Analytical Investigation of the Chemical Reactivity and Stability. Mathieu Etienne, Alain Walcarius
Transcript of 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
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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
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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
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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-
(3)
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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.
(4)
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