Integral equation theory for the four bonding sites model of II. Density profiles and adsorption onto a solid surface associating fluids

Abstract

Density profiles and adsorption isotherms of an associating fluid with four bonding sites near a solid surface are investigated using the Percus–Yevick approximation for the associative Henderson–Abraham–Barker equation. The fluid molecule is represented as a hard sphere with four independent attractive sites. The surface is modelled as a hard wall and also as an adhesive wall. The density profiles are obtained both numerically and analytically (as the Laplace transform) as functions of bulk number density η and of the reduced inverse temperature β*. The features connected with increasing average number n _b of bonds are discussed. An analytical expression for the adsorption coefficient is derived. Since the system exhibits critical behaviour, the gaseous and liquid branches of both the adsorption isotherm and density profiles are described separately. The density profiles and adsorption isotherms are compared with Monte Carlo simulation data. In the case of an adhesive surface the competition between the adhesion and association is studied. The formation of several gaseous adlayers at the wall is demonstrated.