Effects of the repulsive and attractive forces on phase equilibrium and critical properties of two-dimensional non-conformal simple fluids

Abstract

Abstract Molecular simulations in the canonical and isothermal-isobaric ensembles were performed for a two-dimensional simple fluid using a set of non conformal molecular interactions. The specific characteristics of this interaction model give it some advantages over other well known interaction potentials employed traditionally in the study of thermodynamic properties of soft matter systems. The functional form employed for modelling the effective potentials is the one proposed by the so-called ANC theory (Approximate Non-Conformal Theory), which allows to modify separately the contributions of the repulsive and attractive terms by means of a molecular parameter called softness. The effect of both, repulsive and attractive softness, on the liquid-vapour coexistence, liquid-solid transition and vaporization curve as well as on the value of the critical properties were explored. In fact, the linear dependence on the softness parameters exhibited by the critical properties, allows to extrapolate the linear correlations presented in this work, to reproduce the critical point of a two-dimensional Lennard-Jones fluid. With the aim of showing how versatile is the interaction model presented in this work, adequate values of the softness parameters were selected to reproduce qualitatively the liquid-vapour coexistence and critical points for both, a two-dimensional model fluid with a double exponential interaction potential, widely employed in molecular simulations of simple fluids; and a thin monolayer of methane molecules adsorbed on a planar graphite substrate, a system that can be considered in good approximation as two-dimensional and that has been studied in a lot of work for four decades. Finally, the effects of the dimensionality of the system was evaluated by quantifying the relative percentage differences exhibited by the thermodynamic properties explored in this paper with respect to its three-dimensional counterpart. The results presented in this work show not only the predictive capacity of the linear correlations mentioned, but the robustness of the ANC interactions to reproduce thermophysical properties and phase equilibria for model and real two-dimensional fluids. Discriminating the influence of repulsive and attractive contributions in molecular interaction models is undoubtedly of great relevance to have a better understanding of thermodynamic properties of simple and complex fluids.