(309e) Ionic Liquids Behave As Dilute Electrolyte Solutions

Gebbie, M. A., University of California, Santa Barbara
Valtiner, M., Max-Planck-Institut f. Eisenforschung GmbH
Banquy, X., University of California, Santa Barbara
Fox, E. T., North Carolina State University
Henderson, W. A., North Carolina State University
Israelachvili, J., University of California Santa Barbara

Ionic liquids are technologically-important materials that have been demonstrated for numerous applications, including safe, high-efficiency electrochemical storage devices, self-assembly media, and lubrication. Since these liquids are composed solely of electrically-charged ions, ionic liquids are typically expected to exhibit electrostatic screening properties that should be comparable to highly-concentrated electrolyte solutions (1, 2). Here, we combine direct surface force measurements, performed across the common ionic liquid [C4mim][NTf2], with simple thermodynamic arguments to demonstrate that ionic liquids instead behave as dilute weak electrolyte solutions, with typical effective dissociated ion concentrations of less than 0.1% of the maximum ion density at room temperature. Our results clear up several scientific controversies and paradoxes. We also provide a general, molecular-scale framework to guide the design of novel high free ion density ionic liquids.

1. Weingärtner Hermann (2008) Understanding ionic liquids at the molecular level: facts, problems, and controversies. Angew. Chem. Int. Ed. 47:654-670.

2. Kornyshev Alexei A (2007) Double-layer in ionic liquids: paradigm change? J. Phys. Chem. B 111:5545-5557.