The Adsorption of Non-Polar Gases on Alkali Halide Crystals. II. Calculations of The Adsorption Behaviour of Non-Polar Gases on Cubic Sodium Chloride

The potential energy of each isolated atom or non-polar molecule (argon, nitrogen, or carbon dioxide) adsorbed on the (100) plane of cubic sodium chloride has been theoretically calculated at four different positions on the crystal surface. In the present calculations, the repulsive constants of the modified BuckinghamCorner (6-exp) intermolecular potential for these non-polar gases have been used. The quadrupole interaction of the nitrogen or carbon dioxide molecule with a crystal has been taken into consideration. The good agreement between the theoretical and experimental values of the initial heat of adsorption for each adsorbate has been obtained. The present theoretical calculations suggest that the surface heterogeneity caused by the lattice imperfection or crystal defects is of minor importance compared with other types of surface heterogeneity due to the distribution of sites grouped in patches. The present results also suggest that the adsorbed argon atoms or nitrogen molecules easily migrate two-dimensionally along the surface in the low coverage region. These results on the surface migration are in good agreement with the entropy curves of adsorbed argon and nitrogen experimentally determined. The approximate calculation of the heat curve for argon on the (100) plane of cubic sodium chloride has been carried out following a similar method used by Young. The discordance between the experimental and approximately calculated heat curves is considerable, and this is attributed to the fact that the unreliable model (i. e. the uniform surface and the localized adsorption) has been assumed in these approximate calculations.