Double layer for hard spheres with an off-center charge

TL;DR

This study models asymmetric ions in ionic liquids using off-center charges on spherical ions, revealing significant effects on the electrical double layer structure, especially when the cation acts as the counterion.

Contribution

It introduces a novel off-center charge model for spherical ions to mimic asymmetry in ionic liquids, comparing it with existing models.

Findings

Double layer structure is significantly affected by off-center charges.

Off-center charge model differs notably from the primitive and dimer models.

Results highlight the importance of charge placement in ionic liquid simulations.

Abstract

Simulations for the density and potential profiles of the ions in the planar electrical double layer of a model electrolyte or an ionic liquid are reported. The ions of a real electrolyte or an ionic liquid are usually not spheres; in ionic liquids, the cations are molecular ions. In the past, this asymmetry has been modelled by considering spheres that are asymmetric in size and/or valence (viz., the primitive model) or by dimer cations that are formed by tangentially touching spheres. In this paper we consider spherical ions that are asymmetric in size and mimic the asymmetrical shape through an off-center charge that is located away from the center of the cation spheres, while the anion charge is at the center of anion spheres. The various singlet density and potential profiles are compared to (i) the dimer situation, that is, the constituent spheres of the dimer cation are tangentially tethered, and (ii) the standard primitive model. The results reveal the double layer structure to be substantially impacted especially when the cation is the counterion. As well as being of intrinsic interest, this off-center charge model may be useful for theories that consider spherical models and introduce the off-center charge as a perturbation.