Density profiles of a model of dimerizing hard spheres in contact with a crystalline surface: application of Wertheim's theory of association

Abstract

The density profiles of a dimerizing fluid near a crystalline surface are investigated. The model of the bulk fluid is that proposed by Stell and Zhou, which is a generalization of Wertheim's model for dimerization. The formation of dimers results from the interaction between the intracore attractive sites. The fluid–solid interaction consists of two terms. The non-associative term is that proposed by Steele. The directional associative fluid–solid atom interaction is chosen to have a Gaussian form. This provides bonding of the bulk fluid particles with the solid atoms. The solid substrate is assumed to be the graphite basal plane. The density profiles are studied by using the associative singlet Percus–Yevick approximation. When the distance from the surface is greater than the dimer length, the effect of crystalline symmetry is not apparent. The second adlayer tends to disappear when association is present. The relative importance of various positions in the surface unit cell tends to be less when the size of the fluid atom is larger. The dimers prefer to be oriented parallel to the surface but are tilted from the surface plane when bonded to the carbon atoms. Only a small fraction of non-associatively adsorbed dimers have a near-normal orientation.