Binary vapour mixtures adsorbed on a graphite surface: A comparison of mean field density functional theory with results from Monte Carlo simulations

We make a detailed comparison of the predictions of mean field density functional theory with results from isobaric Monte Carlo simulations of a model of binary vapour mixtures adsorbed on a graphite surface. The mean field theory is based upon a recently developed non-local free energy functional for hard sphere mixtures. As in other tests of mean field density functional theory on related systems we have found that it provides a qualitatively correct picture of the adsorption equilibrium. In addition to an inadequate description of the bulk thermodynamic properties, the mean field approximation leads to poor prediction of influence of the attractive forces upon the structure of the adsorbed layers. We show that quantitative agreement between the DFT and simulation results can be obtained by comparing them at the same relative state of undersaturation and by scaling the adsorption excess by that for the completely filled first layer.