Argon Adsorbed on a Graphitic Surface. The Heat of Adsorption from Lattice Liquid Theory

Abstract

A model is proposed to account for the differential or isosteric heat of adsorption which has been observed for argon on graphon at concentrations between 0.5 and 2.0 monolayers for temperatures in the neighborhood of the normal boiling point of argon. The energy of a molecule on the surface is considered to be the sum of two contributions: (1) a site energy for the isolated molecule and (2) an interaction energy with neighbors in the same layer. The site energy for a molecule is calculated assuming atoms and molecules in lower layers fixed with respect to both a configuration in a hexagonally packed planar lattice and the distance between layers. The interaction energy is calculated assuming the behavior in any layer is represented by a two dimensional lattice liquid model utilizing the so‐called Ono approximation for the variation of the generalized free volume with vacancies in the lattice. The distribution of adsorbed molecules in any layer is considered to be completely random. On the basis of this model, an interpretation is made of the two maxima in the isosteric heat which have been observed experimentally.