Neutron scattering of supercooled water in silica gels

Abstract

The effects of temperature on the incoherent quasielastic and inelastic neutron scattering of water in two silica gels (56 and 73 % w/w SiO2) have been measured by time-of-flight spectroscopy from 300 down to 233 K, where the water is in the frozen state. Quasielastic scattering (QENS) data have been analysed using a model applied by Teixeira et al. in a recent study of the diffusive motions in supercooled water. This model involves a fast and a slow component, corresponding to the rotational and translational motions of the water molecule. The temperature dependence of the rotational dynamics of water in the gels is similar to that in the bulk, as is evident from the activation energy (ca. 10 kJ mol–1). At higher temperatures (≳278K), translational diffusion is more restricted in the gels, and possibly reflects the effect of long-range surface interactions. However, in the supercooled region where hydrogen bonding is extensive in the bulk, and leads to marked reductions in the rate of diffusion, the translational mobility of water in the gels is similar to that reported for the bulk. Changes in the inelastic scattering, due to intermolecular modes, reflect the increase in hydrogen bonding which occurs on supercooling and also the structural change on freezing. It is also shown that partial freezing can occur at intermediate temperatures (ca. 260 K), where the water content of the gel is high and thus possibly less influenced by the silica surface.