Monte Carlo investigation of structure of an electric double layer formed by a valency asymmetric mixture of charged dimers and charged hard spheres

Abstract

Canonical and grand canonical Monte Carlo simulation results are reported for the structure of a planar electric double layer containing anisotropic-shaped ions having asymmetric valencies. The model double layer consists of an electrolyte formed by a mixture of charged dimers and charged hard spheres in a dielectric continuum next to a uniformly charged, non-polarisable, planar hard electrode. A dimer is made of two tangentially touching equi-sized hard spheres one of which is positively charged and the other neutral, while the monomer rigid ion of the same size is negatively charged. Results for the electrode-ion, electrode-neutral sphere singlet distributions, and the mean electrostatic potential are obtained for 2:1/1:2 valency asymmetry in the electrolyte solution regime at room temperature, and at a given electrolyte concentration for a series of electrode surface charge densities. Valency asymmetry coupled with anisotropic ion shape leads to a richer double-layer structure than seen previously with dimers in 1:1 symmetric valency situations. Comparisons are also made with the corresponding results for a 2:1/1:2 restricted primitive model electrolyte. The overall asymmetry also leads to a finite, non-zero potential of zero charge indicating a charge separation at the interface.