Influence of anisotropic ion shape on structure and capacitance of an electric double layer: a Monte Carlo and density functional study

The effect of anisotropic ion shapes on the structure and the differential capacitance of an electric double layer in the electrolyte solution regime is studied using the density functional theory and Monte Carlo simulations. The double layer is modelled by a uniformly charged, non-polarizable planar electrode next to an electrolyte where the cation is a dimer consisting of two tangentially touching rigid spheres one of which is positively charged while the other is neutral, the anion is a negatively charged rigid sphere, and the solvent is a dielectric continuum. Numerical results are reported for monovalent electrolytes at room temperature for a series of electrolyte concentrations and varying electrode surface charge densities. Asymmetry in ionic shape leads to more structure near the electrode when its charge is opposite to that of the non-spherical ions. Overall, the theoretically predicted density and mean electrostatic profiles reproduce the corresponding simulation results to a very good degree. The asymmetry of the ion shape also yields asymmetry in the differential capacitance curve plotted as a function of the electrode charge density. The differential capacity evolves from being distorted bactrian camel-shaped (a minimum flanked by a maximum on either side) at low electrolyte concentrations to being bell-like (a single broad maximum) at higher concentrations. The theoretical capacitance results again agree well with the simulations.