Towards the molecular theory of physical adsorption on heterogeneous surfaces

Abstract

The modern theory of adsorption on heterogeneous surfaces is a phenomenological one. The fundamental concepts of this theory are critically reviewed. A molecular theory of physical adsorption on the heterogeneous surfaces has been developed. The theory is grounded on the hypothesis that the amorphous nature of surface layers is the main reason of surface heterogeneity not only in amorphous bodies but in some ultradisperse crystals as well. It differs from the existing molecular theory of physical adsorption in that respect that the latter considers adsorption on (or in) ideal crystals while the former does it for irregular atomic arrangements, e.g., for the Bernal model. Two ways of computer simulation of the heterogeneous surfaces are described: one simulates the surface layer of an amorphous body as a dense random packing of hard balls; the other represents it as a medium which is continuous up to a random surface. The adsorption potential of argon over the dense random packing of hard balls which have adsorption characteristics of oxygen ions have been calculated. The minima of the adsorption potential are considered to be the adsorption sites. The distribution of adsorption sites with respect to energy and thermal entropy as well as the correlation between the latter are determined. The results of prior papers regarding the specific mechanism of the interaction of adsorbed molecules on a heterogeneous surface, the validity of the Dubinin-Radushkevich equation of the isotherm of adsorption, and the reason for the temperature invariance of the Polanyi characteristic curve are recapitulated.