Preparation of High‐Silica Glasses from Colloidal Gels: III, Infrared Spectrophotometric Studies

Abstract

The gel process for making silica was studied by infrared spectrophotometry of solid samples in various stages of dehydration. Spectra were recorded in the high‐frequency overtone region (2500 to 8000 cm‐1) as well as in the region of fundamental absorption (200 to 4000 cm‐1), depending on the nature of the sample. Molecular water was distinguished from silanol groups and the effects of hydrogen bonding were observed. Drying and heat treatment of gels at low temperatures (<150°C) causes a loss of unbonded included water, but significant loss of hydrogen‐bonded water occurs only with relatively high‐temperature heat treatments (>800°C). Hydration of siloxane groups on pore surfaces and attachment of water molecules by hydrogen bonding to these surface silanol groups is reversible for heat treatments up to 1025°C. At higher temperatures (>1025°C), active sintering takes place, converting the porous structure into a homogeneous silica network. In this process, surface silanol groups become isolated within the silica network, giving rise to vibrational frequencies characteristic of internal ‐OH groups. Also, at the higher temperatures silanol groups react to form siloxane groups with loss of H2O. A model is presented for formation of a gel in which hydrogen bonding initiates association of colloidal particles, followed by strengthening through interparticle solid silica precipitation.