Adsorption of a Lennard–Jones gas on random bivariate surfaces

Adsorption of a monatomic gas with Lennard–Jones lateral interactions is studied on a heterogeneous surface represented by a square lattice of sites. Adsorption sites can be of two kinds, strong and weak sites, in equal concentration 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. mperature is fixed in such a way that ordered adsorbate phases can be formed and the adsorption isotherm and isosteric heat of adsorption are studied through 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 simulation and the effective substates approximation (ESA) introduced recently [Riccardo et al., J. Chem. Phys. (1999) in press]. It is found that ESA reproduces very well the coverage dependence of the chemical potential for lattices where only attractive interactions are present. In the case of lattices where order–disorder phase transitions take place due to repulsive interactions, ESA predictions are fairly good only when the heterogeneity is large compared with lateral interactions.