THE VANDERWAALS APPROACH TO ADSORPTION ON HETEROGENEOUS SURFACES

The van der Waals approach is widely used in the theory of liquids and nonlocalized adsorption on homogeneous surfaces. It takes as a reference system of hard or soft repulsive spheres and considers attractive interaction between adatoms as a perturbation. In the present paper, this approach is extended to the case of localized adsorption where adsorbed molecules are located near deep minima of the adsorption potential (at adsorption sites) on a surface. The high temperature expansion of thermodynamic potentials is presented in a form convenient for adsorption applications. With the help of this method, a mean field correction to the Langmuir isotherm on a general (homogeneous or heterogeneous) surface is obtained. This correction is much simpler than that described in the literature (for the lattice model) because it refers not to adsorption, but to chemical potential. It is shown that for the case of a one-dimensional model, the corrected Langmnuir isotherm deviates considerably from the exact isotherm obtained by the transfer matrix method for a homogeneous surface (line), but almost coincides with that obtained for a strongly heterogeneous surface. Computer simulation of adsorption isotherms of Ar atoms deprived of mutual attraction on two irregular and one regular surface also shows that the mean field correction (van der Waals approach) makes it possible to take into account attractive interaction between adatoms and works better on heterogeneous (irregular) surfaces than on a homogeneous (regular) one.