Surface hydration of silicas

The dehydration and rehydration of two silica powders have been studied by infrared, water adsorption and gravimetric methods. The removal of both geminal and single surface hydroxyl groups from a precipitated silica (Manosil) and a silica prepared from the combustion of SiCl₄ (Aerosil) is accompanied by characteristic changes in the infrared spectrum. Mechanisms for the reversible and irreversible removal of geminal hydroxyls, and for the removal and replacement of the single hydroxyl groups at high and low surface hydroxyl densities are proposed. The annealing of silica at temperatures above 400°C is explained in terms of the irreversible removal of geminal hydroxyl sites from the surface of a solid lattice possessing short range structural order. Surface annealing may also be assisted by proton migration at low hydroxyl coverages.