Water-associated dielectric relaxation in oxide nanoparticles

Audio frequency dielectric relaxation studies were carried out over the temperature range 120 to 390K on nanoscale mesoporous silica coated with polystyrene and nanoscale γ alumina. Two features are observed, both of which are attributed to water. A relaxation is observed at about 150K and 1000Hz that is thermally activated. The activation enthalpy is relatively high and the pre-exponential appears to be unusually low and to depend upon the water concentration. A model is presented that accounts for the apparently anomalous behaviour. This relaxation is attributed to water molecules bound at surface hydroxyl groups. A feature is found in the vicinity of 250K and 1000Hz that is associated with conductivity that varies as ω n . This response is attributed to hopping conductivity in loosely bound surface water.