Born-Green-Yvon results for adsorption of a simple fluid on plane walls: Structure, adsorption isotherms, and surface area determination

The Born-Green-Yvon approach using a coarse grained density is applied to study the adsorption of a subcritical Lennard-Jones fluid. In case of a strongly adsorbing wall the adsorbed gas forms several layers with increasing pressure. Close to the vapour pressure the layers are followed by a film of nearly saturated liquid density the thickness of which increases rapidly. At low temperature the layer formation occurs rather suddenly causing type VI (multistepped) adsorption isotherms whilst at high temperature type II (concave one-step BET) adsorption isotherms are obtained. The adsorbed liquid shows the same layered structure as the gas. In case of a weakly adsorbing wall the adsorption mechanism close to saturation at high temperature is similar to that at a strongly adsorbing wall; comparison with the results from the density functional theory of Meister and Kroll is made. At low temperature, however, the gas forms only one layer in approaching the vapour pressure. For all temperatures the adsorption isotherms are of type III (convex). Finally, the accuracy of the surface area determination from adsorption isotherms is discussed.