Long range electrostatic forces in ionic liquids

Abstract

Ionic liquids are pure salts that are liquid under ambient conditions. As liquids composed solely of ions, the scientific consensus has been that ionic liquids have exceedingly high ionic strengths and thus very short Debye screening lengths. However, several recent experiments from laboratories around the world have reported data for the approach of two surfaces separated by ionic liquids which revealed remarkable long range forces that appear to be electrostatic in origin. Evidence has accumulated demonstrating long range surface forces for several different combinations of ionic liquids and electrically charged surfaces, as well as for concentrated mixtures of inorganic salts in solvent. The original interpretation of these forces, that ionic liquids could be envisioned as "dilute electrolytes," was controversial, and the origin of long range forces in ionic liquids remains the subject of discussion. Here we seek to collate and examine the evidence for long range surface forces in ionic liquids, identify key outstanding questions, and explore possible mechanisms underlying the origin of these long range forces. Long range surface forces in ionic liquids and other highly concentrated electrolytes hold diverse implications from designing ionic liquids for energy storage applications to rationalizing electrostatic correlations in biological self-assembly.

Insight into the Electrical Double Layer of an Ionic Liquid on Graphene

“…While EDL forces have been previously measured between charged or biased surfaces in ILs 26 , ion pairs are expected to populate the solid-liquid interface in the absence of surface charge and specific interactions, thereby eliminating overscreening and EDL forces, as it occurs on silica surfaces (Fig. 1C).…”

“…To reconcile the aggregation model with NMR diffusivity measurements, which fail to find evidence for long lived ion pairs 32 , the coordinated cation-anion network is proposed to be highly dynamic and transient. Although the temperature-dependence of the electrical double layer force further strengthens the argument in favor of a diffuse layer, the shape of the reported capacitance and ion conductivity in separate works and in different systems contradicts the assumption of low ion dissociation, and it is still a subject of debate 13, 26 . Nevertheless, our results are consistent with the reported aggregation model.…”

“…Over the past three years some evidence has accumulated demonstrating that long range surface forces on electrically charged surfaces immersed in ILs are of electrostatic origin and they respond to the characteristics of an electrical double layer force of a dilute electrolyte solution 26 . A long-range electrical double layer force with a decay length as large as 13 nm has been measured for several ILs on various substrates 26–31 .…”

“…A long-range electrical double layer force with a decay length as large as 13 nm has been measured for several ILs on various substrates 26–31 . This finding is important since the interfacial capacitance is inversely proportional to the effective thickness of the electrical double layer where the charge is stored, and hence, it affects the stored energy in the supercapacitor.…”

“…This finding is important since the interfacial capacitance is inversely proportional to the effective thickness of the electrical double layer where the charge is stored, and hence, it affects the stored energy in the supercapacitor. The proposed “aggregation” model 26 assumes that ILs are dielectric solvents composed of a strongly correlated ionic network, where each charge is counterbalanced by the sum of the neighbours’ charge and only a small percentage of the ionic network is effectively dissociated. To reconcile the aggregation model with NMR diffusivity measurements, which fail to find evidence for long lived ion pairs 32 , the coordinated cation-anion network is proposed to be highly dynamic and transient.…”