Investigation of step formation in multilayer adsorption isotherms using a lattice model

Abstract

A previously developed lattice model for multilayer physical adsorption has been extended to include the possibility of adsorption on a surface with randomly distributed site energies. Calculations have been carried out for homogeneous adsorbents with various values of temperature, adsorbate energy, and adsorbent surface energy. Heterogeneity was studied for both random and homotattic systems using Gaussian and surface defect models. The main conclusion is that the most important factor in determining stepwise character in the isotherm is adsorbate interaction given here by ϵ/ kT, where ϵ is the adsorbate potential well depth. Both smooth sigmoid and stepwise isotherms can be found for homogeneous substrates for different values of βϵ. Variation of the adsorbent strength causes interesting changes in stepwise character but has a relatively minor effect on the sharpness of steps. Heterogeneity does not necessarily remove steps especially when site energies are randomly distributed. It is concluded that stepwise isotherms do not necessarily imply a homogeneous surface, nor do smooth sigmoid isotherms imply surface heterogeneity.