Don’t Count Your Ions Before They Dissociate

Ionic liquids found to behave differently than expected.

Cartoon depicting how the ionic liquid molecules arrange in electrically charged interfaces (not to scale).
Image courtesy of J. N. Israelachvili
Cartoon depicting how the ionic liquid molecules arrange in electrically charged interfaces (not to scale). The green shading represents the 99.98% of the molecules that exist in a neutral, or “stuck”, state, the blue shapes represent the molecules with a positive charge and the red shapes represent negatively charged molecules. The reaction shown above the cartoon illustrates the fact that the molecules can exist in two states: as a neutral ionic liquid molecule (99.98 % of the molecules) and as dissociated, mobile positive (blue) and negative (red) charged molecules (0.02% of the molecules).

The Science

Researchers have discovered from sensitive measurements of the forces between charged surfaces at the nanoscale that ionic liquids,—liquids made entirely of positive and negative charged particles (i.e., ions)—behave more like solutions of dilute ions than like the commonly accepted description of a concentrated solution of dissociated and highly mobile ions.

The Impact

The new understanding of the behavior of the charged particles in ionic liquids helps explain the low electrical conductivity of many ionic liquids and should lead to a more efficient framework for the design of new ionic liquids to be used in cleaner, more sustainable, and nontoxic batteries, and other forms of electrical energy storage.

Summary

Nano-scale measurements have led to the discovery of a new conceptual framework to describe ionic liquids, which are liquids made entirely of positive and negative charged particles (i.e., ions). The commonly accepted description of ionic liquids is a concentrated solution of dissociated and highly mobile ions, but most ionic liquids actually exhibit low ionic conductivities. The new measurements provide insights into the origin of the low ionic conductivity by showing that, at each instant, ionic liquid molecules exist in two distinct states: neutral, “stuck” ions that do not contribute to ionic conduction, and charge-separated “free” (mobile) ions that can move to conduct electricity. Using a newly-developed experimental apparatus, viz., electrochemical surface force apparatus (EC-SFA), researchers at the University of California, Santa Barbara examined the ways ionic liquid ions distribute in electrically-charged interfaces. By showing that two electrically-charged surfaces “feel” and interact with each other at surface-surface separation distances that greatly exceeded previous expectations, the researchers established that more than 99.9% of the molecules in typical ionic liquids are “stuck” in neutral states. The new framework provides novel strategies for the design of next-generation, high conductivity ionic liquids, which may enable ionic liquids to be utilized in numerous energy storage applications including safer, cleaner batteries for electric automobiles.

Contact

Jacob Israelachvili
University of California – Santa Barbara
[email protected]

Funding

DOE, Office of Science, Basic Energy Sciences program; M.V. was supported by a Marie Curie International Outgoing Fellowship within the European Community Seventh Framework Program.

Publications

Matthew A. Gebbie, Markus Valtiner, Xavier Banquy, Eric T. Fox, Wesley A. Henderson, and Jacob N. Israelachvili; “Ionic liquids behave as dilute electrolyte solutions”, PNAS, 2013, 110 (24), 9674. DOI:10.1073/pnas.1307871110

Related Links

http://www.ia.ucsb.edu/pa/display.aspx?pkey=3034

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