Adsorption interactions and the two-dimensional critical temperature

Abstract

Thermodynamic properties of films depend on both the interactions within the adsorbate and those between the adsorbate and the substrate. We investigate these in relation to the problem of the two-dimensional (2D) liquid–vapour critical point. Our focus is the value of the critical temperature T_c for physically adsorbed atoms and methane molecules. The calculations employ perturbation theory, using the equation of state of a 2D Lennard-Jones (LJ) fluid as a reference. Explicitly included effects are deviations of the two-body potential from LJ shape, substrate modification of the interaction (leading to different 2D and 3D potentials) and three-body interactions within the adsorbate. Each of these contributes a measurable shift of T_c from the value computed with LJ theory and the 3D potential. The final predictions are in semiquantitative agreement with experimental data for gases on graphite.