Heat of adsorption of water on SrF2 : relation to two-dimensional condensation of water adsorbed on SrF2

Abstract

The heat of adsorption and the adsorption isotherm of water on the SrF₂ surface have been measured at 301 K by direct calorimetry to evaluate the energetic properties of the SrF₂ surface and of water adsorbed on it. The differential heats of adsorption, q_diff, reveal an appreciable maximum in the coverage range 0.5–0.8, where a distinct step appears in the adsorption isotherm. These phenomena can be interpreted in terms of a two-dimensional condensation of water molecules resulting from their lateral interactions on the homogeneous solid surface. The behaviour of water molecules on the solid surface was characterized by entropies estimated from the adsorption isotherm and calorimetric data. It was thus revealed that water molecules are in the localized state on the SrF₂ surface and their lateral interaction energy is ca. 7.3 kJ mol^–1. On the other hand, the energy of interaction of water with the SrF₂ surface is ca. 58 kJ mol^–1, which is indicative of a strong interaction in this system.

Water that cannot be desorbed by evacuation at 301 K is present on the SrF₂ sample, and such water once removed by heat treatment at higher temperatures is restored by subsequent exposure of the sample to saturated water vapour. This rehydration energy (ca. 95 kJ mol^–1) seems to be relatively small compared with that of usual metal oxides (ca. 150 kJ mol^–1). From heat of adsorption and IR data, it has become apparent that the rate of rehydration of the SrF₂ surface can be retarded by heat treatment at higher temperatures.