Specific feature of dielectric behavior of water adsorbed on Ag₂O surface

The dielectric behavior of Ag₂O samples with different amounts of adsorbed H₂O was investigated in the temperature range from 308 to 200 K and in the frequency region from 0.1 Hz to 5 MHz. A large relaxation is observed in these ranges, and it is explained by the interfacial polarization due to the heterogeneity of the system. The characteristic frequency (f_m) for this relaxation is lowered with decreasing temperature until a complete monolayer is established. However, such a behavior of f_m in the region beyond the surface coverage of ca. 1 reveals a new specific feature; the f_m value gives a maximal frequency at around 278 K. The cause of this behavior is explored through the measurements of the differential adsorption heat, water content, and Clausius-Clapeyron plots (In p vs 1/T). As a result, a new phenomenon observed in the Ag₂O-H₂O system can be explained by considering that the continuous phase change occurs at around 278 K in the adsorbed layer higher than the monolayer; the cluster-like structure formed on the OH groups at a temperature higher than 278 K is transferred to the two- or three-dimensional network structure formed by H-bonding between adsorbed H₂O molecules in the lower temperature region.