Ionic Liquids Behave As Dilute Electrolyte Solutions

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.