# Primitive models of room temperature ionic liquids. Liquid-gas phase coexistence

## TL;DR

This paper develops primitive models and a mean spherical approximation theory to describe the phase coexistence of room temperature ionic liquids, showing good agreement with simulations and qualitative reproduction of experimental phase behavior.

## Contribution

It introduces new primitive models of RTILs and an AMSA-based theoretical framework to predict their phase diagrams, aligning well with simulation data.

## Key findings

- The models qualitatively reproduce RTIL phase behavior.
- Theoretical phase diagrams agree with computer simulations.
- Increasing asymmetry lowers critical temperature and density.

## Abstract

We propose several versions of primitive models of room temperature ionic liquids (RTILs) and develop a mean spherical approximation (MSA)-type theory for their description. RTIL is modeled as a two-component mixture of hard-sphere anions and flexible linear chain cations, represented by tangentially bonded hard spheres with the charge located on one of the terminal beads. The theoretical description of the model is carried out using the solution of the appropriately modified associative MSA (AMSA). Our solution reduces to solving one nonlinear algebraic equation for the Blum's screening parameter $\Gamma$, which in turn is used to express all thermodynamic properties of the models of interest. We calculate liquid-gas phase diagrams using theoretical and computer simulation methods for two versions of the model, represented by the dimer ($D$) and chain ($C$) models. Theoretical predictions for the phase diagrams appear to be in reasonably good agreement with computer simulation results. It is demonstrated that the models and theory are able to qualitatively reproduce experimentally observed phase behavior of RTILs, in particular the decrease of the critical temperature and critical density with increasing asymmetry of the model in its shape and position of the charge.