Issue 9, 2022

Pressing matter: why are ionic liquids so viscous?


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.

Graphical abstract: Pressing matter: why are ionic liquids so viscous?

Supplementary files

Article information

Article type
Edge Article
08 Dec 2021
06 Feb 2022
First published
08 Feb 2022
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2022,13, 2735-2743

Pressing matter: why are ionic liquids so viscous?

F. Philippi, D. Rauber, K. L. Eliasen, N. Bouscharain, K. Niss, C. W. M. Kay and T. Welton, Chem. Sci., 2022, 13, 2735 DOI: 10.1039/D1SC06857A

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