Thermodynamics of two-dimensional molecular fluids: Discrete perturbation theory and Monte Carlo simulations

Abstract

The recently reported two-dimensional discrete perturbation theory for soft atomic fluids has been extended here for molecular fluids. In particular, the analytical expression for the Helmholtz free energy of intermolecular potentials of arbitrary profile is built within a discrete perturbation theory constructed with a sequence of two-dimensional (2D) square-well and square-shoulder potentials of variable effective widths and ranges. Fluids that comprised both convex and dumbbell particles have been considered. For the former ones, the equation of state was obtained as a function of density, temperature and intermolecular parameters with implicit shape dependence evaluated by a virial rescaling procedure from the compressibility factor of effective hard-disk and two-dimensional square-well/square-shoulder discrete potentials. By varying the intermolecular parameters through their geometrical dependence, some illustrative cases of 2D-square-well and Kihara spherocylinder fluids are explored, and their vapor-liquid phase diagrams and equations of state are tested against new simulation data. For the latter one, an available hard-dumbbell equation of state constitutes the reference term of the perturbation expansion. It is found that these theoretical approaches are able to reproduce the equations of state of the selected fluids quantitatively, but the vapor-liquid equilibrium is only grasped in a qualitative way. Reasons for this drawback are also discussed.