A kinetic study of drying of TEOS-derived gels under nearly isothermal conditions

Abstract

The drying of wet Tetraethoxysilane-derived gels was studied by means of thermogravimetric analysis as a function of the time, from the instant when the samples were placed into a pre-heated oven, kept at 70, 83 and 100 °C. The precursor sol of the gel was obtained by sono-hydrolysis of the Tetraethoxysilane under acid condition and the final [Water]/[Tetraethoxysilane] molar ratio was adjusted to 30. The wet gels were cylindrical (2.5 cm height and 1.2 cm diameter) after 30 days of aging at 30 °C in a hermetically sealed container. The changes in volume and weight during drying are compatible with a mechanism controlled by evaporation from a flat liquid-vapour interface at the surface of the body and contraction of the gel by capillary forces, together with structural changes induced by polycondensation, which partially account for the irreversible shrinkage of the gel. The employed thermogravimetric method does not permit to unequivocally follow the small and slow final loss of weight associated to the falling rate periods.