SCALED-PARTICLE THEORY FOR ADSORBED HARD-CORE MOLECULES.

Molecular fluids such as N//2 and CS//2 adsorb on a graphite surface to form a variety of phases at temperatures below 200 K and submonolayer coverages. To a first approximation, we can model the thermodynamic properties of the adsorbed fluid in terms of a two dimensional fluid of hard-core molecules. However, these systems are not strictly two dimensional, the molecules vibrate in the plane perpendicular to the surface and tilt at an angle to the surface. The tilt of the molecules can be included by projecting the three dimensional adsorbate onto the surface plane. For realistic fluids of linear molecules the two dimensional analogue is a mixture of hard convex molecules with a continuous distribution of bondlengths. This distribution is controlled by the distribution of tilt angles in the three dimensional precursor. In this paper we develop a scaled particle theory for this polydispersed mixture which predicts pressures and chemical potentials which are thermodynamically consistent.