Gels from Hydrolysis of Alkoxides

Tetraalkoxysilanes (usually tetramethoxysilane, TMOS, bp 121 °C, or tetra-ethoxysilane, TEOS, bp 169 °C) for hydrolytic gelling can be made by reaction of SiCU (Section 17.8.2) with the relevant alcohol  

Hydrolysis of tetraalkoxysilanes in pure water is usually incomplete, but it is more effectively carried out in an alcohol-water mixture and can be catalyzed by or weak bases such as ammonia. Polymerization occurs in the range pH 2-7 under neutral conditions, the rate of polymerization is limited by the slowness of hydrolysis, whereas in acidic media hydrolysis is complete before polymerization begins. The nanostructure of the gel (density of particles, fractal dimensions of particle clusters, and degree of cross-linking of particles to form a network) is controlled by these conditions. 

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