Desorbability and reproducibility of chemisorbed water on CaF₂

Desorbability and reproducibility of surface hydroxyls on CaF₂, on which the two-dimensional (2D) condensation of water occurs, have been studied by means of various techniques, such as the measurements of water adsorption isotherm, surface water content, heat of immersion, and IR spectra. It was found that the step in the water adsorption isotherm, due to the 2D condensation of water on CaF₂, did not appear when the sample was evacuated at 600°C and succeedingly exposed to saturated water vapor at room temperature, but was recovered when it was treated at 600°C and succeedingly immersed in water and partially dissolved. On the other hand, the water content of the sample was very small when the sample was treated at 600°C and succeedingly exposed to saturated water vapor, but it increased when the sample was immersed in water. The heat-of-immersion measurement also showed that surface hydroxyls were reproduced on the dehydroxylated surface of CaF₂ when the heat-treated samples were immersed in water. It can be concluded that the surface layers once distorted by a high-temperature treatment is removed by the dissolution, when immersed in water, which brings about the exposure of the ordered surface, and the newly exposed surface is hydroxylated which is effective for the occurrence of the 2D condensation of water.