The Hierarchy of Adsorption Models for Laterally Interacting Molecules on Heterogeneous Surfaces

Abstract

Real adsorbents are heterogeneous, therefore, the computation of the adsorption characteristics for any concentration of adsorbate must take account of the joint effect of the lateral interaction between adsorbed molecules and surface heterogeneity. The local coverage of any adsorption center depends on the adsorption capacity of this center and upon the local coverage of the neighboring centers. As a result, the distribution of the different types of adsorption centers plays an essential role. A comparison of the different approaches based on the fragment, cluster, and pair description of the surface structure is given here. A technique for increasing the precision of calculating adsorption characteristics is found. It uses cluster distribution functions and takes into account the mutual disposition of the different types of sites. As an example of a distributed system, multilayer adsorption is considered. A lattice-gas model for the case of capillary condensation in slitlike micropores gives results in accord with those obtained by molecular dynamics and Monte Carlo techniques. The possibility of using a simplified equation for multilayer adsorption on a heterogeneous surface is analyzed. This equation gives a good description of the experimental data. However, the model parameters are outside the region of their definition in the lattice-gas model. For an exact calculation of the adsorption characteristics on a small fragment of surface, a new method is proposed which allows us to consider fragments that are twice as large as in the well-known matrix technique.