Calorimetric study of water two-dimensionally condensed on the homogeneous surface of a solid

The state of water adsorbed on ZnO, Cr₂O₃ and SrF₂, on which two-dimensional condensation of water occurs, has been studied by a direct calorimetric method. It was found that the differential heat of adsorption exhibits a maximum in the coverage region where a steep increase in the adsorbed amount is observed in the corresponding adsorption isotherm. This suggests the two-dimensional condensation of water due to the strong lateral interaction between water molecules adsorbed on the homogeneous surface in addition to their interaction with the underlying surface. The entropy change for the adsorbed water during the adsorption process, -ΔS_ads, can be estimated from the heat and isotherm data, and compared with the theoretical values. As a result, it was revealed that the localized adsorption of water molecules occurs on the homogeneous surfaces of ZnO, Cr₂O₃ and SrF₂ at 301 K. By applying Fowler's equation, which can be applicable to the localized monolayer adsorption on the homogeneous surface with interactions among the adsorbed molecules, to the present systems and by separating the experimental adsorption isotherm into two components, the adsorptions on the heterogeneous surface and on the homogeneous one, the interaction energies for both types of interactions were evaluated; the lateral interaction energies were 8.8, 9.8 and 8.2 kJ mol^-1 for ZnO, Cr₂O₃ and SrF₂, and the adsorbate-adsorbent interaction energies were 46, 55 and 58 kJ mol^-1, respectively. These results are interpreted in terms of the difference in the structure of these solid surfaces. The adsorbed state of water is also discussed through a dielectric study.