Adsorption of water vapour by magnesium fluoride

Abstract

A xerogel of MgF₂ was prepared by dehydration of a precipitate obtained from aqueous media. In the “air-dry” state the ground sample was associated with a relatively high percentage of water which was not restored by exposure to water vapour once it had been removed. The removal of the gel water did not lead to a porous sample as evidenced by a reversible BET type II N₂ isotherm. Infrared measurements and water vapour isotherms (determined at 298, 288 and 278 K) revealed that the first stage of water adsorption was the chemisorption of hydroxyl ions and/or coordinated water molecules electrostatically bound to the Mg²⁺ ions in several exposed crystal planes. Complete removal of this chemisorbed water by evauation at 973 K caused extensive sintering. The second stage of adsorption was the physisorption of water molecules at localised sites with relatively high heats of adsorption, probably brought about through hydrogen-bonding interactions with the chemisorbed layer.