The dependence of the gelation time on the hydrolysis time in a two-step SiO 2 sol-gel process

Abstract

The gelation time in the basic step of a two-step hydrolytic polycondensation process of tetraethoxysilane (TEOS) in ethanol was strongly influenced by the hydrolysis in the acid step. Generally at short hydrolysis times an extremely long gelation time was measured. Systematic extension of the hydrolysis time results first in a minimum of the gelation time in the basic step, followed by a maximum and subsequently by a gradual decrease in the gelation time. The first decrease in gelation time as a function of the hydrolysis time was ascribed to the formation of silanol monomers during hydrolysis in the acid step. The subsequent increase in gelation time was caused by the decrease of these silanol concentrations, owing to the dimerisation reaction which became dominant because most of the added water had been consumed. For long hydrolysis times an appreciable effect of polymerisation in the acid step resulted in a renewed decrease in gelation time. The effect of parameters such as the hydrolysis temperature and the concentrations of ethanol, water, HCl and NH 4OH on the hydrolysis-condensation processes was investigated.