Micro FT-IR study of the hydration-layer during dissolution of silica glass

Abstract

Hydrothermal hydration of silica glass was experimentally studied with pure water at temperatures of 400–500°C and pressures of 40–96 MPa. The dissolution of silica glass occurred at nearly constant rates based on the weight loss and the hydrated surface layers have optical anisotropy. The thickness of the hydration layer increased gradually during the initial stage of the reaction and reached a steady state thickness (0.1 mm) after tens of hours. Concentration-depth profiles of both OH and H 2O species in the hydration layer were obtained using microscopic Fourier transform infrared spectroscopy (micro FT-IR). The concentration of the OH group was 0.4–1.3 wt% and that of H 2O was about one third of the OH group near the surface. These concentrations decreased exponentially from the surface of the silica glass inward, indicating diffusion-related processes. Overall reaction rates are interpreted to be determined by diffusion of water in the hydration layer. The diffusion coefficients are estimated to be 10 −15−10 −14 m 2/s and exhibit positive correlations with pressure and temperature. The activation energy was approximately 60 kJ/mol.