Nuclear magnetic resonance and dielectric relaxation investigations of water sorbed by Spherisorb silica

Abstract

The Spherisorb silica/sorbed water system has been investigated at coverages 0.018 ⩽Θ/g_H2O g^–1_SiO2⩽ 0.36 using ¹H n.m.r. in the temperature range 190 ⩽T/K ⩽ 300 and also at coverages 0.013 ⩽Θ/g_H2O g^–1_SiO2⩽ 0.033 using dielectric techniques at ambient temperature. In cases where multilayer adsorption occurs there is a discontinuity in the temperature dependence of the n.m.r. linewidth accompanying a freezing phenomenon. Water molecules making up the first loosely packed adsorbed layer (complete at Θ= 0.06 g_H2O g^–1_SiO2) remain mobile at low temperatures at all coverages. Dielectric measurements reveal another discontinuity at a coverage of ca. 0.023 g_H2O g^–1_SiO2. Below that coverage the strength of binding of sorbed water increases with decreasing coverage, while at higher coverages additional water leads to a marked increase in the mobility of the first layer.