Double layer in ionic liquids: Overscreening vs. crowding

TL;DR

This paper presents a continuum theory for ionic liquids that explains the transition from overscreening to crowding in the electrical double layer, aligning well with recent simulations and experiments.

Contribution

It introduces a simple Landau-Ginzburg-type model capturing both overscreening and crowding effects in ionic liquids, providing insights into their structure under different voltages.

Findings

Model predicts overscreening at low voltages.

Model shows crowding dominates at high voltages.

Results agree with recent simulations and experiments.

Abstract

We develop a simple Landau-Ginzburg-type continuum theory of solvent-free ionic liquids and use it to predict the structure of the electrical double layer. The model captures overscreening from short-range correlations, dominant at small voltages, and steric constraints of finite ion sizes, which prevail at large voltages. Increasing the voltage gradually suppresses overscreening in favor of the crowding of counterions in a condensed inner layer near the electrode. The predicted ion profiles and capacitance-voltage relations are consistent with recent computer simulations and experiments on room-temperature ionic liquids, using a correlation length of order the ion size.