Since ionic liquids (ILs) are composed solely of ions, theories of diffuse electric double layers are typically presumed to be inapplicable for modeling their electrostatic screening properties. In this work, we use temperature-dependent, force-distance measurements to demonstrate that the electric double layers formed by ILs can exhibit a near surface regime featuring a pronounced layering of ions that is in series with a longer range, exponentially decaying diffuse double layer regime. Increasing the temperature decreased the measured diffuse layer decay lengths, whereas colloidal interactions that are purely entropic or osmotic in nature will increase in range at higher temperatures. Thus, outside of strongly surface bound ion layers, ILs can be conceptualized as dilute electrolyte solutions, where only a small fraction of the IL ions thermally dissociate to behave as effectively free, diffuse layer ions, while the remaining ions behave as a highly correlated dielectric medium. This framework is analogous to the autoionization of pure water. Our results resolve several scientific paradoxes concerning ILs, and provide a general, molecular-scale framework that may guide the design of high or ultra-low conductivity ILs for energy storage applications.

Ionic liquids and water: the surprising connection

249th National Meeting (2015)

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