The Adsorption of Acetone on Alkali Halides -The Role of Surface Cations as Adsorption Sites-

Abstract

With regard to acetone adsorption on (100) planes of five alkali halide crystals, i. e. LiCl, NaCl, NaBr, KCl and KBr, the adsorbed amounts, the isosteric heats of adsorption, and the adsorption entropies have been determined. The isosteric heat measurements showed that acetone is adsorbed on the surface of these salts by an interaction between the surface cation and an oxygen atom in the carbonyl group with C=O axis oriented perpendicularly to the salt surface. The order of magnitude of the isosteric heats compared at the given equivalent amounts of the adsorbed in low coverage was as follows: NaBr>KBr>NaCl>KCl>LiCI. Since the interaction energies between acetone molecule and the surface cations of NaBr, KBr and NaCl were considerably large, an adsorptive activity of the surface cations being exposed on the surface, i. e. (111) plane, should be taken into account in studies on adsorption of polar substances on alkali halides.
Acetone molecules were recognized to be adsorbed on NaCI and NaBr in the ratio of one acetone to two cations on the (100) planes of the salts, and they were also adsorbed on KCl and KBr, whose lattice spacings are longer than that of NaBr, in a ratio above 1: 2. On the other hand, they were adsorbed on LiCl, whose spacing is shorter than that of NaCl, in a ratio belo; v 1: 2. These phenomena are reasonably interpreted in terms of cross-sectional area of an acetone molecule.