Double layer for hard spheres with an off-center charge

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.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.

Primitive models of room temperature ionic liquids. Liquid-gas phase coexistence

“…We propose to model an RTIL as a two-component mixture of hard-sphere anions and flexible linear chain cations, represented by the tangentially bonded hard spheres with the charge located on one of the terminal beads. For a cation chain length of two, our model reduces to the model studied earlier [12]. More important is that in addition we propose the MSA type of the theory, which is able to provide an analytical description of the structural and thermodynamic properties of the model.…”

“…Although the vast majority of the previous studies have been focused on the description of a RTIL on the detailed atomistic level (see, e.g. [5]), over the last decade several simple models of RTIL have been proposed and investigated [6,7,8,9,10,11,12,13]. Molecular ions (cations) in the framework of these models are represented either as a Lennard-Jones/hard spheres with off-center point charges [7,9,10,11,13], as a Lennard-Jones/hardsphere dimers with point charges located on one or both sites [6,9,12] or as a hard spherocylinder with point charge located on one of its ends [8].…”

“…[5]), over the last decade several simple models of RTIL have been proposed and investigated [6,7,8,9,10,11,12,13]. Molecular ions (cations) in the framework of these models are represented either as a Lennard-Jones/hard spheres with off-center point charges [7,9,10,11,13], as a Lennard-Jones/hardsphere dimers with point charges located on one or both sites [6,9,12] or as a hard spherocylinder with point charge located on one of its ends [8]. In these papers the structural and dynamic properties [6,7,9,10,11,13], density and potential profiles in a planar electrical double layer [12], as well as liquid-solid [10,11,13] and gas-liquid [8,9] phase behavior have been studied.…”

“…Molecular ions (cations) in the framework of these models are represented either as a Lennard-Jones/hard spheres with off-center point charges [7,9,10,11,13], as a Lennard-Jones/hardsphere dimers with point charges located on one or both sites [6,9,12] or as a hard spherocylinder with point charge located on one of its ends [8]. In these papers the structural and dynamic properties [6,7,9,10,11,13], density and potential profiles in a planar electrical double layer [12], as well as liquid-solid [10,11,13] and gas-liquid [8,9] phase behavior have been studied. The common feature of all these studies is that the properties of the models were investigated using computer simulation methods only, either Molecular Dynamics (MD) or Monte Carlo (MC) simulations.…”

Ion pairing and phase behaviour of an asymmetric restricted primitive model of ionic liquids

“…5 Malvaldi and Chiappe introduced a dumbbell model. 6 Slivestre-Alcantara and co-workers performed analyses on mixtures of off-centre charged cations and centrally charged anions 7 , as well as on ions composed of hard-sphere dimers, where only one of the tangentially connected spheres carry a charge. 8 Similar models, also including those with a trimer structure, were considered by Fedorov and Kornyshev.…”