Structural studies of gel phases. Part 2.—Infrared spectroscopic study of silica monoliths; the interaction of water with surface species

Abstract

Near-infrared (NIR) spectroscopy has been used to investigate the interaction of water molecules with monolithic silica gels and glasses prepared by the sol–gel route. Studies of monoliths thermally treated at 180 °C (specific surface area, 627 m² g^–1) and 1000 °C (specific surface area 120 m² g^–1) has enabled a model describing interactions between silanol groups and water molecules during hydration of the gel phases to be proposed. At least four kinds of water species can be identified and assignments for S^′₀(monomeric), S^′₁(partially hydrogen bonded), S^′₂(fully hydrogen bonded) and S_v(vapour-like) species are given. S^′₀ and S^′₁ water species exist even after dehydration and the coexistence of S^′₀, S^′₁ and S^′₂ species at early stages of rehydration indicates that associated species are formed well before completion of the first water layer by adsorption of S^′₀ species. S_v species appear only during the final stages of hydration and give rise to rotation-like features observed on the water combination band. Samples treated at 1000 °C show a reduction in the level of silanol functionality, some of which are inaccessible to the external environment. The hydroxyl component of the silanol bond is shortened and shows a reduced capability in the promotion of hydration.