Critical behaviour of ionic fluids

TL;DR

This paper develops a microscopic approach using collective variables to analyze the critical behavior of ionic fluids, specifically the restricted primitive model, revealing phase instabilities and classical critical behavior consistent with simulations.

Contribution

It applies a collective variable method to ionic fluids, deriving a functional for the grand partition function and analyzing phase behavior beyond mean-field approximation.

Findings

Phase diagram shows gas-liquid and charge ordering instabilities.

Critical temperature matches Monte Carlo simulation data.

Critical density is underestimated, indicating limitations of the model.

Abstract

Recently we proposed a microscopic approach to the description of the phase behaviour and critical phenomena in binary fluid mixtures. It was based on the method of collective variables (CV) with a reference system. The approach allowed us to obtain the functional of the Ginzburg-Landau-Wilson (GLW) Hamiltonian expressed in terms of the CV (fluctuating densities). The corresponding set of CV included the variable connected with the order parameter. In this paper we use this approach to the study of the critical behaviour of ionic fluids. For the restricted primitive model (RPM) we obtain the functional of the grand partition function in the phase space of the two fluctuating fields conjugate to the fluctuating densities. First we calculate the phase diagram of the RPM in the mean-field (MF) approximation and then we do this calculation taking into account the terms of the higher orders in the effective Hamiltonian. In the both cases the phase diagrams demonstrate the gas-liquid (GL) and charge ordering phase instabilities. In the latter case, the obtained value for the GL critical temperature is in good agreement with the MC simulation data whereas the critical density is underestimated. The explicit expression found for the grand thermodynamic potential in the vicinity of the GL critical point implies a classical critical behaviour of the RPM.