Silylation of laponite clay particles with monofunctional and trifunctionalvinyl alkoxysilanes{

Norma Negrete Herrera,a Jean-Marie Letoffe,b Jean-Pierre Reymondc and Elodie Bourgeat-Lami*a

Received 8th October 2004, Accepted 17th November 2004

First published as an Advance Article on the web 13th December 2004

DOI: 10.1039/b415618h

We report in this work the grafting of laponite clay particles with monofunctional

c-methacryloxypropyl dimethyl methoxy silane (c-MPTDES) and trifunctional

c-methacryloxypropyl trimethoxy silane (c-MPTMS) coupling agents. The evolution of the grafted

amount and of the grafting yield was monitored as a function of the reaction time and of the initial

silane concentration. We showed that the grafted amount increased with time and with the silane

content up to a plateau value. The amount of chemisorbed silane at saturation varied from 0.56 to

1.9 mmol g21 depending on the nature of the coupling agent, the reaction time and the grafting

conditions. While the trifunctional silane was capable of both reaction with the clay edges and

formation of complex polysiloxane oligomers in the bulk which were further deposited on the

particulate surface, the monofunctional silane formed a monolayer coverage on the border of the

clay plates with the carbonyl groups being directed toward the surface as attested by Fourier

transform infrared (FTIR) spectroscopy. The properties of the organosilane-modified laponite were

examined by various analytical techniques such as wide-angle X-ray diffraction (WAXD), nitrogen

adsorption and thermogravimetric analysis (TGA). The monofunctional silane exhibited nearly no

effect on the physicochemical properties of the clay whereas grafting of the trifunctional silane

resulted in decreased porosity, increased interlamellar distance and higher hydrophobicity.

Introduction

Nanocomposite materials have attracted significant attention

in recent years because the intimate combination of organic

and inorganic components at the nanoscale offers prospects of

new and synergistic properties. In general, the key step in the

synthesis of these materials is the establishment of a chemical

link between originally non-mixing phases. Although bonding

of organic polymers to inorganic surfaces has long been a

common operation, a major need for new bonding techniques

arose in 1940 when glass fibers were first used as reinforcing

agents in organic resins. Organically modified alkoxysilanes

with the general formula Rn–Si–X(42n), where X is a

hydrolysable group and R is a functional terminal group, are

particularly well suited for this purpose, and are often used to

provide covalent bonding between inorganic fillers and

polymer matrices, enhance interfacial adhesion and improve

the mechanical properties of composite materials.1,2 In the

past, there has been a huge number of works on this topic

which mainly focused on the mechanistic aspects of the

grafting reaction. Nishiyama et al. studied for instance the

hydrolysis and condensation mechanisms of silane coupling

agents in the presence of colloidal silica using 29Si and 13C

solid state NMR.3 Ishida and coworkers reported evidence for

covalent bonding between vinyl silanes and the surface of silica

beads.4 However, most of these studies concern silica or glass

fibers and few works have been reported so far on the grafting

of silane coupling agents on clay surfaces.5–14 Pioneering

works in this domain were reported in 1975 by Ruiz-Hitzky

et al. who described the reaction of organochlorosilanes with

the surfaces of sepiolite and chrysotile: two silicate materials.5

The grafting reaction mainly occurred at the external surface

of the clay and was highly dependent on the accessible surface

area. Five years later, the same group reported the silylation

reaction of a layered silicic acid, H-magadiite, expanded with

dimethyl sulfoxyde or N,N-dimethyl formamide to provide

access to the internal silanols.6 H-Magadiite contains a lot of

OH groups located in the interlayer space of the silicic acid

compound, the surface hydroxyls of which were reacted

with various chlorosilanes bearing different functionalities. A

similar strategy was followed by Kuroda et al. who again

performed the silylation reaction of magadiite using dodecyl

trimethyl ammonium-intercalated magadiite compounds as

intermediates.7 The space created in the expanded interlayer

enabled the grafting of bulky molecules such as diphenyl-

methyl chlorosilanes and perfluoroalkyl silanes.8 But contrary

to magadiite, clays of the smectite group, such as montmor-

illonite or laponite, contain a relatively low amount of

hydroxyls located at the edge of the individual particles, and

can therefore bind only a small proportion of organics.9 To

overcome the low reactivity of smectite clay surfaces toward

organic molecules, several authors have reported the direct

synthesis of organoclays by the sol–gel process using

organoalkoxysilanes as the silica source.10 Although this

approach allows a high degree of organosilane incorporation,

it may cause distortion and introduce structural defects within

{ Electronic supplementary information (ESI) available: 13C NMRspectra of MPTMS and MPDES-functionalized laponite clay particles.TGA graphs for bare laponite and laponite that has been reacted withvarious amounts of MPTMS and MPDES coupling agents, respec-tively. See http://www.rsc.org/suppdata/jm/b4/b415618h/*bourgeat@lcpp.cpe.fr

PAPER www.rsc.org/materials | Journal of Materials Chemistry

This journal is � The Royal Society of Chemistry 2005 J. Mater. Chem., 2005, 15, 863–871 | 863

the clay sheets. Therefore, the direct grafting of smectite-type

clay samples with organoalkoxysilanes still appears to be of

interest.11–14 Tonle et al.12 reported for instance the chemical

modification of a series of natural Cameroonian clays of the

smectite group with amine and thiol functionalities by reaction

of aminopropyl and mercaptopropyl trimethoxysilanes and

highlighted the potential interest of the functional clays for

electroanalytical applications. Wassermann et al.13 described

the silylation reaction of bentonite with octadecyl trimethoxy-

silane and the effect of the chemical modification on the inter-

layer chemistry of iron. It is worth mentioning also the recent

work of Bourlinos et al.14 on the chemical modification of

montmorillonite with a–v bridging organosiloxane. However,

none of these studies concerns laponite although laponite has

gained considerable interest in various fields of applications.

As a part of a program devoted to the synthesis of polymer/

layered silicate nanocomposites, we are interested in the

present work in the surface modification of laponite clay

platelets by grafting of organic derivatives. In a preliminary

paper, we have demonstrated that organo alkoxysilanes

carrying a terminal double bond could be satisfactorily

attached to the laponite particles surface and that the resulting

materials were potential systems for the synthesis of water-

based colloidal nanocomposites through emulsion polymeriza-

tion.15 We report herein a more in-depth investigation of the

reaction of the trifunctional c-MPTMS and the monofunc-

tional c-MPDES coupling agents with the laponite clay

surface. Various analytical techniques were used to assess the

efficiency of the grafting process. The amount of grafted silane

was determined by elemental analysis and FTIR was applied

to characterize the configuration of grafting. WAXD, BET

and TGA were used to study the effect of the grafted amount

on the final properties of the functionalized clay materials.

Experimental

Materials

Unless stated otherwise, laponite RD, a synthetic hectorite

from Rockwood Additives Ltd. (UK), was used as supplied.

The trifunctional c-MPTMS (C9H20O5Si, structure 1) and the

monofunctional c-MPDES (C10H22O3Si, structure 2) silylating

agents from Gelest Inc, were used without further purification.

Toluene from Aldrich was of synthetic grade and used as

received or distilled over activated molecular sieve under a

nitrogen atmosphere before use (so-called anhydrous toluene).

Functionalization

Reaction of hydrated laponite with c-MPTMS and

c-MPDES in toluene. Silylation of laponite was carried out

in toluene (as received) by varying the reaction time and the

initial silane concentration. In a typical run, laponite (as received)

was suspended in toluene at the concentration of 10 g L21

and the required amount of the coupling agent ([silane] 5

3.7 mmol g21), was introduced in the reaction flask and

allowed to react for various periods of time (3, 6, 10, 17 and

21 days) at room temperature. The reaction was reproduced

for various initial silane concentrations comprised between

0.75 and 7 mmol g21 and a fixed reaction time (21 days). The

grafted laponites were filtered, extensively washed with toluene

in order to remove the silane in excess, and dried overnight in a

vacuum oven at 40 uC before characterization.

Reaction of dehydrated laponite with c-MPTMS in anhydrous

toluene. 1 g of laponite dried at 200 uC for 2 hours in a vacuum

(so-called dehydrated laponite) was introduced into a reaction

flask containing 100 mL of anhydrous toluene. The required

amount of c-MPTMS (corresponding to 0.75, 3 and

5.2 mmol g21, respectively) was introduced into the suspension

medium and the reaction mixture was stirred at room tempera-

ture under an argon atmosphere for 21 days. The grafted

laponite was then washed and dried as described above.

Characterization

29Si and 13C solid state NMR were performed on a Bruker

DSX-300 spectrometer operating at 59.63 and 75.47 MHz,

respectively, as described previously.15 Infrared spectra were

recorded using a Nicolet FTIR 460 spectrometer on powder-

pressed KBr pellets. Thermogravimetric analysis of the treated

clays was performed on a Mettler TG 50/TA 3000 thermo-

balance, controlled by a TC10A microprocessor. Samples were

heated at the rate of 10 uC min21 under a nitrogen flow

(150 mL min21). Surface areas were obtained by using the

Brunauer–Emmet–Teller (BET) equation.16 The hydroxyl

group concentration of the clay was determined by titration

by measuring the volume of ethane produced from the reaction

of triethyl aluminium with the dehydrated clay.17 X-Ray

powder diffraction patterns were obtained using a

Siemens D500 diffractometer (Ni-filtered CuKa radiation,

l 5 1.5405 A). The d001 basal spacings were calculated from

the 2h values using the EVA software. A JEOL JCXA 733

electron microprobe analyzer (EPMA) was used to determine

the carbon content of the bare and the functionalized clay

platelets. The grafted amount (expressed in mmoles of grafted

silane per g of bare laponite) was determined from the

difference DC (wt%) of carbon content after and before

grafting as follows:18

Grafted amount mmol g{1� �

~103|DC

1200NC{DC M{1ð Þð Þ (1)

where Nc and M (g mol21) designate the number of carbon

atoms and the molecular weight of the grafted silane molecule,

respectively (Nc 5 7 and M 5 206 for MPTMS while Nc 5 9

and M 5 202 for MPDES).

The grafting yield, which corresponds to the percentage of

silane molecules that effectively participated to the coupling

reaction, was calculated as follows:

Grafting yield (%) 5 Grafted amount 6 100/[silane] (2)

864 | J. Mater. Chem., 2005, 15, 863–871 This journal is � The Royal Society of Chemistry 2005

where [silane] (mmol g21) designates the initial silane

concentration.

Results and discussion

Laponite RD is a fully synthetic clay similar in structure and

composition to natural hectorite of the smectite group. The

chemical composition (%w/w) is as follows: SiO2, 66.2; MgO,

30.2; Na2O, 2.9; LiO2, 0.7, which corresponds to the molecular

empirical formula:19 Si8{Mg5.5Li0.4H4.0O24}0.72Na0.70.7+. Each

layer is composed of three sheets: two outer tetrahedral silica

sheets and a central octahedral magnesia sheet. Isomorphous

substitution of magnesium with lithium in the central sheet

creates a net negative charge compensated by intralayer

sodium ions located between adjacent layers in a stack. The

interlayer also contains water molecules, some of which are

complexed to the metal cations. The dimensions of the elemen-

tary platelets are the following: diameter 30 nm and thickness

0.9 nm. In the dry state or in organic solvents, the platelets are

piled up into tactoıds of around 2–3 layers thick held together

by long-range attractive forces. However, in contrast to clays

with a large aspect ratio that tend to aggregate in a face-to-face

lamellar fashion, laponite has a tendency to form partially

delaminated disordered aggregates through edge-to-face and

edge-to-edge interactions. Reactive silanols, corresponding to

structural defects, are located at the broken edges of these

disordered stacks while Mg–OH groups are contained in the

internal space of the individual clay sheets.20

The covalent attachment of either the c-MPTMS or the

c-MPDES silane compounds to the clay edges was evidenced

by a number of analytical techniques such as 29Si solid state

CPMAS NMR and FTIR spectroscopies. The NMR data were

extensively discussed in our preliminary work.15 The different

species are named according to the conventional Qn, Tn and

Mn notation where Q, T and M designate tetra, tri and

monofunctional units, respectively, and n is the number of

bridging O atoms surrounding the silicon atom. The NMR

spectrum of raw laponite is characterized by two resonances at

294.7 and 284.8 ppm which correspond respectively to Q3

trioxo coordinated framework silicon, and Q2 sites attributed to

isolated silanol groups present at the silicate sheet edges. The

appearance in the 29Si NMR spectra of signals assigned to M1

(15 ppm) and T2,3 (256.9, 266.5 ppm) silicate units derived from

mono and trialkoxysilanes, respectively) gave clear evidence of

the presence of chemically anchored MPDES and MPTMS

groups on the clay surface. In the present work, FTIR is used to

provide deeper insights into the configuration of grafting for

both the trifunctional and the monofunctional silane.

The FTIR spectrum of unreacted laponite is shown in Fig. 1

(spectra a or a9). The spectrum exhibits two bands due to the

presence of physisorbed water, namely the nOH stretching

frequency at around 3450 cm21 and the dOH deformation

Fig. 1 FTIR spectra of MPTMS (top) and MPDES (bottom) -functionalized laponite with increasing silane contents. a,a9) bare laponite, b,b9–

e,e9) silane-functionalized laponite and f, f9) pure MPTMS and MPDES silane molecules. The silylation reaction was performed under wet

conditions. The reader must refer to Table 1 for quantitative attributions.

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band at 1640 cm21. The FTIR spectrum of laponite is also

characterized by a small shoulder at around 3680 cm21 which

can be assigned to surface hydroxyls. This poorly resolved

signal presumably consists of the overlapping of two

components at 3675 and 3686 cm21 corresponding to Sis–

OH and Mgs–OH stretching vibrations, respectively (see the

enlargement spectra of Fig. 2).6b,21 Upon grafting of the

c-MPTMS silane compound, this shoulder became sharper,

shifted to the Mgs–OH vibration frequency and increased in

intensity, while concurrently the area under the broad band at

3450 cm21 decreased. In addition, absorption peaks, ascribed

to the C–H stretching vibrations of the methyl and the

methylene groups of the coupling agent molecule, appeared

between 2850 and 2950 cm21. Similar trends can be observed

on the FTIR spectra of the MPDES-functionalized laponites

(Fig. 1 and 2). The disappearance of the signal at 3675 cm21

together with the appearance of signals in the aliphatic region

argue for the covalent attachment of the silane molecules to

the clay edges via Sis–O–SiC bonds formation. The presence of

residual Mgs–OH groups after grafting indicates that, as

expected from their location in the clay structure, these groups

are not accessible to the coupling agent molecules. The

increase of the Mgs–OH peak intensity with increasing the

amount of chemisorbed silane is due to the diminution of

the amount of physisorbed water after grafting, an assumption

which is supported by quantitative measurements (see below).

Since the intensity of the peak vibration at 3450 cm21

corresponding to H-bonded hydroxyls decreases, the signal

at 3686 cm21 appears proportionally more intense.

The FTIR spectra of the grafted clay samples in the

carbonyl region are shown in Fig. 1 (right). In addition to an

adsorption peak at 1634–1635 cm21 characteristic of the CLC

double bond stretching vibration overlapping with the dOH

deformation band of physisorbed water mentioned above, the

FTIR spectra of the MPTMS-grafted laponites (top) show a

strong band at 1700 cm21 attributed to hydrogen-bonded

carbonyl groups of the silane moiety.22 This band increased in

intensity with increasing the initial silane concentration, while

simultaneously, a new band at the same position as in pure

c-MPTMS, and attributed to ‘‘free’’ non-bonded carbonyl,

appeared at 1720 cm21. This result shows that the degree of

condensation of the chemisorbed silane increases going from

submonolayer coverages at low concentrations, the carbonyl

groups forming hydrogen bonds with hydroxyls present on the

surface of the clay plates, to multilayer coverages at higher

concentrations characterized by the appearance of ‘‘free’’

carbonyl groups. In contrast to the trifunctional silane, the

monofunctional silane cannot condense in solution and the

FTIR spectra of the MPDES-funtionalized laponites exhibit a

unique signal at 1700 cm21 whatever the initial silane content.

The formation of a multilayer coating is improbable in this

case and the carbonyl groups of the MPDES molecule interact

by hydrogen bonds with the clay surface in such a way that the

organic chains are parallel to the silicate layers, the carbonyl

group being directed toward the surface. The above inter-

pretation is supported by 13C solid state NMR analysis which

shows two signals at 172 and 175 ppm corresponding to free

and H-bonded carbonyls for the MPTMS-functionalized

laponite samples and only one signal at 175 ppm for the

MPDES-functionalized clay (see electronic supplementary

information{).

In summary, the above FTIR analysis gives evidence of

covalent attachment of the silane molecules to the clay edges.

While the trifunctional silane forms a submonolayer coverage

at low concentrations and a multilayer coating at higher

concentrations, the monofunctional derivative is attached to

the clay plates as a molecular film lying flat on the surface

whatever the initial silane concentration. Although there are

obviously some similarities between the present work and the

studies from Ishida et al. on the grafting reaction of silane

molecules on E-glass fiber4a and a series of clay substrates,4b,23

this is to our knowledge the first report on the silylation

reaction of laponite clay surfaces, and as such is clearly

differentiated from these previous studies.

Quantitative analysis

Carbon elemental analysis provides quantitative data concern-

ing the amount of organic molecules grafted to the clay surface

and was used to give additional insights into the structure of

the grafted silanes. To proceed further in the interpretation of

these quantitative data requires knowledge of the surface

hydroxyl group concentration.

Determination of the laponite surface hydroxyl group

concentration. Much effort has been done in the literature to

develop analytical methods for determination of the surface

hydroxyl group concentration of mineral oxides.17,24–26

However, despite the obvious interest of scientists in the

reactivity of clay surfaces, there are surprisingly very few

quantitative data in the literature on the determination of the

OH group concentration of clay minerals.9

The surface hydroxyl concentration of laponite was

determined by titration with triethyl aluminium (TEA). For

accurate determination of the actual amount of OH groups,

the clay surface was dehydrated at 200 uC for 10 hours under a

Fig. 2 Enlargement FTIR spectra of the hydroxyl region of MPTMS

(left) and MPDES (right) -functionalized laponite with increasing

silane contents. a,a9) bare laponite and b,b9–e,e9) silane-functionalized

laponite. The silylation reaction was performed under wet conditions.

The reader must refer to Table 1 for quantitative attributions.

866 | J. Mater. Chem., 2005, 15, 863–871 This journal is � The Royal Society of Chemistry 2005

vacuum before analysis to remove physisorbed water. The

chemical titration was performed in duplicate and gave a

hydroxyl group content of 0.36 ¡ 0.03 mmol g21. We can

reasonably assume that only external hydroxyls are titrated by

this method. Indeed, interlayer Mg–OH groups and also edge

silanols physically trapped within two neighboring clay plates

in a stack are presumed to be inaccessible to the organome-

tallic reagent. The above value is therefore only an approxi-

mate value and must be considered with caution. These data

can be used nevertheless to estimate the silanol group density

on the crystal edges. Given an overall specific surface area

of 370 m2 g21 (as determined by BET) and a density of

2.6 g cm23, we can assume the clay stacks in cylinders

composed of 2 to 3 densely packed elementary platelets, with a

diameter of 30 nm and a height of 2.4 nm. The edge surface of

these cylindrical clay stacks then represents 90 m2 g21.27

Assuming that the hydroxyl groups are randomly distributed

on the edge surface, one can calculate a Si–OH density of

4 mmol m22 (i.e., 2.4 SiOH nm22). This value is two times

lower than the silanol group concentration of fully hydro-

xylated silica surfaces but is similar to the hydroxyl groups

concentration of sepiolite reported by Celis et al.28 We can

therefore conclude that despite its plate-like geometry, laponite

contains a substantial amount of silanol groups corresponding

to around half the amount of exchangeable cations.29 This is

mainly due to the fact that laponite has a low aspect ratio

(around 30) and offers consequently much more external edge

surface than clays of similar structure with a higher aspect

ratio (e.g., for instance montmorillonite).

Effect of reaction time. The kinetics of the silylation reaction

of laponite with the monofunctional and the trifunctional

silanes under wet conditions are shown in Fig. 3. The data

clearly show that the grafting reaction is time-dependent

whatever the nature of the silane compound although the two

systems exhibit significantly different kinetics. While a maxi-

mum grafted amount of around 0.5 mmol g21 was reached for

the monofunctional silane after a relatively short period of

time and remained nearly constant even after a long period of

time, the grafted amount and the grafting yield increased

continuously with time when the trifunctional MPTMS

compound was used as silane coupling agent.

These quantitative measurements are in perfect agreement

with the FTIR data described above and support the view

of the formation of a polysiloxane multilayer upon self-

condensation of the trifunctional molecule and subsequent

grafting of the polycondensate on the clay surface. The rate

of grafting is limited in this case by the rate of formation of

the polysiloxane oligomers and by the rate of diffusion of

these oligomers to the clay surface. In contrast, when the

monoalkoxy silane is used as coupling agent, the grafted

amount appears to be limited to the formation of a monolayer

coverage corresponding to a grafted amount of around

0.5 mmol g21. It is worth noticing here that this value is

slightly higher than the OH group density determined above.

The difference between the two sets of data may be due to

difference in reactivity and accessibility of the TEA and

MPDES reagents to the clay surface. Indeed, the silane

compound forms hydrogen bonds with water molecules

located at the vicinity of the surface. Since the clay layers are

expected to be face to edge rather than face to face stacking,

the silane molecule can penetrate to some extent these

‘‘disordered’’ clay stacks and reach physically entrapped

silanols. According to this, we can reasonably assume that

the actual amount of Si–OH groups in laponite is not that

determined by chemical titration with TEA but is at least equal

to that obtained from the reaction of the clay powder with the

monofunctional MPDES silane that is 0.5 mmol g21 (i.e.,

5.5 mmol m22 or 3.3 SiOH nm22 assuming that the reaction is

taking place exclusively on the clay edges). Although we

cannot completely exclude the possibility of the presence of

non-reacted silanols, the fact that the MPDES grafting density

on the clay edge is three times larger than the commonly

accepted value for optimum packing of molecules of similar

structure on mineral oxide surfaces suggests that nearly all of

the hydroxyl groups have been involved in the grafting

reaction. Accessibility of the MPDES reagent to the surface

hydroxyls is suspected to be promoted in this case by the

geometry of the surface and the localization of the reactive

sites at the extremity of the basal faces of the individual

platelets. There is less steric restriction than on a planar surface

since the molecules can orientate both toward the clay edges

and the basal faces of the clay plates.

Effect of silane concentration. In a second set of experiments,

we varied the initial silane concentration and maintained the

reaction time constant. We began our study with the reaction

of MPTMS and MPDES with laponite in toluene used as

Fig. 3 Evolution of the MPTMS (a) and the MPDES (b) grafted

amounts (—) and grafting yields (- - -) as a function of time. The

silylation reaction was performed under wet conditions. Initial silane

concentration: 3 mmol g21.

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supplied, i.e., without drying either the solvent or the clay

powder. The data plotted in Fig. 4a and b show that the

grafted amount increases with increasing the silane concentra-

tion in the treating solution, while concurrently, the grafting

yield decreases. As expected from the previous kinetics data,

the grafted amount is again significantly higher for the trifunc-

tional than for the monofunctional silane. As before, these

results indicate that the trifunctional c-MPTMS silane can

potentially react in the bulk to yield polysiloxane oligomers

which further deposit on the clay surface. Of course oligomer

formation is highly dependent on the experimental conditions

and the presence of a substantial amount of water in the

solvent may account for the relatively high grafted amounts.

Concerning the monofunctional silane, it is to be noted that

the plateau value is similar to that determined in the kinetic

study which supports the assumption of saturation of the

surface, each MPDES molecule reacting with one silanol

group.

With the intention to control the surface coverage of the

grafted silane molecules in the case of the trifunctional

compound, a new set of experiments was performed under

conditions which enabled extensive condensation of the

MPTMS precursor to be avoided, i.e., in anhydrous toluene

and using a dehydrated clay sample. As expected, reaction of

MPTMS with laponite under anhydrous conditions resulted in

a substantial decrease of the grafted amount (Fig. 4c) although

the silane content at saturation was still two times higher than

a single layer coverage indicating that we were unable to

prevent the formation of polysiloxane oligomers. It is likely

that the thermal treatment of the clay at 200 uC was

insufficient to provide a completely dehydrated clay sample.

Indeed, it is well established that silane modification of mineral

substrates is sensitive to drying conditions. It is known from

previous works that very small changes in the experimental

procedure and more particularly in the water content may

significantly affect the grafting reaction. Such a sensitivity

makes it difficult to ensure reproducible surface coverages, and

obviously accounts for the large disparity of the literature data

on the grafting reaction of organo alkoxysilanes to mineral

surfaces. Although we take care to remove physisorbed

water by heating the clay powder at 200 uC, water molecules

physically entrapped within the interlayers of the clay stacks

may be still present after the thermal treatment. This hypo-

thesis holds if we consider that at least one third of the total

amount of water adsorbed onto laponite surfaces is released

above 200 uC.30 These firmly bound water molecules are

suspected to progressively escape from the clay surface upon

grafting of the coupling agent molecules. Indeed, as shown

above, the silane compound competes with the water

molecules for hydrogen bonding to the surface. The initially

attached silane molecules may penetrate to some extent into

the clay stacks and create a steric hindrance between the

individual clay plates which will promote the displacement of

water from the interlayer gallery space. In addition, grafting of

the silane molecules renders the surface more and more

hydrophobic which also contributes to reduced interactions

between the clay surface and the water molecules and

promotes therefore water removal from the inter-gallery space

and clay expansion in organic solvent. As will be demonstrated

below, these assumptions are supported by i) the regular

increase of the interlamellar distance, and ii) the decrease of

the water content of the organoclay with increasing the

amount of chemisorbed silane.

Impact of grafting on the physicochemical properties of

functionalized laponites. Table 1 shows the different properties

of the surface-grafted laponite particles as a function of the

grafted amount for both the monofunctional and the trifunc-

tional coupling agents. Wide angle X-ray diffraction (WAXD)

showed an increase of the interlamellar distance d with

increasing the silane content in case of the trifunctional silane

compound for silylation reactions performed both under wet

and anhydrous conditions. In contrast, grafting of the

monofunctional silane compound had nearly no influence on

the d spacing. Expansion of the clay galleries is suspected to

occur via the formation of a polysiloxane multilayer that links

together the clay plates and penetrates to some extent the

interlayer space of the gallery. The formation of a bulky

polysilsesquioxane structure on the border of the clay plates

Fig. 4 Evolution of the MPTMS (a) and the MPDES (b) grafted

amounts (—) and grafting yields (- - -) as a function of the initial silane

concentration. a) and b) Silylation reaction performed under wet

conditions. c) Silylation reaction performed under anhydrous condi-

tions. Reaction time: 21 days.

868 | J. Mater. Chem., 2005, 15, 863–871 This journal is � The Royal Society of Chemistry 2005

also has a drastic influence on the specific surface area of the

clay (Sspec). The data in Table 1 indicate that Sspec decreases

with increasing the grafted amount which suggests limited

access to the internal porosity due to the multilayer con-

figuration of grafting. The bulkiness of the grafted poly-

siloxane species blocks the access of nitrogen to the micropores

of laponite.5a It is worth noting that both the increase of the d

spacing and the decrease of the specific area are nearly

proportional to the quantity of grafted moiety (Fig. 5).

Additional informations on the clay properties were

provided by TGA (see electronic supplementary informa-

tion{). While bare laponite shows a significant weight loss

below 200 uC which can be attributed to weakly bonded water,

the functionalized laponite samples contain significantly

lower amounts of water indicating that the surface has been

rendered hydrophobic. It is worth noticing again that,

independently of the nature of the silane coupling agent, the

higher the grafted amount, the lower the amount of

physisorbed water and the more hydrophobic is the clay

surface. Those quantitative data support the above FTIR

observations on the diminution of the OH group stretching

vibration intensity upon grafting.

From the data reported above, we can give a general

description of the grafting reaction occurring on the surface of

the laponite clay particles. A schematic representation is given

in Fig. 6 for illustration.

It is known from previous works that a minimum amount of

physisorbed water is necessary for efficient grafting of the

silane compound on the inorganic surface. The silane molecule

initially contained in the organic solution hydrolyses in the

bulk or in the vicinity or the surface depending on the amount

of available water and gradually adsorbs on the clay plates in a

horizontal configuration with the carbonyl groups forming

hydrogen bonds with the surface hydroxyls. The reactive

alkoxysilyl head group of the silane molecule also adsorbs on

the clay and can form a covalent bond with the edge silanols. If

the silane compound is monofunctional, the reaction cannot

proceed further and the surface coverage is limited in this case

by the accessibility of the coupling agent to the surface

hydroxyls. Although steric constraints often predominate and

control the extent of grafting, such restrictions seem not to be

operative in the present system presumably because of the

geometry of the clay plates, the reaction occurring at the end of

the crystals which cannot be assimilated to a surface. When the

trifunctional molecule is used as coupling agent, the excess of

water promotes the formation of polysiloxane oligomers the

size of which increases with increasing the initial silane con-

centration. These species progressively intercalate within the

clay platelets giving rise to a gradual increase of the interlayer

d spacing. The intercalation reaction starts at the edges of the

laponite microcrystals and the molecules progressively pene-

trates towards the interior. At low concentrations, the interac-

tion proceeds as reported above for the monofunctional

derivative, but as the reaction continues a multilayer coating,

characterized by the appearance of free carbonyls, is formed

around the clay plates. Not only can these oligomers link together

the individual platelets but they can also create a chemical bond

between neighboring clay stacks thus providing limited accessi-

bility to the internal clay porosity and surface area.

Conclusion

Laponite clay particles were functionalized with monofunc-

tional and trifunctional methacryloxypropyl organoalkoxy-

silanes. FTIR and solid state NMR showed successful grafting

of the silane molecule on the border of the clay plates. The

extent of grafting was estimated by elemental analysis and

plotted as a function of the silane concentration and the

reaction time. Optimum organic loadings ranging from 0.56 to

Fig. 5 Evolution of the interlamellar distance and of the specific

surface area of the MPTMS-functionalized laponite as a function of

the MPTMS grafted amount for silylation reactions performed under

wet conditions.

Table 1 Interlamellar distance, specific surface area and water uptake as a function of the grafted amount of functionalized laponites

FTIRspectra

Silaneconcentration/mmol g21

Graftedamount/mmol g21 (EA) d001/A (XRD)

Sspec/m2 g21

(BET)Water uptake(wt%) (TGA)

Laponite RD a, a9 0 0 12 370 10.2MPTMS-grafted laponitea b 0.78 0.80 14.9 212 4.8

c 2.98 1.54 15.1 95 3.9d 3.73 1.66 16.8 91 3.0e 5.22 1.86 16.9 76 2.9

MPDES-grafted laponitea b9 0.75 0.29 13.0 264 7.8c9 2.24 0.40 13.6 263 7.1d9 5.22 0.51 13.0 238 6.6e9 6.71 0.56 13.0 201 5.0

MPTMS-grafted laponiteb — 0.75 0.59 12.7 295 —— 2.98 0.83 12.6 227 —— 5.22 1.13 12.7 195 —

a Silylation reactions performed under wet conditions. b Silylation reactions performed under anhydrous conditions.

This journal is � The Royal Society of Chemistry 2005 J. Mater. Chem., 2005, 15, 863–871 | 869

1.9 mmol g21 were achieved depending on the nature of the

coupling agent. Whereas the trifunctional silane coupling

agent was grafted as polysiloxane oligomers pillaring the clay

plates, the monofunctional silane formed a monolayer cover-

age on the clay surface. Examination of the organically-

modified products by FTIR enabled ones to give a qualitative

description of the grafting process involving the organosilanes

and the edge-hydroxyl groups of the clay samples. At low

concentrations, the silane molecules form a monolayer lying

down on the surface with the methacryloxy groups in close

contact with the border of the clay plates. Further addition of

the coupling agent in case of the trifunctional compound leads

to the formation of a multilayer coverage and orients the

carbonyl group of the silane moiety away from the clay

surface. WAXD, BET and TGA analyses showed a significant

influence of grafting on the laponite structure properties in

agreement with the quantitative analysis. The specific surface

area decreased significantly upon grafting while, concurrently,

the interlamellar distance increased. The effect was much more

pronounced for the trifunctional silane than for the mono-

functional derivative in agreement with all other observations.

Acknowledgements

The authors are grateful to the SFERE-CONACYT exchange

program for its financial support of this work. The gift of a

sample of laponite RD by Rockwood Additives is also greatly

acknowledged.

Norma Negrete Herrera,a Jean-Marie Letoffe,b Jean-Pierre Reymondc

and Elodie Bourgeat-Lami*a

aLaboratoire de Chimie et Procedes de Polymerisation, UMR 140CNRS/CPE, Bat. 308, 43, Bd. du 11 Nov. 1918, BP 2077, 69616,Villeurbanne Cedex, France. E-mail: bourgeat@lcpp.cpe.frbLaboratoire des Multimateriaux et Interfaces, UMR CNRS 5615,Universite Claude Bernard Lyon 1, 69622 Villeurbanne Cedex, FrancecLaboratoire de Genie des Procedes Catalytiques – CPE, Bat. 308F, 43,Bd. du 11 Nov. 1918, BP2077, 69616, Villeurbanne Cedex, France

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