The characterization of molecular water on silica surfaces

Abstract

The existence of two types of molecular, adsorbed water on the surface of precipitated silica has been confirmed. It has been shown that the physically adsorbed water gives a Type II isotherm and requires an activation energy of 6.6, 7.4, and 8.2 kcal./mole for its removal, on three different precipitated silica samples studied. The second type of water is probably H-bonded to SiOH groups and conforms to a Langmuir isotherm, requiring about 10 kcal. activation energy for its removal. The data are consistent with a bridged structure involving one water and two SiOH groups. The data suggest that past reports of water adsorption on silica surfaces have shown monolayer coverage to be complete at spurious partial pressures because the two types of water adsorption have not been clearly separated.