INFLUENCE OF SEVERAL METAL-IONS ON THE GELATION ACTIVATION-ENERGY OF SILICON TETRAETHOXIDE

The effects of nine metal cations (Li+, Na+, Mg2+, Ca2+, Sr2+, Cu2+, Al3+, La3+, and Y3+) on silica gel formation have been investigated by studying the hydrolysis and polycondensation of silicon tetraethoxide (TEOS) in the presence of metal nitrates. The influence of the water: TEOS mole ratio, metal ion concentration, and the reaction temperature has been investigated. The overall activation energy for gel formation has been determined from the temperature dependence of the time of gelation for each system. The activation energy for network formation is 54.5 kJ/mol. The gel formation time as well as the activation energy sharply increases in the presence of Cu2+, Al3+, La3+, and Y3+. In contrast, the presence of Li+, Na+, Mg2+, Ca2+, or Sr2+ lowers the gelation time but has no appreciable effect on the activation energy. This difference may be attributed to the participation or nonparticipation of the metal ions in the formation of the three‐dimensional polymeric network during polycondensation. The concentration of metal ion (Mg2+, Ca2+, Y3+) or the water: TEOS mole ratio had no appreciable effect on the gelation activation energy. A simple test has been proposed to determine whether a metal ion would act as a network intermediate or modifier in silica and other glassy networks. Copyright © 1990, Wiley Blackwell. All rights reserved