RELATION BETWEEN THE ACIDITY AND REACTIVITY OF A TEOS, ETHANOL AND WATER MIXTURE

The relation between the hydrolysis time in the acid step and the gelation time in the basic step of a two-step silica sol-gel process of tetraethoxysilane (TEOS), ethanol and water mixtures was investigated using different acids in the concentration range of 10-6 - 10-1 M. The values and positions of the extremes in gelation time were found to depend on the concentration and degree of dissociation of the catalyst used. Decreasing the catalyst concentration resulted in a gradual weakening of the extremes in gelation time. At very low catalyst concentrations, no extremes in gelation time were found and gelation already occurred in the acid step. We also found that by measuring the potential difference in the hydrolyzing mixture continuously, the hydrolysis-condensation process in the acid step could be followed directly. The reaction products formed during this process were investigated with 29Si NMR spectroscopy. The disappearance of the extremes in gelation time at low catalyst concentrations was ascribed to the transition from an acid to a base-catalyzed reaction mechanism for the participating hydrolyzed silicon compounds. The isoelectric point of each silanol determines the potential difference in the hydrolyzing mixture at which the transition from one to the other mechanism takes place. © 1990.