Long-range electrostatic screening in ionic liquids

Abstract

Electrolyte solutions with high concentrations of ions are prevalent in biological systems and energy storage technologies. Nevertheless, the high interaction free energy and long-range nature of electrostatic interactions makes the development of a general conceptual picture of concentrated electrolytes a significant challenge. In this work, we study ionic liquids, single-component liquids composed solely of ions, in an attempt to provide a novel perspective on electrostatic screening in very high concentration (nonideal) electrolytes. 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. We use our quantitative results to propose a general model of long-range electrostatic screening in ionic liquids, where thermally activated charge fluctuations, either free ions or correlated domains (quasiparticles), take on the role of ions in traditional dilute electrolyte solutions. This picture represents a crucial step toward resolving several inconsistencies surrounding electrostatic screening and charge transport in ionic liquids that have impeded progress within the interdisciplinary ionic liquids community. More broadly, our work provides a previously unidentified 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.electrostatic interactions | intermolecular interactions | interfacial phenomena | Boltzmann distribution | activation energy E lectrolyte solutions are multicomponent liquids that are composed of ions (solutes) dissolved in a liquid phase (solvent), a classic example being salt water. Like any ideal mixture, the driving force for the dissolution of ions in electrolyte solutions is entropic. Unlike ideal mixtures, the long-range nature and high interaction free energy of the electrostatic interactions between ions ensures that the physical properties of all but the most dilute electrolyte solutions exhibit pronounced deviations from ideal behavior. These deviations primarily arise from the steric "crowding" of ions and the electrostatic correlation of ions, for example the formation of neutral ion pairs, both of which increase the range of electrostatic interactions, compared with ideal solutions (1). As a result, the development of a general conceptual picture of concentrated electrolyte solutions remains challenging. Nevertheless, electrolytes with high ionic concentrations are prevalent in biological systems (2) and technological applications, such as energy storage devices (3-5), so overcoming this challenge remains an important task.In this work, we study room-temperature ionic liquids (RTILs), ...

Mean-Field Theory of Electrical Double Layer In Ionic Liquids with Account of Short-Range Correlations

“…It is interesting to note that experimentally measured nonmonotonic dependence of Debye length on concentration has been reported, suggesting a necessity for further experimental tests and thereby possible need for a more general theory [41]. The recent papers by Isrealashvili et al [42,43] have reported that ionic liquids may be considered as a mixture of ion pairs and free ions, which interexchange between these two states, with the equilibrium shifted in favour of ion pairs (for discussion of those findings see Refs. At the same time Lee et al published estimates of the degree of ion-paring in ionic liquids which suggests that on average only 1/3 of the total number of ions are bound in ionic-pairs [40].…”

“…We can show the difference in differential capacitance curves however, if we took for orientation the values of γ corresponding to estimates of the concentration of free charge carriers following Ref. 42,43 or 40, as seen in Fig. 5.…”

“…A renowned Feature Article Ref. 7 has set a tone in this development and triggered a flow of theoretical , as well as experimental [37][38][39][40][41][42][43], works in this area. A simple theory was presented in that paper which predicted 'bell' or 'camel' shaped differential capacitance-potential curves, instead of Gouy-Chapman (GC) U-shape differential capacitance-potential dependence.…”

Ion pairing and phase behaviour of an asymmetric restricted primitive model of ionic liquids

“…9,10 Models with Gaussian-distributed charges have also been considered. 11,12 In room temperature ionic liquids (RTILs), a high density and low dielectric screening promotes large electrostatic correlations and the increased importance of steric interactions. Here the RPM (and modifications to it) can serve as a basic model that can be treated semi-analytically or numerically with simulations.…”

“…There are surface force, conductivity and voltammetry measurements that do support the existence of ion pairs, or higher order clusters, in ionic liquids. [11][12][13][14][15][16] For example, Gebbie et al 11,12 used the Surface Force Apparatus, SFA, to measure interactions between charged surfaces, separated by ILs. They found long-ranged forces, persisting to separations of 10-12 nm.…”