Isotope effect on physical adsorption on a non-homogeneous surface

Abstract

Non-homogeneity of adsorption sites is shown, by a model calculation, to account for a non-monotonic temperature dependence of the isotope separation factor α in adsorption of a monoatomic gas on a solid adsorbent. Such a non-monotonic dependence was experimentally observed by Purer, Kaplan and Smith.

The model calculation shows that a non-monotonic temperature dependence of α arises when there are two types of adsorption site, one for which the energy of interaction with the adsorbed atom is characterized by a shallow but narrow potential well and the other for which the well is deeper and wider. It is also shown that wide holes or planar sections in the solid adsorbent and narrow holes correspond, respectively, to these two types of adsorption site which exist when the adsorbent is a porous glass.