Chapter 9 - Localised Monolayer Adsorption with Interactions between Adsorbed Molecules

Abstract

This chapter describes monolayer adsorption of gases on heterogeneous surfaces by interacting between adsorbed molecules. The basic thermodynamic quantities—such as the average surface coverage v , the Helmholtz free energy F , the internal energy U , the isosteric enthalpy of adsorption q st , and the heat capacity c v —can be calculated directly from the canonical system partition function. The pressure p c at which the two-dimensional condensation takes place is determined from the two-dimensional analogue of Maxwell's rule, where the integration is taken along the continuous isotherm. Thus, the thermodynamic functions at subcritical temperatures contain three distinct segments. The behavior of adsorption systems is characterized by a random surface topography and a rectangular energy distribution and an exponential energy distribution. The rectangular energy distribution should represent the features of the actual solid–gas adsorption systems at high surface coverages. The nonconfigurational part contributes a term to the total heat capacity, which is proportional to the adsorbed amount (surface coverage). The effect of the surface heterogeneity on the critical temperature T e in systems is characterized by the exponentially decreasing adsorption energy distribution. The solid lines were obtained by using the quasi-chemical QC approximation, whereas the broken lines correspond to the mean-field Bragg–Williams BW approximation.