Vapour-liquid phase diagram for an ionic fluid in a random porous medium

TL;DR

This paper develops a theoretical framework to analyze the vapour-liquid phase diagram of an ionic fluid confined in a porous medium, revealing how porosity and particle size influence critical points and phase coexistence.

Contribution

It introduces a combined theoretical approach using collective variables and scaled-particle theory to study ionic fluids in disordered porous media, extending existing models.

Findings

Decreased porosity lowers critical temperature and density.

Critical points approach bulk values as matrix particle size increases.

Both approximations accurately capture effects of porous media on phase behavior.

Abstract

We study the vapour-liquid phase behaviour of an ionic fluid confined in a random porous matrix formed by uncharged hard sphere particles. The ionic fluid is modelled as an equimolar binary mixture of oppositely charged equisized hard spheres, the so-called restricted primitive model (RPM). Considering the matrix-fluid system as a partly-quenched model, we develop a theoretical approach which combines the method of collective variables with the extension of the scaled-particle theory (SPT) for a hard-sphere fluid confined in a disordered hard-sphere matrix. The approach allows us to formulate the perturbation theory using the SPT for the description of the thermodynamics of the reference system. The phase diagrams of the RPM in matrices of different porosities and for different size ratios of matrix and fluid particles are calculated in the random-phase approximation and also when the effects of higher-order correlations between ions are taken into account. Both approximations correctly reproduce the basic effects of porous media on the vapour-liquid phase diagram, i.e., with a decrease of porosity the critical point shifts toward lower fluid densities and lower temperatures and the coexistence region is getting narrower. For the fixed matrix porosity, both the critical temperature and the critical density increase with an increase of size of matrix particles and tend to the critical values of the bulk RPM.