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Journal of Sol-Gel Science and Technology 8, 173–176 (1997)c© 1997 Kluwer Academic Publishers. Manufactured in The Netherlands.

Synthesis and Characterization of the Organic Surface Modificationsof Monodisperse Colloidal Silica

C. GELLERMANN, W. STORCH AND H. WOLTERFraunhofer-Institut für Silicatforschung, Neunerplatz 2, D-97082 Würzburg, Germany

Abstract. Monodisperse colloidal silica spheres were prepared from tetraethylorthosilicate in mixtures of water,ammonia and ethanol. The surfaces of the spheres were successfully modified by chemical reaction with silanecoupling agents. Several qualitative and quantitative methods were employed to analyse the organic surface modi-fications. As a result, the surface coverage of silica spheres with silane coupling agents could be calculated usingdifferent characterization methods.

Keywords: colloidal silica, monodisperse particles, surface modification, silane coupling agents, characterization

1. Introduction

Applications involving submicron particles are of inter-est not only in the academic field of physical chemistry,dealing with stability and interactions in dispersions,but also in numerous industrial fields including ceram-ics, catalysis, chromatography, pigments, and pharma-ceuticals. Over the years several techniques have beendeveloped for the synthesis of powders with uniformsize, shape and composition. The most convenientmethod for the preparation of so called monodisperseparticles is the control of chemical reactions in ho-mogenous solutions. One of the basic processes, whichyields monodisperse inorganic spheres, was publishedby Stober et al. [1], who synthesized colloidal silicawith diameters of 50–2000 nm by a simple one-step-procedure from tetraethylorthosilicate (TEOS). Theseparticles were so extraordinarily spherical that manyother research groups used them as model system forfurther studies of rheology [2], electrorheology [3],light scattering [4], sintering [5] or sedimentation [6].Modifications of the hydrophilic surface have been re-alized by polymer adsorption [7], graft polymerization[8], esterification [9] and silylation with silane cou-pling agents [10] in order to adjust the properties andto improve dispersibility in organic media.

The long-term objective here is to study particleswhich can be used as fillers in composite materials, e.g.,

inorganic-organic copolymer systems (ORMOCER1s[11]). Therefore, we have prepared monodisperse col-loidal silica which has been modified with silane cou-pling agents by an in-situ multistage process.

2. Experimental

Particle size was measured by dynamic light scatter-ing (DLS) in very dilute dispersions at 25◦C, using aMalvern Autosizer 4700. Transmission electron mi-crographs were obtained using a Scanning Transmis-sion Electron Microscope Philips CM 12. The sampleswere prepared by dipping copper carrier grids coveredwith a carbon film into the dilute dispersion. Particlesizes were determined by measuring at least 400 par-ticles on the micrograph and calculating the averagesize and the standard deviation. Nitrogen adsorption-desorption isotherms (BET) were determined with aSorptomat Micromeritics ASAP2400. Spectrometrywas conducted with a Biorad FTS 25 fourier trans-form infrared spectrometer in combination with a dif-fuse reflectance infrared Fourier transform (DRIFT)apparatus from Spectra Tech. Shimadzu spectrome-ter UV-3100 and the Ulbricht accessory MPC-3100were used to record the UV-spectra. Zeta potentialswere measured with a Zetamaster of Malvern Instru-ments, in aqueous dispersions, as a function of the pH.

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Quantitative analyses of amino or epoxy groups werecarried out with a Mettler titrator DL 25, according toDIN 53188. Titrations of C C-double bonds were car-ried out according to a method of Byrne and Johnson[12]. Elemental analyses were obtained with a HeraeusCHN O-Rapid. Thermogravimetric (TGA) and dif-ferential thermal (DTA) analyses were conducted witha Netzsch STA 429, by heating the samples at 20 K/minfrom 30 to 800◦C.

2.1. Particle Synthesis

Monodisperse silica particles were prepared in the av-erage diameter range of 50–200 nm, according to amethod of St¨ober et al. [1]. Here, particles of one size(60 nm) are highlighted as a typical example. In thefirst step, 180 ml (2.18 mol) of 12.1M ammonia and3600 ml (61.7 mol) of ethanol were mixed and stirredat 21◦C. Then 180 g (864 mmol) of TEOS was rapidlyadded. During the following 20 min the reaction mix-ture became turbid as the silica sol (sample U1–U3)was formed.

2.2. Surface Modification

Surface modifications were carried out in situby the use of silane coupling agents purchasedfrom Fluka Chemie: vinyltrimethoxysilane (VTMO),3-methacryloxypropyltrimethoxysilane (MEMO), 3-aminopropyltriethoxysilane (AMEO) and 3-glyci-doxypropyltrimethoxysilane (GLYMO). The appropri-ate amount of alkoxysilane (Table 1) was quickly added

Table 1. Samples obtained by centrifugation (C) and evaporation(E) after in situ modification of silica samples (U) with given amountsof different silane coupling agents.

Unmodified Silane AmountSample sample coupling agent [mmol/l]

E1 U1 VTMO 10.3

C1 U1 VTMO 10.3

E2 U1 MEMO 10.3

C2 U1 MEMO 10.3

E3 U2 AMEO 10.3

C3 U2 AMEO 10.3

E4 U3 GLYMO 8.60

E5 U3 GLYMO 22.1

C4 U3 GLYMO 10.3

Figure 1. Flow chart of particle syntheses.

to 500 ml of the silica sol at 21◦C and stirring was con-tinued for 1 h. The isolation of the modified particleswas achieved by two different methods: centrifugationat 2500 rpm or evaporation of solvent. Four subsequentadditional washing steps were required for both iso-lation methods, including redispersion in ethanol andcentrifugation (Fig. 1). The resulting colorless precip-itates or sediments were dried under vacuum at 70◦Cfor 8 h.

3. Results and Discussion

Size measurements of the synthesized particles by dy-namic light scattering showed average diameters rang-ing between 58 and 66 nm. In spite of the relativelysmall standard deviation of 12–18%, reflecting narrowsize distributions, the surface modification cannot bedefinitely proven by DLS.

The specific surface areas were measured by BET,e.g., 69–75 m2/g for U1-3. The values obtained for themodified particles varied, depending on the isolationmethod, from 38–50 m2/g for E1-5 and from 51–62m2/g for the centrifuged samples C1-4.

A typical TEM photograph of an organically modi-fied colloidal silica is shown in Fig. 2. The morphol-ogy of the particles is evident: they are homogeneouslyspherical which indicates a surface regularly coveredby silane coupling agents. A second nucleation did notgenerally occur.

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Monodisperse Colloidal Silica 175

Figure 2. TEM photograph of MEMO modified silica particles (E2)with a diameter of 56± 4 nm.

The organic surface modifications were qualitativelyconfirmed using DRIFT spectrometry. Characteristicsignals in the C O stretching region were observed be-tween 1725–1690 cm−1 for MEMO modified samples.Sample E2 for example, shows bands at 1725, 1709 and1692 cm−1, indicating different bondings of MEMOmolecules on the particle surface. Relatively weak butdefinite vinyl bands at 1415–1400 cm−1 confirm thepresence of CC double bonds on VTMO modifiedcolloidal silica, e.g., 1410 cm−1 in E1. Measurementsin the UV region support the qualitative DRIFT analy-ses. In spectra of MEMO modified samples, e.g., E2,an absorption with a maximum at ca. 210 nm confirmsthe presence of methacrylate groups and a shoulderbetween 190–230 nm supports the evidence for vinylgroups on the particle surface of E1.

Zeta potential measurements clearly demonstrate thelarge influence of modification on the surface proper-ties (Fig. 3). As expected, basic functionalities, such asamine groups (sample E1), cause a considerable shiftof the isoelectric point to high pH in contrast to vinyl(E1) or silanol groups (U1).

Several quantitative methods, including CC dou-ble bond, amine or epoxide titration, elemental (CHN)and thermogravimetric (TGA) analysis were employedto determine the amount of surface functional groups.The values obtained by titration reflect the number ofreactive functionalities. It is possible that an additional

Table 2. Quantitative analyses of organic surfacemodifications inµmol/g (molecules/nm2).

Sample Titrations CHN TGA

E1 567 (8.8) 891 (13.8) 562 (8.7)

C1 155 (1.7) 146 (1.6) 148 (1.6)

E2 485 (7.7) 372 (5.9) 370 (5.9)

C2 147 (1.7) 30 (0.4) 24 (0.3)

E3 491 (5.9) 500 (6.0) 534 (6.4)

C3 198 (1.9) 8 (0.1) 17 (0.2)

E4 269 (3.6) 482 (6.4) 399 (5.3)

E5 732 (15.7) 1195 (25.7) 831 (17.9)

C4 54 (0.6) 29 (0.3) 52 (0.5)

Figure 3. Zeta potentials as a function of pH.

amount of groups is present which is inaccessible totitration. The surface coverages of samples could becalculated, by use of the specific surface areas, values inparentheses (Table 2). It is obvious from a general com-parison of evaporated and centrifuged samples, that themethod of evaporation led to a multilayer coating. Atypical example is E1 with 567µmol/g and a surfacecoverage of 8.8 molecules/nm2 in contrast to C1 with155µmol/g and 1.7 molecules/nm2.

Elemental (CHN) analyses of U1–U3 resulted in4.31–4.35% of carbon, 2.21–2.53% of hydrogen and0.62–0.74% of nitrogen. With the assumption thatthe amount of unhydrolysed ethoxy groups remainsconstant after modification the number of functionalgroups was calculated after subtracting the carbon per-centage of unmodified from modified samples.

The amounts of organic groups were obtainedin TGA/DTA measurements from the differences in

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weight loss of unmodified (7.10%) and modifiedsamples between 250 and 800◦C. In this temperaturerange, two exothermic processes are generally known:oxidation of organics and dehydroxylation. Quantita-tive results were obtained on condition that the organicsurface modification does not influence the weight lossof the silica core (Table 2).

Good agreement was observed between titrations,elemental (CHN) and thermogravimetric (TGA) anal-yses; C1 is an excellent example with constantvalues of 146–155µmol/g and surface coverages of1.6–1.7 molecules/nm2.

4. Conclusion

Organic surface modifications of monodisperse col-loidal silica particles were obtained in situ by the useof four different silane coupling agents. For the sur-face characterization of these particles the analyticalmethods used are generally suitable: qualitative deter-minations with DRIFT-, reflectance UV-spectrometryand zeta potential measurements definitely confirm theexistence of organic modifications. Additional quanti-tative analyses of the functionalities were in excellentagreement for different methods.

With regard to the wide field of applications, anincorporation of the described modified particles intodifferent polymer matrices, e.g., ORMOCER1 resins,opens up new possibilities in the area of composites.

Acknowledgment

We gratefully thank Dr. G. Neumann for valuable dis-cussions.

Note

1. Trademark of Fraunhofer Gesellschaft e.V., Munich.

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