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 refere nce system of hard or soft repulsive spheres and considers attractive interaction between adatoms as a perturbation. In the present paper, t his approach is extended to the case of localized adsorption where ads orbed molecules are located near deep minima of the adsorption potenti al (at adsorption sites) on a surface. The high temperature expansion of thermodynamic potentials is presented in a form convenient for adso rption applications. With the help of this method, a mean field correc tion to the Langmuir isotherm on a general (homogeneous or heterogeneo us) surface is obtained. This correction is much simpler than that des cribed in the literature (for the lattice model) because it refers not to adsorption, but to chemical potential. It is shown that for the ca se of a one-dimensional model, the corrected Langmnuir isotherm deviat es considerably from the exact isotherm obtained by the transfer matri x method for a homogeneous surface (line), but almost coincides with t hat 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 c orrection (van der Waals approach) makes it possible to take into acco unt attractive interaction between adatoms and works better on heterog eneous (irregular) surfaces than on a homogeneous (regular) one.