Surface modification of silica gels with aminoorganosilanes

COLLOIDS

AND Colloids and Surfaces A: Physicochemical and Engineering Aspects 98 (1995) 235 241

ELSEVIER

A

SURFACES

Surface modification of silica gels with aminoorganosilanes K.C. Vrancken *, K. Possemiers, P. Van Der Voort, E.F. Vansant Laboratory of Inorganic Chemistry, University of Antwerpen ( U.I.A.), Universiteitsplein 1, B-2610 Wilrijk, Belgium Received 11 November 1994; accepted 6 February 1995

Abstract

The surface modification reaction of silica gel with aminoorganosilanes proceeds in two steps. For both the reaction step and the curing step, the chemical and physical interactions of the silane molecules with the silica surface have been modelled. From ethanol leaching tests, the reaction phase interaction, in dry conditions, may be characterized as 22% proton transfer, 10% hydrogen bonding and 68% siloxane bonding. The deposition of the aminosilane molecules is governed by the specific surface area and surface hydration. For the bifunctional N-/~-aminoethyl-7-aminopropyltrimethoxysilane, a two-step deposition is observed. The rate of siloxane bonding in the curing phase is limited by the number of alkoxy groups, and results in a turnover of the aminosilane molecules. A new application of aminosilane-modified silica gel is developed in converting the aminosilane layer to SiC. Thus, the liquid phase path of the chemical surface coating process, for the controlled synthesis of advanced ceramics, is set up.

Keywords: Aminoorganosilanes; Silica gels; Surface modification

1. Introduction

Aminoorganosilanes have the general formula H2N-R-Si-(OR')3. They differ from the general organosilanes in carrying an aminofunctional group in the organic chain. This group is responsible for the specific chemical reaction behaviour and high reactivity of the aminosilane molecules. The electron-rich nitrogen centre of the amine group can enter into hydrogen bonding interactions with hydrogen donating groups, such as hydroxyl groups or other amines. Mixing of an aminosilane with silica gel results in fast adsorption, by hydrogen bonding of the amine to a surface hydroxyl group [ 1]. After adsorption, the amine group can catalyze the condensation of the * Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved S S D I 0927-7757(95)03119-7

silicon side of the molecule with a surface silanol. Thus siloxane bonds with the surface may be formed in the absence of water [2,3]. For other silanes the siloxane bond formation requires an initial hydrolysis of the ethoxy groups or the addition of an amine to the reaction mixture [4]. The three most widely used and studied aminosilanes are [5] ?-aminopropyltriethoxysilane (APTS; (CH3CHzO)3SiCHzCHzCHzNH2), ~-aminopropyldiethoxymethylsilane (APDMS; (CH3CH20)2 CH3SiCH2CH2CH/NH2), and N-/%aminoethyl-7aminopropyltrimethoxysilane (AEAPTS; ( C H 3 0 ) 3 SiCH2CH2CH/NHCH2CH2NH/). In the broad field of oxide modification, aminosilane-modified silica gel is the most widely used combination. The modified silica may be used as such, or after a secondary treatment. Pure aminosilane-modified silica gel is used as a stationary

236

K. C Vrancken et al./Colloids Surfaces A: Physieochem. Eng. Aspects 98 (1995) 235 241

phase in liquid and gas chromatography [6,7]. Trace levels of metals, such as copper, in aqueous solution are separated and concentrated before analysis. Aminosilane-modified silica serves very well for this process [8,9]. In this application, AEAPTS is often used, because of the high coordination capacity of the bifunctional organic chain. Aminosilanes find a broad range of applications as coupling agents in fibre-reinforced plastics [ 10]. The aminosilane-silica couple is used for the reinforcement of natural and synthetic rubbers and elastomers [ 11 ]. The amino group may also serve as an active site on the silica surface to bind other molecules. These molecules may impart new interaction capacities to the surface. A major field of research and application is the immobilization of enzymes on the silica surface [12,13]. Apart from their increased stability, isolated enzymes are easier to handle, are more specific in their function and are more predictable in their activity. The bonding of metal complexes on aminosilane-silica allowed the production of heterogeneous catalysts. Various types of catalysts have been developed [14]. Aminosilane-modified silica layers are used as a starting layer for integrated circuit (IC) buildup [15]. All these applications explain the wide interest in the chemistry of aminosilane-modified silica. A fundamental understanding of the reaction mechanism may assist the further optimization of all these uses. Besides this, there is a large interest in the development of still new applications of the aminosilane-modified silica. Any new development may be quickly implemented on the industrial scale, since all basic operations are already wellestablished. This article deals with both topics. A fundamental study of the reaction of aminosilanes with silica gel is followed by the development of a new application of aminosilane-modified silica.

was performed under vacuum. At higher temperatures, an ambient atmosphere was used. The pretreated silica gel was stirred for 2 h in a 1% (v/v) silane/toluene (p.a. Merck) solution. For nbutylamine (Fluka) an equimolar 0.4% (v/v) solution was used. The aminosilanes APTS, AEAPTS (OSi chemicals) and A P D M S (Hills AG) were distilled at reduced pressure before use. Toluene was stored on molecular sieve (5A) to keep it dry. The reaction was performed under a CaC1 z guard tube. After filtration, the modified substrate was transferred to a vacuum sample holder and cured at 423 K for 20 h. Leaching tests were performed by stirring 1 g of the modified silica in a 0.3% salicyclic aldehyde/ ethanol (Merck) solution. The supernatant was centrifuged and absorbance was measured at 404 nm. The total coverage was measured by taking three 10 ml samples of the filtrate and adding 150 gl of salicyclic aldehyde and 150 gl of diethyl ether (Merck). The absorbance at 404 nm was measured after 1 h of reaction. The APTS concentration was calculated using a linear calibration curve (r= 0.996). Freeze sampling was performed as reported previously [ 16]. Non-pretreated silica was modified with a 1% APTS/toluene solution and was cured at 383 K under an ambient atmosphere. The sample was heated at 10 K min -1 to 1873 K, and the temperature was kept constant for 2 h. After cooling, the sample was stored under dry nitrogen until analyzed. X-ray photoelectron spectra (XPS) were obtained, using a SSI-SSX 100 spectrometer, with an A1 K~ X-ray source (hv= 1486.6 eV), a pass energy of 50eV, a spot size of 600gm and a pressure of 10 -8 Torr.

3. Results and discussion

3.1. Fundamental study of the reaction mechanism 2. Experimental Kieselgel 60 (Merck) was thermally pretreated before reaction. Treatment temperatures ranged from 298 to 1073 K. For T~<673 K, pretreatment

For a fundamental understanding of the processes occurring during the modification, a distinction has to be made between processes taking place in the reaction step and in the post-reaction curing.

K~C. Vrancken et al./Colloids Surfaces A." Physicochem. Eng. Aspects 98 (1995)235 241

In the reaction phase, three types of interaction of the aminosilane molecule with the silica surface have been reported [-17,18] (Fig. 1). The dmine may enter into a hydrogen bonding interaction with a surface hydroxyl group. The basic amine function may abstract a proton from a silanol group and form an ionic bond. This type of interaction is much more stable than the first one. The hydrogen-bonded molecules may self-catalyze the condensation of the silicon side of the silane molecule. Thus, a covalent siloxane bond is formed. In order to determine the extent of all three of these interaction types in the reaction phase, a leaching test [-19,20] was performed on a noncured sample. Upon stirring in ethanol, the weakly bonded silane molecules desorb and the amount is measured quantitatively by means of a colour reaction. The amine group reacts with salicyclic aldehyde, forming a yellow Schiff's base with '~max = 404 nm. Physisorbed molecules desorb, while ionic and covalently bound molecules are stable towards this ethanol leaching. In Fig. 2, the relative percentage of silane lost from the surface is displayed as a function of leaching time. From

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Fig. 1. Various aminosilane interaction types in the reaction phase. (a) Hydrogen bonding; (b) proton transfer; (c) condensation to siloxane.

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Fig. 2. Ethanol leaching curves of uncured modified silica. Curve a, APTS; curve b, n-butylamine.

the APTS leaching curve (Fig. 2 (curve a)), we obtain a relative amount of about 10% of the coating which is only physically bonded to the surface before curing. In order to distinguish the ionic bonding from the covalent attachment, the same test was performed using n-butylamine (Fig. 2 (curve b)). This molecule is a carbon analogue of APTS. The amine group interaction of butylamine is similar to that of APTS, but the amine group has no silicon atom to form covalent linkages. Twenty two per cent of the butylamine appears to be stable towards the ethanol leaching. Therefore it may be concluded that after 2 h of reaction and before curing, 22% of the coating is in ionic interaction with the surface, 10% is hydrogen bonded and the remaining 68% is covalently bonded. The course of the aminosilane deposition during the reaction phase was measured using freeze sampling analysis. After certain reaction times, samples are taken from the reaction mixture, which are then frozen to stop the reaction. After melting and separation, the amount of reacted silane is measured. The reaction profiles of APTS and AEAPTS are displayed in Fig. 3. Both compounds reach an equilibrium adsorption within 1 min of reaction. This reflects the quick adsorption of the amine group, forming hydrogen bonds with the surface silanol groups.

K. C Vrancken et al./Colloids Surfaces A: Physicochem. Eng. Aspects 98 (1995) 235 241

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For the AEAPTS, carrying two amine groups, a two-step adsorption is found. This indicates that an amine group of the organic chain remains free in the first step and is able to adsorb an additional layer of silane molecules. Since secondary amines are better acceptors for hydrogen bonds, it will be the primary amine function, at the end of the organic chain, that remains free. Upon adsorption of the secondary layer, an equilibrium situation is again reached. For the monofunctional silane, the first equilibrium is followed by an additional adsorption, which does not have a step-wise profile. This may not be observed in this figure, but became evident from additional experiments, as reported previously [ 16]. An explanation for this increase will be given below. The equilibrium situation is related to the localized adsorption of silane molecules on the surface hydroxyl groups, thus forming a monolayer coating on the surface. In order to study the effect of substrate-related parameters, the pretreatment temperature of the silica substrate may be varied. In Fig. 4, the total coverage, expressed as number of APTS molecules per nanometre squared, is displayed as a function of the pretreatment temperature. The total coverage is a measure for both chemically and physically adsorbed silane species. The degree of surface hydration and hydroxylation, as well as the specific surface area of the silica, varies with varying pretreatment temperature.

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Fig. 4. Total APTS coverage as a function of silica pretreatment temperature.

In the low-temperature range (298 473 K), i.e. upon dehydration of the silica gel, a decrease of surface loading with increasing temperature is observed. If surface water is present, surface adsorbed molecules hydrolyze and condense with other silane molecules. Thus, a multilayer coating is obtained. At higher pretreatment temperatures, a constant loading is observed. While the degree of hydroxylation decreases in this temperature region, the specific surface area remains constant. Therefore, the total coverage is controlled by the specific surface area of the silica, rather than the hydroxyl group content. Silane molecules are deposited on the silica surface, with each molecule covering 50 ~2. The previously mentioned course of APTS deposition, may be interpreted as a silanol groupspecific initial deposition, reaching equilibrium, followed by a filling of the free space on the silica surface. Non-specific adsorbed molecules will desorb quickly in the curing phase. For silica gel pretreated at 1073 K, an increased surface coverage is observed. This may be due either to the structure of the coating layer, involving multilayer formation or a change in the molecular orientation at the surface, or to the different porous structure of the 1073 K pretreated silica. None of both hypotheses can be excluded on the basis of these data. The participation of strained siloxane groups [21] is another plausible explanation. It has been previously reported that those

K. C. Vrancken et al./Colloids Surfaces A: Physicochem. Eng. Aspects 98 (1995) 235-241

maximal stability is reached. The difference in condensation behaviour is clearly due to the different number of ethoxy groups in the silane molecule. The higher number of ethoxy groups of the APTS molecule causes a much faster stabilization. Concerning the amine side of the molecule, valuable information can be drawn from 13C solidphase N M R spectra. The results have been published elsewhere [-20] but we would like to recapitulate the conclusions. From the position of the peak due to the /?-C atom of the propyl chain, information on the mobility of the aminopropyl chain may be obtained. It appeared that upon curing, the amine group relinquishes its interaction with the silica surface. Therefore the aminosilane molecule turns from the original amine-down position in the reaction phase (Fig. 6, sketch a), towards an amine-up position after condensation (Fig. 6, sketches b and c). This is called the flip mechanism.

siloxanes may enter into physical and chemical interactions with silanes [-22,23] and ammonia [24,25]. Here it appears that aminosilanes also are able to interact with the strained siloxane bridges [26]. It has been generally accepted that the majority of the silane-to-surface siloxane bonds are formed in the curing phase. Above, we have demonstrated that already in the reaction phase, 68% of the APTS molecules have formed at least one chemical bond with the surface. The formation of covalent bonds in the curing phase has been probed by a similar ethanol leaching test. In Fig. 5, the stability towards ethanol leaching is plotted as a function of curing time. Curves for both APTS (Fig. 5 (curve a)) and APDMS (Fig. 5 (curve b)) are displayed. It can be seen that the APTS reaches its maximal stability within 3 h of curing. Only 2% of the coating remains merely physisorbed. For the APDMS, maximal stability is reached after a much longer time. APDMS needs 20 h of curing before

3.2. Development of new applications of aminosilane-modified silica

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As we have characterized the processes occurring during the reaction of silica gel with aminosilanes, progression towards the development of new applications of aminosilane-modified silica gel can be made. In view of the research on the synthesis of thin layers of advanced ceramic coatings, by means of the chemical surface coating method [-27], the modified substrate was subjected to hightemperature treatment. APTS-modified silica gel was exposed to temperatures up to 1873 K under a pure argon atmosphere. From thermal analyses [28], two regions of weight loss could be observed.

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Fig. 5. Amount of physisorbed aminosilane molecules per gram of silica as a function of curing time. Curve a, APTS; curve b, APDMS.

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K. C Vrancken et al./Colloids Surfaces A." Physicochem. Eng. Aspects 98 (1995) 235 241

240

The most important loss in weight occurred below 1273 K. A second, minor loss was observed upon heating from 1273 to 1873 K. The total mass lost from the sample appeared to be higher than the mass of the coating. In accordance with the hightemperature conversion of organosiloxanes, this may be attributed to a rearrangement of the modification structure. As discussed by Burns et al. [-29], carbothermic reduction involves the loss of oxygen from the substrate, as CO. Infrared spectral analyses indicated the loss of N H and CH vibrational bands, while the silica structural vibrations are maintained. From XPS data we are able to confirm the formation of silicon carbide on the silica surface. The C is region of the spectra shows that at 1273 K, a relative amount of 40% of the total carbon content is present as silicon carbide. At 1873 K, we obtain a 80% conversion to silicon carbide. The C ls region of the corresponding XPS spectrum is displayed in Fig. 7. Therefore, we can conclude that we have developed a new synthesis route to advanced ceramic silicon carbide coatings. This new method is characteristic for the chemical surface coating (C.S.C.) process, in that it involves

chemical modification of the silica substrate, followed by a controlled thermal treatment. However, while previous C.S.C. syntheses involved subsequent chemical reactions of gaseous compounds [24], this route uses a one-step liquid phase reaction.

4. Conclusion The modification of silica gel with aminosilanes has been characterized with respect to physical interactions and chemical reactions of the silane molecules with the silica surface. A new application of APTS-modified silica has been developed, in converting the silane layer to an SiC advanced ceramic coating (C.S.C. method).

Acknowledgements The authors would like to thank Frederique Bodino and Christophe Girardeaux of the LISE laboratory at the University of Namur, Belgium for the XPS analyses, Dr. Bob Gilissen of VITO,

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K. C. Vrancken et al./Colloids Surfaces A: Physicochem. Eng. Aspects 98 (1995) 235-241

Mol, Belgium for the thermal conversion experiments and Pascale Jacquet of the University of Orleans, France, for her experimental assistance. K.C.V. and P.V.D.V. are indebted to the NFWO/FNRS as a research assistant and senior research assistant, respectively. K.P. acknowledges the financial support of the IWT.

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[13] K.M.R. Kallury and M. Thompson, in J.J. Pesek and I.E. Leigh (Eds.), Chemically Modified Surfaces, The Royal Society of Chemistry, Cambridge, U.K. 1994, p. 58. [14] T.J. Pinnavaia, J.G-S. Lee and M. Abedini, in D.E. Leyden and W.T. Collins (Eds.), Gordon and Breach Science Publ., New York, 1988, p. 35. [15] J.H. Helbert and N. Saha, in Silanes and Other Coupling Agents, VSP, Utrecht, The Netherlands, 1992, p. 439. [16] K.C. Vrancken, E. Casteleyn, K. Possemiers, P. van der Voort and E.F. Vansant, J. Chem. Soc. Faraday Trans., 89 (1993) 2037. [17] G.S. Caravajal, D.E. Leyden, G.R. Quinting and G.E. Maciel, Anal. Chem., 60 (1988) 1776. [18] D.J. Kelly and D.E. Leyden, J. Colloid Interface Sci., 147 (1991) 213. [19] T.G. Waddell, D.E. Leyden and M.T. DeBello, J. Am. Chem. Soc., 103 (1981) 5303. [20] K.C. Vrancken, P. van der Voort, K. Possemiers, P. Grobet and E.F. Vansant, in J.J. Pesek and I.E. Leigh (Eds.), Chemically Modified Surfaces, The Royal Society of Chemistry, Cambridge, U.K., 1994, p. 46. [21] B.A. Morrow and I.A. Cody, J. Phys. Chem., 80 (1976) 1998. [22] P. van der Voort, I. Gillis-D'Hamers, K.C. Vrancken, E.F. Vansant, J. Chem. Soc. Faraday Trans., 87 (1991) 3899. [23] L.H. Dubois and B.R. Zegarski, J. Phys. Chem., 97 (1993) 1665. [24] P. van der Voort, K.C. Vrancken, E.F. Vansant and J. Riga, J. Chem. Soc. Faraday Trans., 14, (1993) 2509. [-25] P. Fink, I. Plotski and G. Rudakoff, Wiss. Z. FriedrichSchiller-Univ. Jena Math. Naturwiss., Reine 39, (1990) 217. E26] K.C. Vrancken, P. van der Voort, K. Possemiers and E.F. Vansant, J. Colloid Interface Sci., in press. [-27] P. van der Voort, I. Gillis-D'Hamers, K.C. Vrancken and E.F. Vansant, Ceramic Ind. Int., 102 (1992) 17. [28] K.C. Vrancken, unpublished results, 1994. [29] G.T. Burns, R.B. Taylor, Y. Xu, A. Zangvil and G.A. Zank, Chem. Mater., 4 (1994) 1313.