Pressing matter: why are ionic liquids so viscous?

Frederik Philippi,Kristine Niss,Daniel Rauber,Christopher W M Kay,Tom Welton,Kira Lieberkind Eliasen,Nathalie Bouscharain

doi:10.1039/d1sc06857a

Abstract

Room temperature ionic liquids are considered to have huge potential for practical applications such as batteries. However, their high viscosity presents a significant challenge to their use changing from niche to ubiquitous. The modelling and prediction of viscosity in ionic liquids is the subject of an ongoing debate involving two competing hypotheses: molecular and local mechanisms versus collective and long-range mechanisms. To distinguish between these two theories, we compared an ionic liquid with its uncharged, isoelectronic, isostructural molecular mimic. We measured the viscosity of the molecular mimic at high pressure to emulate the high densities in ionic liquids, which result from the Coulomb interactions in the latter. We were thus able to reveal that the relative contributions of coulombic compaction and the charge network interactions are of similar magnitude. We therefore suggest that the optimisation of the viscosity in room temperature ionic liquids must follow a dual approach.

Highlights

In recent years, ionic liquids have transformed from a scienti c curiosity to extensively used functional uids, both in academia and industry.[1,2,3,4,5] the practical applicability of most ionic liquids is limited by their high viscosity compared with conventional molecular solvents
The densities of the ionic liquid and the molecular mimic were measured at ambient pressure as a function of temperature
We have measured the viscosity of the ionic liquid and the molecular mimic at ambient pressure

Summary

Introduction

Ionic liquids have transformed from a scienti c curiosity to extensively used functional uids, both in academia and industry.[1,2,3,4,5] the practical applicability of most ionic liquids is limited by their high viscosity compared with conventional molecular solvents. This is a key aspect for applications such as batteries, gas separation or biomass processing. The neutral system has been called the ‘molecular mimic’[6,7] and is a mixture of neutral analogues of the anionic and cationic molecular constituents.[6,7,8] To ensure similarity, the molecular

Methods

Results

Conclusion