Thermogravimetric investigation of the kinetics and thermodynamics of dehydration of amorphous silica

Abstract

Several commercial and laboratory-synthesized specimens of silica gel have been studied, using thermogravimetry. Two forms of adsorbed water have been found: the first form, which evaporates in the temperature range 100–200°, is polymolecular water layer in open pores and loosely-bonded molecules on the surface and in the bulk of the globules; the second form, which evaporates in the temperature range 200–700°, comprises water molecules hydrogen-bonded to silanol groups, and evidently coordinated to silicon atoms. The dehydration heat of the first form, determined from the DTA curves, depends on the ratio of the two forms, and varies from 3.7±0.6 kcal/mole H2O for coarse-grain silica gel produced at pH=7 to 12±1 kcal/mole H2O for fine-grain silica gel of commercial “chromatography” grade.