Gas-liquid critical point in ionic fluids

TL;DR

This paper develops a statistical field theory for ionic fluids to analyze their gas-liquid critical points, successfully predicting phase diagrams that align qualitatively with simulation data for various charge asymmetries.

Contribution

It introduces a new method to calculate the chemical potential considering higher order fluctuations, enabling accurate phase diagram predictions for asymmetric ionic models.

Findings

Critical temperature decreases with increasing charge asymmetry.

Critical density increases rapidly with charge asymmetry.

Theoretical results qualitatively agree with Monte Carlo simulations.

Abstract

Based on the method of collective variables we develop the statistical field theory for the study of a simple charge-asymmetric $1:z$ primitive model (SPM). It is shown that the well-known approximations for the free energy, in particular DHLL and ORPA, can be obtained within the framework of this theory. In order to study the gas-liquid critical point of SPM we propose the method for the calculation of chemical potential conjugate to the total number density which allows us to take into account the higher order fluctuation effects. As a result, the gas-liquid phase diagrams are calculated for $z=2-4$. The results demonstrate the qualitative agreement with MC simulation data: critical temperature decreases when $z$ increases and critical density increases rapidly with $z$.