Adsorption of water vapour by calcium fluoride, barium fluoride and lead fluoride

Abstract

Crystalline samples of CaF₂, BaF₂ and PbF₂ were prepared by precipitation from aqueous media and an infrared examination of pressed discs of CaF₂ and BaF₂ showed that these materials contained occluded water. Both compounds were completely cleared of i.r. bands in the 4000–3000 cm^–1 region by evacuation at 673 K. It was further shown by the i.r. technique that the CaF₂ surface is capable of binding adsorbed water molecules relatively strongly (i.e. they could not be desorbed by prolonged evacuation at room temperature), but that such molecules can be desorbed without incurring a surface hydrolysis reaction. In contrast, the BaF₂ surface cannot similarly bind water molecules and adsorption on this surface is brought about solely by a physisorption process. This finding was endorsed by adsorption isotherm results for BaF₂. Adsorption isotherms on BaF₂ and PbF₂ were determined volumetrically for water vapour at 278, 288 and 298 K, and for krypton at 77.5 K. Reversible B.E.T. type II water isotherms were obtained consistent with a simple physisorption process. Integral entropies were consistent with an immobile model for the adsorbed water. Possible packing configurations involving hydrogen-bonding interactions with surface fluoride ions are discussed.