Influence of anisotropic ion shape, asymmetric valency, and electrolyte concentration on structural and thermodynamic properties of an electric double layer

Abstract

Grand canonical Monte Carlo simulation results are reported for an electric double layer modelled by a planar charged hard wall, anisotropic shape cations, and spherical anions at different electrolyte concentrations and asymmetric valencies. The cations consist of two tangentially tethered hard spheres of the same diameter, d . One sphere is charged while the other is neutral. Spherical anions are charged hard spheres of diameter d .The ion valency asymmetry 1:2 and 2:1 is considered, with the ions being immersed in a solvent mimicked by a continuum dielectric medium at standard temperature. The simulations are carried out for the following electrolyte concentrations: 0.1, 1.0 and 2.0 M. Profiles of the electrode-ion, electrode-neutral sphere singlet distributions, the average orientation of dimers, and the mean electrostatic potential are calculated for a given electrode surface charge, σ, while the contact electrode potential and the differential capacitance are presented for varying electrode charge. With an increasing electrolyte concentration, the shape of differential capacitance curve changes from that with a minimum surrounded by maxima into that of a distorted single maximum. For a 2:1 electrolyte, the maximum is located at a small negative σ value while for 1:2, at a small positive value.

Monte Carlo Study of a Planar Electric Double Layer Formed by Ions with Off-Center Charge

“…The differential capacitance behavior is consistent with what is well-known in the literature, viz., a transition from a double-humped shape to a singlehumped shape as the concentration increases. 26,29 However, the striking features seen in this work are the increases in the magnitude of the capacitance and in the asymmetry of the curves with increasing charge displacement.…”

“…Cations have been approximated by beads, , coarse-grained models, , and pentameric ions . In our recent studies of EDL, we have also used an advanced model of an electrolyte. − Its molecules were assumed to be charged dimers composed of two tangentially touching hard spheres, one charged and the other one neutral. In one instance, these spheres were also assumed to be able to fuse into each other .…”

“…Recently, we have studied the structural and thermodynamic properties of EDL formed by shape-anisotropic organic cations such as N -methyl-pyridinium and 1-butyl-3-methylimidazolium. In much of our former work, we modeled these ions by a dimer consisting of two tangentially tethered hard spheres, one of which has a point electric charge immersed at its center and the other being neutral. − A natural extension of this dimer model is the situation when the two spheres can fuse into one another, leading to dumbbell-shaped ions …”

The Electrode-Electrolyte Interface from the perspective of Density Functional Theory

“…Within a density functional theory that captures Coulomb and hard-sphere interactions accurately [2] for aqueous electrolytes, we calculate the differential capacitance for several electrolytes, changing the size and valency of the ions similar to work done in Refs. [152,160,161,162,163,164,165,166,167,168,169]. We also derive a new relation for the differential capacitance, which explicitly shows that the response of the first layer of ions near the electrode to the applied potential on the electrode is key to its understanding.…”