Adsorption of a Lennard-Jones gas on random bivariate surfaces

Adsorption of a monatomic gas with Lennard-Jones lateral interactions is st udied on a heterogeneous surface represented by a square lattice of sites. Adsorption sites can be of two kinds, strong and weak sites, in equal conce ntration and distributed at random. The lattice parameter (distance between nearest-neighbor sites) is also varied to obtain different situations for the lateral interaction among first, second, etc. order neighbors. The temp erature is fixed in such a way that ordered adsorbate phases can be fond an d the adsorption isotherm and isosteric heat of adsorption are studied thro ugh Monte Carlo simulation, changing the different parameters of the model. A rich variety of different behaviors is found and analyzed in the context of the lattice-gas model. Further comparison is carried out between the si mulation and the effective substates approximation (ESA) introduced recentl y [Riccardo et al., J. Chem. Phys. (1999) in press]. It is found that ESA r eproduces very well the coverage dependence of the chemical potential for l attices where only attractive interactions are present. In the case of latt ices where order-disorder phase transitions take place due to repulsive int eractions, ESA predictions are fairly good only when the heterogeneity is l arge compared with lateral interactions. (C) 2000 Elsevier Science B.V. All rights reserved.