Adsorption of water vapour on ionic solids containing preadsorbed sodium oleate. Part 2.—Calcium carbonate

Abstract

Krypton adsorption isotherms at 76 K on calcium carbonate prepared by thermal decomposition of calcium oxalate have a stepwise appearance, and the step is eliminated by pre-adsorption of sodium oleate from aqueous solution. The effective cross-sectional are of an adsorbed water molecule at 293 K on pure CaCO₃ is ca. 28 × 10^–20 m², which together with the low coverage isosteric heats of ca. 33 kJ mol^–1, increasing to just above the heat of liquefaction of water with increasing coverage, indicates that cooperative adsorption of water occurs at localized sites. Pre-adsorption of sodium oleate at surface coverage below the theoretical monolayer decreases the subsequent adsorption of water vapour, but at higher concentrations the trend is reversed and a film of oleate, equivalent to ca. 6 theoretical layers, shows a marked increase in water vapour adsorption with hysteresis persisting to very low pressures. It is suggested that the first two layers of oleate ions have a laminar structure, with higher layers forming a similar structure to that of solid sodium oleate.