Application of the ionic association concept to the study of the phase behaviour of size-asymmetric ionic fluids in disordered porous media

Abstract

We study the vapour-liquid phase behaviour of a two-component size-asymmetric ionic fluid confined in a disordered porous matrix formed by uncharged hard spheres (HS) or overlapping (OHS) hard spheres. The ionic fluid is described as a monovalent primitive model consisting of opposite charged hard spheres of different diameters. We propose the theoretical approach which combines the scaled particle theory and the associative mean-spherical approximation based on the simplified mean-spherical approximation. Our results show that the fraction of free ions (dissociation degree) decreases in a matrix. On the other hand, the size asymmetry of ions strengthens this effect and makes it more pronounced at low porosities. It is also observed that the region of phase coexistence of an ionic fluid gets narrower when the porosity of matrices decreases. At the same time, the critical temperature and the critical density get lower. It is shown that the critical parameters of both the symmetrical and asymmetrical ionic fluids confined in the OHS matrix are higher than in the HS matrix if the porosity is fixed.