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.

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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.Key words: electrical double layer, simulations, density functional theory, off-center charged spheres PACS: 61.20.Qj, 82.45.Fk, 82.45.Gj, 82.45.Jn This article is dedicated to our colleague and friend, Stefan Sokołowski, in commemoration of his 65th birthday. DH first met and collaborated with "Don Esteban" in Mexico City but had admired his work long before that. Stefan has been our good friend and frequent collaborator since that time. We wish him a happy birthday and continued good health and productivity.

The structure of a planar electric double layer formed by a 2:2 valency dimer electrolyte in the vicinity of a uniformly charged planar hard electrode is investigated using density functional theory and Monte Carlo simulations. The dimer electrolyte consists of a mixture of charged divalent dimers and charged divalent monomers in a dielectric continuum. A dimer is constructed by two tangentially tethered rigid spheres, one of which is divalent and positively charged and the other neutral, whereas the monomer is a divalent and negatively charged rigid sphere. The density functional theory reproduces well the simulation results for (i) the singlet distributions of the various ion species with respect to the electrode, and (ii) the mean electrostatic potential. Comparison with earlier results for a 2:1/1:2 dimer electrolyte shows that the double layer structure is similar when the counterion has the same valency.

In this paper we present the results of the ion mobility measurements made in pure carbon tetrafluoride (CF4) and gaseous mixtures of argon with carbon tetrafluoride (Ar-CF4) for pressures ranging from 6 to 10 Torr (8–10.6 mbar) and for low reduced electric fields in the 10 Td to 25 Td range (2.4-6.1 kV⋅cm−1⋅bar−1), at room temperature. The time of arrival spectra revealed only one peak throughout the entire range studied which was attributed to CF3+. However, for Ar concentrations above 70%, a bump starts to appear at the left side of the main peak for reduced electric fields higher than 15 Td, which was attributed to impurities. The reduced mobilities obtained from the peak centroid of the time-of-arrival spectra are presented for Ar concentrations in the 5%–95% range.