Electrolyte solutions containing nominally free ions are prevalent in biological systems, the ocean, and numerous energy storage technologies. The development of a general conceptual, and quantitative, model of concentrated electrolytes remains challenging due to the need to clearly distinguish between “free” and “bound” ions, and their effective concentrations. In this work, we study ionic liquids – single component liquids composed solely of ions – to provide a perspective on electrostatic screening in such systems. We use temperature-dependent surface force measurements to demonstrate that the long range, exponentially decaying diffuse double-layer forces observed across ionic liquids exhibit a pronounced temperature dependence. Increasing the temperature decreases the measured exponential (Debye) decay length, implying an increase in the thermally driven effective free-ion concentration in the bulk ionic liquids. Based on these quantitative results, we propose a general model for the long-range electrostatic screening in ionic liquids, where thermally activated charge fluctuations, either free ions or correlated domains of ions (quasiparticles), take on the role of free ions in traditional dilute electrolyte solution theories. This model represents a crucial step toward resolving several inconsistencies surrounding the current understanding of electrostatic screening and charge transport in ionic liquids. We are currently studying the dynamics of these effects in confined ionic liquids, for example under the action of oscillating electric fields and/or moving surfaces. Our work provides a way of envisioning highly concentrated electrolytes, with implications for diverse areas of inquiry, ranging from designing electrochemical devices to rationalizing electrostatic interactions in biological systems.

Long-range electrostatic forces in ionic liquids

255th National Meeting (2018)

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