Effect of Water on Acid- and Base-Catalyzed Hydrolysis of Tetraethylorthosilicate (TEOS)
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EFFECT OF WATER ON ACID- AND BASE-CATALYZED HYDROLYSIS OF TETRAETHYLORTHOSILICATE (TEOS) L. C. KLEIN* AND G. J. GARVEY" *Rutgers-The State University of New Jersey, Ceramics Department, P.O. Box 909, Piscataway, NJ 08854; **Room 12-007, MIT, Cambridge, MA02139 ABSTRACT Two series of gels were prepared by hydrolyzing TEOS with HCl or NH40H catalyst. In the first series, the molar ratio of water to TEOS was 4:1 and the catalyst addition was (1 x 10-3), (4 x 10-3), (1 x 10-2), (4 x 10-2), In the second series, the catalyst and (1 x 10-1). addition was (1 x 10-3) moles and the molar ratio of water Dried samples to TEOS was 2:1, 4:1, 8:1, 16:1, and 32:1. were characterized for surface area, porosity and C and H 0 content. Samples were heated to 800 C and thermogravimetric weight loss was recorded. Both HC1 and NH40H are catalysts for hydrolysis, but during polymerization, the relative rates of hydrolysis and condensation determine resulting dried microstructures. INTRODUCTION The mechanisms for hydrolyzation in acid- and base-catalyzed solutions of ethanol-water-tetraethylorthosilicate (TEOS) have been studied directly with titration and gas chromatography [1,2,3,4] and indirectly by monitoring changes in physical properties, most often viscosity, [5,6,7,8]. The mechanism for acid-catalyzed hydrolysis is electrophilic attack and the mechanism for base-catalyzed hydrolysis is nucleophilic attack [9]. Not only are the mechanisms different, the kinetics are different. For acid-
catalysis, hydrolysis is complete and the number of unreacted hydroxyls per silicon decreases with decreasing acid concentration [1]. For basecatalysis, hydrolysis is incomplete and the effect of polymerization is large. At the same time, the solubility of silicic acid in basic solutions makes dissolution and rearrangement of the growing polymer possible [10]. If acid and base had equivalent catalytic effect, it would be easier to compare the results using a parameter such as pH. However, the catalysts commonly used, HCI and NH4 0H, do not have equal strength and an alcohol-water mixture is a further complication. Several attempts have been made to compare the effects of acid and base. One scheme that qualitatively predicts physical properties of dried gels is that acid-catalyzed solutions yield linear polymers which become tangled at the sol-gel transition, while base-catalyzed solutions yield branched clusters which coalesce [11]. Accordingly, the dried gels can be classified by texture, either fine or coarse. Likewise, acid-catalyzed gels have higher bulk densities [12],and base-catalyzed gels are very friable. Acid-catalyzed gels should have hydroxyl contents that scale with surface area, while base-catalyzed gels do not, since the solubility of silicic acid leads to internal condensation [10]. In the course of rearrangement in base-catalyzed gels, the NH40H content is reduced and the polymer does not gel as a unit. Rather, what appears to be particles sediment from the solution, though in detail the particles are not colloids [9]. Of course, t
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