The Effects of Solvents and Drying Process on the Dry Gel Structure in the Sol-Gel Method

Abstract

Dry gels were synthesized by the sol-gel method from a mixture of Si(OCH3)4, water and several kinds of alcohols. Gelation of the solution was carried out at 45°C and the obtained wet gel was allowed to dry by making a pinhole on the closed container at 60°C. The shrinkage and surface area of the dry gel were measured to obtain porosity and average pore diameter. Residual carbon in the dry gel was analyzed. As the molecular weight of the alcohol increased, the pore diameter and porosity of the dry gel increased (Table 1). When a mixture of methanol and tert-butanol was used as solvent, the dry gel's porosity increased with increasing tert-butanol ratio, even if tert-butanol was added to the wet gel after the gelation (Fig. 1). Addition of water, methanol, tert-butanol or a mixed solvent to the wet gel before the drying process starts affected the porosity and residual organic component of the dry gel (Figs. 3 and 4). The change in composition of the wet gel solution during the drying process was calculated (Fig. 7) using a model analysis of the wet gel's drying process (Figs. 5 and 6). The porosity of the dry gel was correlated with the calculated water concentration of the wet gel solution when the solution had reached one-tenth reduction in weight ([H2O]1/10) by drying. When methanol or tert-butanol was added to the wet gel, the [H2O]1/10 decreased and the gel's porosity increased (Fig. 8). When several kinds of alcohols were used as a solvent for the solution, the gel's porosity was inversely related to the [H2O]1/10 (Fig. 9). Residual carbon content was related to the [H2O]1/10 (Fig. 10). When wet gel involved only water and methanol, the residual carbon content was inversely related to the [H2O]1/10. When wet gel involved tert-butanol, the residual carbon preferred the form of the tert-butoxy group and its content increased (Figs. 10 and 11).