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.