Effect of chemisorbed water on the two-dimensional condensation of water and argon on CaF2

Abstract

The desorbability and regeneration of surface hydroxyls on CaF₂, on which the two-dimensional condensation of water occurs, have been investigated by measuring the water adsorption isotherm, the surface water content and the i.r. spectra. Three water desorption peaks were found: A(25–150 °C), B(150–250 °C) and C(250–500 °C). Chemisorbed water exhibiting peaks A and B could be regenerated by exposing the heated surface to water vapour and are responsible for the occurrence of the two-dimensional condensation of water. Chemisorbed water corresponding to peak C could not be reproduced and it is considered to be in inner layers of the crystal. The homogeneous surface responsible for the two-dimensional condensation of water and argon was estimated for samples treated at various temperatures, and it was found that the homogeneous surface for water adsorption decays markedly on heat treatment at 250–500 °C, but recovers to the same extent as the original surface on exposing the surface to water vapour. The homogeneous surface for water adsorption is not homogeneous but heterogeneous for Ar adsorption, and vice versa.