A comparison of density functional and integral equation theories vs Monte Carlo simulations for hard sphere associating fluids near a hard wall

Abstract

We make a comparison of a perturbation density functional (DF) theory and an integral equation (IE) theory with the results from Monte Carlo simulations for nonuniform fluids of hard spheres with one or two association sites. The DF used applies the weighting from Tarazona’s hard sphere density functional theory to Wertheim’s bulk first order perturbation theory. The IE theory is the associative form of the Henderson–Abraham–Barker (HAB) equation. We compare results from the theories with simulation results for density profiles and adsorption of one- and two-sited associating fluids against a hard, smooth wall over a range of temperatures and molecular densities. We also report fraction of monomers profiles for the DF theory and compare these against simulation results. For dimerizing fluids, the DF theory is more accurate very close to the wall, especially at higher densities, while the IE theory has more accurate peak heights and positions away from the wall, also especially at higher densities. Accuracy of the IE theory increases with an increasing degree of association. For two-sited hard spheres, the DF theory is more accurate than the IE theory at lower densities; at higher densities accuracies are similar to that of dimerizing hard spheres.