)00024-9
mailto:[email protected]:[email protected]


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silicas have recently attracted considerable atten-

tion and gave rise to a wide range of applications,

mainly in analytical separations, electrochemistry

and sensors [5/10]. Such success is partly due tothe versatility of the sol/gel process to prepare a

wide range of materials with predesigned composi-

tion and structure, in various shapes, and with

many different properties [11,12]. For example,

this process was exploited for enzyme immobiliza-

tion in inorganic matrices without activity loss and

permitting, therefore, biosensor development [13],

or to produce ceramic-carbon composite electro-

des [14], or associated to the screen-printed

technology to get disposable sensors [15]. Incor-

poration of as-synthesized materials into carbonpaste electrodes for preconcentration analysis of

metal ions was also reported [16/18]. Moreover,

the use of template molecules associated to the

sol/gel process has allowed the huge development

of ordered mesoporous silicas during the 1990s

[19], for which the organically modified forms are

very promising, e.g., for solid-phase extraction of

heavy metal species from diluted solutions [20/22].

Amine-functionalized silicas have been widely

studied in their solid phase [23], and silica-based

materials containing either aminopropyl groups ormore complex ligands bearing amine functions,

which were either covalently attached to the

inorganic network or simply impregnated on a

silica surface, have been often proposed as solid

extractants for heavy metal species [24/32]. Ex-

amples are available for CuII [24/30], CdII and

HgII [31,32], and some other such as CoII, NiII,

ZnII or PbII [24,27,30/32]. The binding ability of

amine-bearing silicas was also exploited in electro-

analysis, e.g., for the voltammetric detection of

trace CuII after accumulation at electrodes mod-

ified with such solids [16,17]. To be efficient, allthese applications would require the chemical

stability of the hybrid material, at least in the

particular conditions and within the time scale of

the experiments. This aspect in relation to trace

metal extraction from aqueous medium was,

however, sparingly considered in the past, and

most often not at all.

The chemical stability of silica in aqueous

medium is pH-dependent and decreases signifi-

cantly in alkaline solutions [33]. When the silica

surface is functionalized with amine groups, it is

expected that the basic character of these functions

would affect the overall chemical stability (and

reactivity) of the resulting hybrid aminopropyl-functionalized silica (APS) material. Covalent

coupling between a silica network and aminopro-

pyl groups usually proceeds with using the APTES

precursor that is either grafted on an as-synthe-

sized silica in organic solvent [23] or co-conden-

sated with another silica precursor (e.g.,

tetraalkoxysilane) in hydroalcoholic medium lead-

ing to the one-step formation of aminopropylsi-

loxane gels [34]. Interest in the covalent linkage

between the inorganic structure and the organic

groups arises from the non-hydrolyzable Si/Cbond in the organosilane, which prevents from

leaching of the immobilized reagent in the external

solution contrary to impregnation [25]. However,

this advantage is only valid if no other degradation

pathway (i.e., alkaline attack) is liable to transfer

gradually the organic modifier into the solution.

This may occur with APS materials in aqueous

medium via the hydrolysis of siloxane bonds

owing to the basic properties of the amine func-

tions [34].

The acid-base properties of APS hav

e beenpreviously characterized by Zhmud et al. [34/36].

It was especially shown that hydrogen bonds

between amine groups and residual silanols (hy-

droxyl groups present on the silica surface) can

arise from sprawling aminopropyl tails on the

surface [35]. In the presence of water, this interac-

tion promotes proton transfer from silanols to

amine groups, which leads to the formation of

zwitterion-like moieties (/SiO(,'H3N/) on the

silica surface [35,36]. The amine groups can be

protonated in acidic medium, this process being,

however, rather slow owing to restricted diffusionin the porous material [37]. Moreover, they are

soluble to some extent in aqueous medium (owing

to hydrolysis of siloxane bonds, which is favored

at high pH values), pointing out the lack of

stability of such materials in water [34]. This

relative chemical instability was also mentioned

in some other reports [38/40]. Except these few

investigations, and despite the large record of

works dealing with the use of amine-functionalized

silicas for removal of heavy metal ions [24/32], no

M. Etienne, A. Walcarius / Talanta 59 (2003) 1173/11881174


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detailed studies on the main parameters affecting

the chemical stability of these materials in aqueous

medium are available.

In this work, we have thus examined the acid-base reactivity and chemical stability of silica gel

samples grafted with aminopropyl groups in aqu-

eous solutions. Various APS samples have been

prepared, containing different amounts of grafted

ligands. They have been characterized in solution

by acid-base titration and quantitative analysis of

their degradation products. Effects of pH and

contact time with water were thoroughly investi-

gated and critically discussed by taking into

account the high local concentration of amine

groups in APS. A mercaptopropyl-grafted silicagel (MPS) was used for comparison purpose.

Better understanding the basic chemistry of APS

in solution in a wide range of experimental

conditions would contribute to better defining

those required for optimal applications of these

materials as solid-phase extractants or as electrode

modifiers.

2. Experimental

2.1. Chemicals and solutions

All solutions were prepared with high-purity

water (18 MV cm) from a Millipore milliQ water

purification system. Nitric acid (min. 65%) and

sodium acetate were purchased from Riedel de

Haen, HCl was obtained from Prolabo. Silica gel

was the chromatographic grade Kieselgel Geduran

60 from Merck (average particle size: 70 mm). The

reactants 3-aminopropyl-triethoxysilane (APTES)

99% and 3-mercaptopropyl-trimethoxysilane

(MPTMS) 95% were, respectively, purchasedfrom Aldrich and Lancaster. Hg(NO3)2 (Fluka),

BuNH2 (!/99%, Aldrich), dry toluene (99%,

Merck), and ethanol (95/96%, Merck) were used

as received.

2.2. Synthesis of grafted silicas

Grafting the silica surface by covalently attach-

ing aminopropyl or mercaptopropyl functional

groups proceeds via a reaction between silanol

groups and an appropriate organosilane (APTES

or MPTMS) in dry toluene [23,32]. Typically, 5 g

of silica sample are dispersed in 50 ml dry toluene

and stirred for a few minutes at room temperature;selected amounts of APTES (ranging between 20

and 0.060 ml), or 5 ml MPTMS, is then slowly

added to the suspension and refluxed for 2 h

(APTES) or 24 h (MPTMS). After slow cooling,

the resulting solids are filtered, washed with

toluene, and dried under reduced pressure for 24

h. The aminopropyl-grafted samples are heated at

120 8C for 12 h. The grafted materials are called

afterwards APS (aminopropyl-silica) and MPS

(mercaptopropyl-silica).

2.3. Apparatus

Modified and unmodified silica materials have

been characterized by various techniques. The

total pore volume and specific surface area of

grafted silica gels were estimated on the basis of

nitrogen adsorption/desorption isotherms at the

temperature of liquid nitrogen (BET method).

These measurements were performed using the

Coulter SA 3100 apparatus. The amine content of

the APS samples was determined by acid-basetitration [32,37], which was monitored with the

Metrohm 691 pHmeter (electrode No. 6.0222.100)

and by elemental analysis (Central Service for

Analysis, CNRS, Lyon). APS samples were also

characterized by X-ray Photoelectron Spectro-

scopy (XPS) and Infrared spectrometry (IR).

XPS measurements have been performed at a

residual pressure lower than 10(9 mbar, with

using a VSW HA150 MCD electron energy

analyzer operating with a Mg Ka non-monochro-

matic source. IR experiments were carried out in

the diffuse reflectance mode, with the aid of aPerkin/Elmer 2000 apparatus, by reflection on a

KBr powder containing 10% of the silica-based

material.

Quantitative analysis of silicon in aqueous

solution was made by inductively coupled

plasma-atomic emission spectroscopy (ICP-AES,

plasma 2000, Perkin/Elmer), and total amounts of

amine in solution were quantified by pH-metry.

Zeta potentials were measured by Doppler veloci-

metry (Malvern Instruments) on APS suspensions

M. Etienne, A. Walcarius / Talanta 59 (2003) 1173/1188 1175


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adjusted at selected pH values by HNO3 or NaOH

additions. A potential of 50 mV was applied

between two palladium-plated electrodes in a

quartz suprasil chamber of 75 mm effective length.Measurements were performed after some minutes

equilibration. Solution-phase HgII was determined

by anodic stripping differential pulse voltammetry

on gold electrode, using a m-Autolab potentiostat

associated to the GPES electrochemical analysis

system (Eco Chemie). Measurements were per-

formed in a conventional single-compartment cell

assembled with a rotating gold electrode, a Ag/

AgCl reference electrode (Metrohm, No.

6.0733.100), and a Pt wire auxiliary electrode.

2.4. Procedures

2.4.1. pH-metric titration

Direct titration of each APS material was

carried out by automatic addition of 10(2 M

HCl in a reactor containing 40 ml of deionized

water and 100 mg of solid sample. The titration

speed was adjusted in order to neutralize all the

amine groups in the modified silica material. A

speed of typically 0.03 ml min(1 was selected as a

good compromise to ensure diffusion of thereactant to all the active centers in the porous

solid while keeping a reasonable experiment time.

Complete neutralization was checked by back

titration of 100 mg APS in an excess HCl, by a

standardized NaOH solution, after 24 h reaction

and filtration of the solid phase. The differences

observed between direct and back titration were

less than 2%. The amine group content was also

determined by elemental analysis for confirmation

purpose.

2.4.2. Monitoring the stability/degradation of APSin solution

0.1 g of silica was added to 200 ml of aqueous

solution. After selected reaction times, 1 ml of the

solution was taken out with a syringe and filtered

off with a 0.45 mm HV Millipore filter. The silicon

concentration in solution was then directly deter-

mined by ICP-AES. For each figure presented in

this paper, measurements of all data points have

been performed in a single set of experiments and

with the same ICP-AES parameters. The etalon

curve was prepared with tetraethoxysilane (!/

98%, Merck).

Loss of amine groups in solution was also

measured to evaluate the rate and extent of APSinstability. Typically, 0.1 g APS was placed in 50

ml of aqueous or ethanolic (96%) solution. After a

selected equilibration time, the solid was filtered

off and the filtrate was titrated by the pH-

potentiometric method with using a standardized

HCl solution.

2.4.3. Mercury uptake by APS in HCl solutions

0.1 g of the APS material was placed in 50 ml of

aqueous solution containing initially 10(4 M

Hg(NO3)2 and 0.10 M (or 0.02 M) HCl. After aknown time in solution, the solid was filtered off

and the solution was analyzed. The mercury(II)

determination has been performed by anodic

stripping differential pulse voltammetry on rotat-

ing gold disk electrode (0/4 mm, v0/500

min(1, electrolysis at 0.3 V for 30 s) in 100 ml of

electrolytic solution (72 mM NaCl, 12 mM dis-

odium-EDTA, 2.8 M HClO4), according to a

published procedure [41].

3. Results and discussions

3.1. Grafting and solid-phase characterization

The procedure used for the chemical modifica-

tion of silica gel by APTES is referred to the work

of Vansant et al. [23] and that of Waddell et al.

[38]. It is schematically represented in Fig. 1 and

involves two successive steps. During the first step,

APTES is allowed to react with the silica surface in

toluene under nitrogen atmosphere and constant

stirring while refluxing. These conditions must bekept for 2 h in order to permit the diffusion of the

organosilane molecule in all the pores of the

material; even those located deeper in the interior

of the porous solid [37]. During this step, the

surface hydroxyl groups of silica (silanol groups)

are condensing with the ethoxy groups of APTES,

liberating EtOH in the medium. The second part

of synthesis is a curing step at elevated tempera-

ture, which is required to increase the degree of

condensation of the grafted layer [23]. Appropriate



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cross-linking is achieved after 2 h curing at 120 8C

[38].

Usually, the surface modification of silica by

APTES is performed after complete dehydration

(by thermal treatment). Indeed, adsorbed water on

the silica surface has a great effect on the grafted

layer because it can participate to the hydrolysis of

ethoxy groups carried by APTES. Moreover, the

presence of water often leads to increasing the

extent of condensation between neighboring ami-

nopropylsilane molecules and could also generate

hierarchical polymerization of the grafting agent

with itself. On the other hand, it can contribute to

increase the stability of the grafted layer [23]. For

this reason, we have chosen to perform the surfacemodification without thermal pre-treatment be-

cause great stability is useful for applications in

which the material is in contact with aqueous

solutions.

If operating in excess APTES, the quantity of

grafted ligands is directly related to the amount of

silanol groups on the silica surface because they

are primarily involved in the grafting process. The

number of silanols can be varied by changing the

degree of hydroxylation of the silica surface, e.g.,

decreasing upon thermal treatment to condenseadjacent /SiOH groups into siloxane bonds /Si/

O/Si/, which requires high temperatures (!/

200 8C) [23], or increasing by rehydration in acidic

medium [20]. In the present case, a single state of

the silica surface was used throughout, containing

4.2 mmol OH g(1 (as measured by thermogravi-

metry), and the quantity of grafted amine groups

was adjusted by changing the amount of APTES

introduced in the reaction vessel containing dry

toluene as the solvent. Fig. 2A depicts the relation-

Fig. 1. Schematic representation of the silica surface modification by grafting APTES.

Fig. 2. (A) Variation of the amount of grafted groups on the

silica surface, determined by acid-base titration as a function of

the initial quantity of APTES in the medium. (B) Correspond-

ing variation of the total pore volume of grafted materials

expressed with respect to the amount of grafted groups.



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immobilized aminopropyl groups. These bands

were attributed to both the symmetric and asym-

metric stretching of CH3 and CH2 groups

(nas(CH3)0/2975 cm(1 (small intensity),

nas (CH2)0/2928 cm(1, ns(CH3)0/2886 cm

(1

(small intensity), ns(CH2)0/2870 cm(1), and of

NH2 (nas0/3358 cm(1, ns0/3279 cm

(1) [42/44].

Presence of bands of CH3 (small intensity) indi-

cates the presence of some remaining ethoxy

groups that have not been hydrolyzed. The XPS

analysis of the APS surface shows the presence of

silicon (Si 2p at 102.7 eV, 27.4%), oxygen (O 1s at

531.9 eV, 55.8%), carbon (C 1s at 284.6 eV,

11.7%), and nitrogen (N 1s at 399.0 eV, 5.2%).

These results are in good agreement with thosepreviously reported in the literature [45].

3.2. Characterization in aqueous solution

The chemical modification of silica by grafting

APTES dramatically changes its surface proper-

ties. The surface of unmodified silica is intrinsi-

cally acid owing to the presence of silanol groups

that are readily deprotonated in alkaline medium

(pKa0/6.89/0.2, as determined by Schindler and

Kamber [46] at 25 8C in 0.1 M NaClO4). Thisreaction, however, is not quantitative as the

apparent pKa values of silanols are increasing

significantly in proportion as deprotonation is

going on [47,48]. When grafted with aminopropyl

groups, the silica surface is expected to display

basic properties. Fig. 3A shows illustrative titra-

tion curves for an APS sample by HCl at various

speeds of reactant addition, from 1 ml min(1

down to 0.02 ml min(1, corresponding to experi-

ment times ranging from about 15 min to 12 h. It is

clearly noticeable in this figure that a fast addition

of hydrochloric acid leads to lowering pH in the

medium in proportion to the quantity of added

protons. At so short experiment times, all the

added protons have no enough time to be con-

sumed by the aminopropyl-modified silica (curves

a/c in Fig. 3A). This limitation is owing to

restricted diffusion inside the porous structure of

APS, which requires rather long times for the

protons to reach all the basic sites located deeper

in the material [37]. For titration rates lower than

0.03 ml min(1, identical potentiometric curves

were obtained, corresponding to steady-state si-

tuation: a speed of proton addition of 0.03 ml

min(1 is sufficiently slow to allow the reactant todiffuse in the APS material and to reach all the

active sites without creating an unsteady low pH in

solution. Under conditions of complete titration,

two different acid-base reactions are observed (two

successive pH jumps, as shown in curve d in Fig.

3A). The first one is characterized by a pKa of

about 9.6, and is consistent with the protonation

of the free amine groups (into their corresponding

ammonium form, Eq. (1)). The second one has a

pKa of about 6.7 and is attributed to the protona-

tion of the zwitterion-like species /SiO(

,'H3NC3H6/Si/ (Eq. (2)) that are known to exist

in APS materials [34,35].

These two pKa values are in agreement with

those observed for titration of pure APTES in

aqueous medium, which forms octameric species,

displaying a 91:9 percent ratio between the two

successive steps [49]. In the case of APS, however,

the free amine-to-zwitterion ratio was close to

60:40. It seems, therefore, that 40% of the amino-

propyl groups in APS strongly interact with

(1)

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residual silanols to enable proton transfer (forma-

tion of /SiO(,'H3NC3H6/Si/) while 60% of

them remain in the free amine form, which is

probably hydrogen-bonded with a surrounding

silanol group, as suggested by the shift IR bands

of NH2 (nas0/3358 cm(1, ns0/3279 cm

(1) to-

wards values lower than those corresponding to

free n -propylamine (nas0/3365 cm(1, ns0/3297

cm(1).

All the APS materials containing various quan-

tities of ligands have been titrated at low speed

(equilibrium conditions). Fig. 3B shows that a

linear relationship was observed between the first

and the second equivalent points for all these

materials, whatever the amine loading in the range0.015/1.71 mmol g(1. Therefore, the two distinct

forms of aminopropyl groups grafted on silica gel

are coexisting in aqueous suspension at a constant

ratio of about 60:40 (owing to the slope of the

straight line in Fig. 3B), independently on the

grafting extent. However, it is difficult to draw an

exact representation of the APS surface at this

stage as long as the stability of this material in

solution is not better understood (see Section 3.3).

On the other hand, electrophoretic mobility

measurements (Zeta potentials) carried out fromaqueous suspensions of APS subjected to an

electric field have brought additional information

on the surface properties of this material. A major

difference between unmodified and amine-grafted

silica gels was indeed observed in the variation of

the surface charge of particles with pH. The

isoelectric point (IEP) of silica gel is close to pH

2; above this value the silica surface is negatively

charged owing to the presence of silanolate groups

[33]. On the opposite, the surface of APS samples

was found to be positive on a wider pH range, as

explained by the fact that the great part of the

amine population is protonated at pH lower than

10. When measuring Zeta potentials of APS as afunction of pH, directly (i.e., a few min) upon

dispersion of particles in solution to avoid sig-

nificant degradation of the material, an IEP close

to pH 10 was measured and the APS surface was

positive at lower pH values (e.g., '/60 mV at pH

8). Such Zeta potentials and IEP are consistent

with the pKa value of the grafted aminopropyl

groups that are mainly protonated at pHB/10.

This IEP value is, however, noticeably higher than

those reported for other APS materials when

awaiting for equilibration before starting theZeta potential measurements (IEP0/8 [50] and

IEP/7 [35,36]). In these latter cases, very long

times were required to reach steady-state values

(i.e., 3 days [36]) so that significant degradation of

the APS materials is expected to have occurred

[38/40]. In addition, local pH in the porous APS

structure might be different as that in solution as a

consequence of the high concentration of pH-

sensitive ligands in a confined environment whose

accessibility to the external solution is time-depen-

dent [37]. This indicates that the surface propertiesof APS are exposed to variation over prolonged

contact with an aqueous phase, as discussed here-

after.

3.3. Stability in solution

3.3.1. Influence of pH on the dissolution of APS

materials in aqueous medium

A first characterization of the APS stability in

aqueous medium was provided by monitoring the

(2)



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total soluble silicon-containing species that have

leached in solution as a function of time. Their

concentration is expected to increase as a result of

partial hydrolysis of the silica network as well as

from the liberation in solution of aminopropylsi-

lane moieties arising from the hydrolysis of the

chemical bond between the organosilane and the

silica surface. This latter reaction can be catalyzed

by amine groups [39]. This preliminary stability

study has been performed at various pH with an

APS material grafted with 1.7 mmol g(1 amino-propyl groups; similar results were obtained for

solids characterized by lower capacity. Fig. 4A

shows the evolution over 2 h (inset: over 4 days) of

the silicon concentration in solution when 0.1 g

APS was placed in suspension into 200 ml of three

different solutions: 0.1 M HNO3 (pH 1), acetate

buffer (pH 5.7) and pure water (in this last medium

the solution pH is compelled by the intrinsic

basicity of the amine-bearing APS material at a

value of about 9.5). It is clearly shown in this

figure that initial pH of the suspension has adramatic effect on the quantity of silicon species

liberated in solution, even during the first minutes.

The extent of solubilization is very low at pH 1

(Fig. 4A, curve a), much higher at pH 5.7 (Fig. 4A,

curve b), and maximal when the solution pH is not

controlled (Fig. 4A, curve c), enabling the material

to express its total basic power. Rationalization of

these results is quite easy at pH 1 (where the basic

action of amine groups is prevented because they

are totally protonated) and at pH 9.5 (where silica

is expected to dissolv

e to significant extent), but israther surprising at pH 5.7 as unmodified silica is

usually stable at this pH value and as most of the

amine groups of APS are expected to be proto-

nated (!/99.99%). Despite these latter facts, the

APS material displays a significant rate of dissolu-

tion during the first hour of suspension in a

buffered solution at pH 5.7 (Fig. 4A, curve b).

Note that in this case (pH 5.7) the dissolution

extent, as expressed by mass ratio with respect to

the mass of starting material, remains low: about

1% degradation after 1 h, and 6% after 10 h in

suspension. It seems, therefore, that the graftedaminopropyl groups at the silica surface are

playing a key role in the instability of the APS

material in aqueous medium, even in non-basic

environment.

This is further confirmed by comparing the

behavior of silica-based materials in the absence

and in the presence of amine groups, either

dispersed in solution (soluble base) or immobilized

at the silica surface (grafted bases). Fig. 5A

summarizes the results of three experiments car-

Fig. 4. (A) Extent of dissolution (soluble Si) of 0.1 g APS in

three different media (200 ml): (a) 0.1 M HNO3; (b) acetate

buffer at pH 5.7; (c) pure water (pH is imposed by the intrinsic

basicity of the material, which was 9.5 at the end of the

experiment). Inset: same experiment prolonged over 4 days. (B)

Long-term (in)stability of 0.1 g silica-based materials in 200 ml

acetate buffer at pH 5.7: (a) APS containing 1.7 mmol amine

groups per gram; (b) unmodified silica gel in the presence of

butylamine in solution in the same quantity as the amine groups

in (a). Inset: difference between curves (b) and (a). Data areexpressed in the form of variation of soluble silicon concentra-

tions in solution.



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ried out on the same time scale as in Fig. 4A, inacetate buffer at pH 5.7. Curve a shows the

hydrolysis of 0.1 g of unmodified silica gel in this

medium, which is very low and slow. Curve b

illustrates the hydrolysis of 0.1 g of APS material

in the same conditions and, as seen previously in

Fig. 4A, the quantity of soluble silicon increases

immediately after the material was suspended in

solution. The last curve c shows the behavior of a

suspension containing 0.1 g of the unmodified

silica in the presence of soluble BuNH2, at an

amount corresponding to about the molar quan-

tity of the amine grafted on the surface of 0.1 g of

APS. Although the quantity of amine in solution is

the same as that immobilized within the APSmaterial, the hydrolysis of silica in the presence of

free amine (non-grafted) is comparable to that of

the same material without base (comparison

between curves a and c in Fig. 5A). The acetate

buffer at pH 5.7 is thus able to neutralize the

basicity of soluble BuNH2 distributed into the

whole volume of solution (0.85)/10(3 M), but it

is not sufficient to counterbalance the basicity

arising from the APS material. In this last case, the

amine groups are not dispersed into the whole

solution but they are confined within the APSmaterial at a high concentration (2.2 M, as

estimated from the amine loading of 1.7 mmol

g(1 and total pore volume of 0.76 ml g(1).

When performing a similar set of experiments as

in Fig. 5A, but in pure water (i.e., floating pH)

instead of buffer solution, the dissolution of

unmodified silica was still very slow (Fig. 5B,

curve a), that of APS was very fast (Fig. 5B, curve

b), but that of unmodified silica in the presence of

BuNH2 led to a sharp increase in the silicon

concentration in solution, contrarily to whathappened in the buffer. This demonstrates the

ability of free amine in solution to dissolve silica

when it is not neutralized by an acid buffer, in

agreement with the increase in the hydrolysis of

silica when rising pH [33]. By comparing Figs. 4A

and 5A and B, it appears that APS dissolution in

aqueous medium is mainly due to the presence of

amine groups, but their quantity is not the only

parameter explaining the high rate of dissolution

of the material in acetate buffer. Indeed, the high

local concentration of amine groups in APS

materials can not be buffered efficiently to avoidhydrolysis. It was only possible to counter effi-

ciently the high basicity of the concentrated

aminopropyl groups in APS by dispersing the

material into a 0.1 M nitric acid solution (Fig.

4A, curve a); for such an external medium, the

concentration of residual unprotonated amine

groups in the material was as low as 5)/10(9 M.

The behavior of APS in buffered solution (pH

5.7) was also investigated at longer equilibration

times. Fig. 4B compares the evolution of the

Fig. 5. Dissolution kinetics of 0.1 g silica-based materials in

various conditions in (A) 200 ml acetate buffer at pH 5.7 and

(B) 200 ml pure water: (a) unmodified silica gel alone; (b) APS

containing 1.7 mmol amine groups per gram; (c) unmodified

silica gel in the presence of butylamine in solution in the same

quantity as the amine groups in (b). Other conditions as in Fig.

4.



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silicon concentration measured as a function of

time in suspensions containing, respectively, an

unmodified silica gel in the presence of BuNH2

(curve a) and the APS material (curve b), during 2

days. In agreement with what was observed

previously (Fig. 5A), a sharp increase of the

soluble silicon appeared during the first 250 min

(/4 h) when the APS material was suspended in

the buffer solution, while a continuous slow

dissolution of the unmodified silica was observ

edover the entire time range (Fig. 4B, curve b). After

the first 4 h, the degradation rate of APS was

slower, displaying a speed of dissolution compar-

able to that of unmodified silica gel. The dissolu-

tion of the unmodified silica gel in this medium is

owing to the high ionic strength generated by the

buffer [33,51], while this parameter is not rate-

determining in the dissolution of APS. The inset in

Fig. 4B depicts the difference between curves a and

b. It allows clearly to distinguish a breakthrough

during APS dissolution in aqueous medium: a fast

degradation at short time due to the presence ofamine groups in the material, followed by a slow

dissolution at longer times similar to that of

unmodified silica. Steady state situation appears

after several hours.

Because of possible destruction of the Si /O/Si

bond by nucleophilic attack of amine groups

(catalyzed by water molecules), one can suggest

that liberation of silicon in solution arises from the

deterioration of chemical bonds between the silane

layer and the silica surface leading to leaching of

free aminopropylsilane. This is expected to occur

to the substrate which is either singly, doubly, or

triply bonded to APTES; the last step being

illustrated by the following equation:

Indeed, the amount of aminopropylsilane that

has leached out of APS after 4 h equilibration in

aqueous medium (i.e., just before the break-

through in curve b of Fig. 4B) is about 0.9 mmol

g(1, which is less than the initial loading of the

APS material used for this experiment (1.7 mmolg(1). Aminosilane liberation, however, cannot be

the sole mechanism involved in the degradation

process as the quantity of silicon in solution after

48 h equilibration is higher than 2 mmol g(1,

which exceeds the amount of APTES that has been

grafted on the material. Some other silicon-con-

taining species originating from the bulk material

have also passed in solution. To distinguish

between these two processes, the quantity of amine

liberated in pure water (drastic conditions con-

cerning the stability) has been determined by

potentiometric titration (Fig. 6, curve b). By thisway it is possible to characterize quantitatively the

extent of leaching of aminopropylsilane in solu-

tion, as a function of time, and to compare it with

the amount of total soluble silicon. As shown, a

fast liberation of aminosilane was observed during

the first 2 h with the APS material in solution. A

steady state was reached after typically 4 h and did

not change later on, even after several days (data

not shown). The quantity of amine liberated at the

equilibrium does not correspond to all the amino-

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silanes available within the material. At the max-

imum, only 67% of the total aminopropylsilane

content of APS were liberated in solution: one-

third of them was still remaining at the surface of

the silica material after several days in closed

reactor. A possible interpretation of APS degrada-

tion involves a fast initial step resulting from theleaching of aminopropylsilane species in solution,

and a subsequent slow degradation event which is

essentially owing to the attack of the bulk silica

that is catalyzed by the basic conditions generated

by the solution-phase aminopropylsilane (hydrox-

ide anions produced by hydrolysis of amine

groups).

Interestingly, the maximal amount of amino-

propylsilane species that pass in solution when

suspending APS particles in pure water (i.e., 67%

of them) is close to the amount of those groupsthat are in the form of free amine in the material

(those having reacted in the first part of titration

curve; see Section 3.2 and Fig. 3A). It seems,

therefore, that zwitterion-like species (/SiO(,'H3NC3H6/Si/) are much more stable and less

subject to leaching in solution, in agreement with

the fact that ammonium functions do not possess

the pair of electrons of amine functions that is

responsible for the hydrolysis of the grafted

aminopropylsilanes (Eq. (3)).

3.3.2. Influence of proton concentration on the long-

term stability of APS in acidic solutions and

restricted uses in alkaline medium*/interactions

with metal ion species

As shown above, APS particles exhibit notice-

ably long stability when immersed in acidic solu-

tions. This stability was further studied withrespect to the binding of the negatively charged

chloro-complexes of mercury(II) to protonated

APS in acidic medium, which can occur via

electrostatic interaction with the ammonium

groups /NH3',Cl( that are formed in the

presence of HCl. In this medium, both HgCl3(

and HgCl42( species are liable to exist in a

proportion depending on the chloride ion concen-

tration. These anionic complexes are liable to

exchange chloride ions in protonated APS (see

Eq. (4), as an illustrative case for HgCl3().

The percentage of accumulated mercury(II)within the protonated APS material has been

followed over several weeks in two different acidic

media, 0.02 M HCl (Fig. 7, curve a) and 0.1 M

HCl (Fig. 7, curve b). Immobilization of the

mercury(II) complexes by ion exchange in the

material was rather fast and an equilibrium was

observed after some hours in both solutions,

corresponding to the consumption of about 65%

of the initial solution-phase metal ion concentra-

tion. These HgII-loaded APS particles were then

Fig. 7. Extent of mercury(II) adsorbed by 0.1 g APS, as a

function of time, in solutions containing initially 10(4 M

Hg(NO3)2 and two different HCl concentrations: (a) 0.02 M; (b)

0.10 M.

Fig. 6. Influence of solvent on the degradation rate of APS(expressed through the variation of concentration of amino-

propyl groups that have leached in the external solution with

time): (a) ethanol at 96%; (b) pure water. Data were obtained

from 0.1 g solid in 50 ml solution.

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allowed to equilibrate further in the same solutions

while performing a continuous monitoring of the

solution-phase metal ion concentrations over sev-

eral days, in order to compare the stability of HgII-

loaded APS for these two different acidic

strengths. As shown in Fig. 7, the materials

capacity did not change significantly during more

than 5 weeks equilibration in 0.1 M HCl, while a

slow decrease of the accumulated mercury(II) was

recorded when working in less diluted HCl (0.02

M). This latter behavior is explained by a slow loss

of ligands in solution due to some remaining

free amine groups that are not fully eliminated

in 0.02 M HCl because of the high local concen-

tration of ligands in the APS material; a higher

HCl concentration (e.g., 0.1 M) is required to

entirely compensate this remaining local basicity.

Long-time immobilization of metal ions by APS

requires, therefore, a rather strong acidic medium.

At this point of the discussion, it is of interest to

compare Figs. 4B, 6 and 7. In pure water, the

release of aminosilane occurs very quickly and asteady state is observed after several hours in the

solution (Fig. 6, curve b). In acetate buffer at pH

5.7, a steady state seems to appear after 2 days in

solution (inset of Fig. 4B). Finally, in 0.02 M HCl

solution, a very slow decrease of the ligand loading

in APS is observed within several weeks (Fig. 7,

curve a). These three experiments show the rela-

tionship existing between pH of the aqueous

solution and the stability of the chemical bonds

relying on the aminopropylsilane and the silica gel

surface. Depending on the time of contact required

for a target application of this kind of material in

aqueous medium, the results presented in this

study enable to select carefully the experimental

conditions that must be used in order to keep a

sufficient reactivity and to limit the APS degrada-

tion during the time scale of the experiment. For

example, we have proposed recently the use of an

APS-modified carbon paste electrode for the

electrochemical detection of copper(II) ions inaqueous medium at pH 7 [17]. In spite of the

Fig. 8. Effect of grafting the silica surface with mercaptopropyl

groups on its chemical stability in aqueous medium as a

function of pH. Extent of dissolution (soluble Si) of 0.1 g

MPS in 0.1 M acetate buffer at pH 5.0 (a), and in 0.1 M

phosphate buffer at pH 8.3 (b); curve (c) depicts the case of 0.1

g of unmodified silica gel in phosphate buffer at pH 7.9, for

comparison purpose.

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analytical investigation of the chemical reactivity and stability. mathieu etienne, alain walcarius

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