Are Room Temperature Ionic Liquids Dilute Electrolytes?

TL;DR

This paper presents a model indicating that ionic liquids contain a majority of free ions, challenging the view that they are primarily ion pairs, and thus are concentrated electrolytes rather than dilute ones.

Contribution

The authors develop a mean-field model incorporating various ion interactions to clarify the ion pairing dynamics in ionic liquids.

Findings

Approximately two-thirds of ions are free at any time.

Ion pairs have short lifetimes comparable to diffusion timescales.

Ionic liquids are better characterized as concentrated electrolytes.

Abstract

An important question in understanding the structure of ionic liquids is whether ions are truly "free" and mobile which would correspond to a concentrated ionic melt, or are rather "bound" in ion pairs, that is a liquid of ion pairs with a small concentration of free ions. Recent surface force balance experiments from different groups have given conflicting answers to this question. We propose a simple model for the thermodynamics and kinetics of ion pairing in ionic liquids. Our model takes into account screened ion-ion, dipole-dipole and dipole-ion interactions in the mean field limit. The results of this model suggest that almost two thirds of the ions are free at any instant, and ion pairs have a short lifetime comparable to the characteristic timescale for diffusion. These results suggest that there is no particular thermodynamic or kinetic preference for ions residing in pairs. We therefore conclude that ionic liquids are concentrated, rather than dilute, electrolytes.