Simulation of Fluid Phase Equilibria in Square-Well Fluids: From Three to Two Dimensions

Abstract

We study the influence of geometric restrictions on vapour/liquid coexistence properties and critical data of square-well fluids. Starting with three-dimensional bulk systems, we model the confinement by slit-like pores with decreasing slit widths arriving finally at planar (two-dimensional) fluid layers. For both bulk and confined fluids, we use a uniform approach performing series of canonical ensemble Monte Carlo simulations with Widom-like (virtual) particle insertions to estimate chemical potential versus density isotherms. By estimating the corresponding vapour/liquid coexistence densities using a Maxwell-like equal area rule for the subcritical chemical potential isotherms, we are able to study the influence of the confinement not only on chemical potentials but also on the coexistence properties. Critical point data are calculated from the coexistence densities by means of scaling relations. In particular, we study the change of the critical temperature and critical density varying the slit width and including the two- and three-dimensional bulk fluids as limiting cases. While the difference between the bulk and the slit critical temperature is found to decay exponentially with an exponent reciprocal to a linear function in the slit width, no comparable simple relation describing the influence of the confinement on the critical density is found.