Novel Processing of Silica Hydrosols and Gels

The extensive research activity on sol-gel processing of silica glass has been reviewed by several authors [l-4]. The typical alkoxide route to silica sols comprises the catalyzed reaction of tetraethoxysilane (TEOS) and water in a mutual solvent. The TEOS alkoxy groups hydrolyze to form silanol monomers which condense and crosslink to form the silica gel network. The nature of the sol and the resultant gel is determined by reactant concentrations and the type and amount of catalyst (acid or base) [5]. Special additives [6] or elaborate drying procedures [7] are generally required to obtain gel monoliths. It has long been held [8] and is now commonly assumed that a mutual solvent (e.g., a short-chain alcohol) is essential for sol formation because TEOS and water are immiscible. However, direct reaction of water with tetramethoxysilane [9], or with TEOS under intense ultrasonication [lo], have been reported. We describe here a novel technique for producing silica hydrosols from TEOS without either solvent addition or intensive agitation; and a procedure for inducing rapid and homogeneous hydrosol gelation with syneresis that yields xerogel monoliths on simple air drying [ll]. In the solventless hydrosol procedure, as-received TEOS * is added to distilled water and stirred magnetically to form a gray two-phase