The anatomy of the double layer and capacitance in ionic liquids with anisotropic ions: Electrostriction vs. lattice saturation

We investigate the basic mechanisms of the electrical double layer formation in ionic liquids near a flat charged wall, by performing Monte Carlo simulations of several model liquids with spherical and elongated ions that contain charged ‘heads’ and neutral ‘tails’. We analyze the structural transitions in response to the variation of the voltage across the double layer and their effects on the formation of the characteristic ‘camel shape’ capacitance recently observed in experiments [1] and [2]. The camel shape was predicted earlier by the mean-field theory [3] for the case of an ionic liquid with large percentage of voids which allowed for substantial ‘electrostriction’ and thereby increase of the local charge density in the double layer. However, we show that in contrast to a liquid composed of spherical ions, in a liquid with anisotropic ions the double-hump ‘camel shape’ of the capacitance curve does not require high compressibility of the liquid. Many ionic liquids have elongated shape of ions, with charged ‘heads’ and neutral ‘tails’. We show that these neutral tails play a role of space fillers, or ‘latent voids’ that can be replaced by charged groups, following rotations and translations of ions. This feature allows an electric field-induced increase of the counter charge density without a substantial compression of the liquid. It causes rising branches of capacitance at small positive and negative electrode polarizations. However, we show that at large positive and negative voltages it has to be inevitably replaced by the ‘lattice saturation’, a universal effect, which does not depend on the model, in which the capacitance decreases inversely proportional to the square root of the voltage. This conclusion is supported by our analysis of the experimental results of Lockett et al. (2008) [1] that appear to follow this law staring from the potentials ∼±0.7–0.9 V. The competition between the two trends, one prevailing at low voltages and the other one at high voltages, results in the camel shape of capacitance, seen in our simulations.