GRAND CANONICAL ENSEMBLE COMPUTER-SIMULATION OF ADSORPTION OF ARGON ON A HETEROGENEOUS SURFACE

Grand canonical ensemble Monte Carlo (GCEMC) computer simulations of adsorption of Ar on the surface of a model amorphous oxide are reported. The oxygen anions of the oxide form a dense random packing of hard spheres, i.e., the Bernal model. In the calculation of the argon-solid interaction, pairwise summation over the anions only was taken. The isotherms of adsorption in the submonolayer region (ca. 0.2-0.6 of the monolayer) are linear on a semilogarithmic scale and manifest the temperature invariance of characteristic curves, as first suggested by Polanyi and Dubinin et al. Differential heats of adsorption were computed by four independent methods: (i) from the slopes of isosteres, (ii) by the numerical differentiation of the coverage-dependent average energies for GCEMC simulations, (iii) by the same for canonical ensemble Monte Carlo (CEMC) computer simulations, and (iv) from the correlation between the fluctuations of energy and number of particles in the GCEMC. It is shown that the isosteric method gives the best results. It is shown that the CEMC computer simulation of adsorption on heterogeneous surfaces at small coverages and low temperatures may manifest nonergodicity. The distributions of the adsorbate-adsorbent and adsorbate-adsorbate energies of adsorbed atoms were obtained and discussed.