Phase behaviour of primitive models of molecular ionic liquids in porous media: effects of cation shape, ion association and disordered confinement

TL;DR

This paper develops a theoretical framework to study how cation shape, ion association, and porous confinement influence the phase behavior of ionic liquids, revealing that cation shape significantly affects critical properties and phase coexistence.

Contribution

It introduces a novel theoretical model combining multiple approaches to analyze ionic liquids with different cation shapes in porous media, including spherocylinder-shaped ions.

Findings

Lower critical temperature and density for spherocylinder cations compared to chain cations.

Narrower phase coexistence region for spherocylinder-shaped ionic liquids.

First theoretical study of spherocylinder-shaped ions in disordered porous media.

Abstract

The phase behaviour of room-temperature ionic liquids (ILs) confined in disordered porous media is studied using a theoretical approach that combines an extension of scaled particle theory, Wertheim's thermodynamic perturbation theory, and the associative mean spherical approximation. Two models, differing in the shape of the molecular cation, are considered: one with cations formed as charged flexible chains and the other with cations modelled as charged hard spherocylinders. Each model is described by a mixture of dimerized and free ions, while the porous medium is represented as a disordered matrix of hard spheres. We focus on the effects of the molecular cation shape, partial ion association, and disordered confinement on the liquid-vapour-like phase behaviour of the model ILs. In the approximation considered, we find that both the critical temperature and critical density in the model with spherocylinder cations are lower than those in the model with chain cations, and the phase coexistence region is narrower. This is the first theoretical attempt to describe an IL model with molecular ions shaped as spherocylinders, particularly in a disordered porous medium.