Adsorption of water vapour on ionic solids containing preadsorbed sodium oleate. Part 1.—Calcium fluoride

Abstract

The adsorption of water vapour on calcium fluoride has been measured between 273 and 293 K. The surface area calculated from water B.E.T. plots agrees closely with the values obtained from krypton and nitrogen adsorption at 76 K. Outgassing at 673 K causes an irreversible change, the surface becoming less hydrophilic and more uniform to krypton, which is attributed to surface hydrolysis. The mechanism of adsorption of water is discussed in terms of hydrogen-bonding with the surface. Films of sodium oleate, preadsorbed on calcium fluoride from aqueous solutions, cause a significant decrease in the isosteric heats of adsorption of water vapour at low coverage, and the ratio A_H2O/A_Kr, where A represents the specific surface area of the solid determined from water and krypton B.E.T. plots, decreases from unity to limiting value of 0.30 with increasing concentration of oleate. The results suggest that oleate ions are adsorbed initially as a uniform close-packed monolayer covering most of the surface, the effective cross-sectional area of an adsorbed oleate ion being similar to the value for a condensed film on a liquid surface. Infra-red spectroscopy, used primarily to determine amounts of pre-adsorbed oleate, indicates that the preadsorbed films contain chemisorbed calcium oleate.