ABSTRACT
The influence of temperature on the structure and dynamics of the [BMIM][PF(6)] ionic liquid/graphite interface has been investigated by molecular dynamics simulations. The performed simulations cover a 100 K wide temperature interval, ranging from 300 K to 400 K. It was shown that the magnitudes of density peaks of anions in the vicinity of the surface decrease with increasing temperature while in the case of cations anomalous temperature behaviour of the density profile is observed: the magnitude of the second peak of cations increases with the increase of temperature. To characterize interface dynamics the local self-diffusion coefficients D(x) of ions in the normal direction to the surface and the residence time of ions in the first and second interfacial layer have been estimated. It was shown that the local self-diffusion coefficients in the vicinity of the surface correlate with the local ion density; the maxima of the function D(x)(x) for the cations (anions) coincide with the regions of reduced cation (anion) density and vice versa. Finally, the influence of temperature on the screening potential in the vicinity of a charged graphite surface has been studied. It was shown that the increase of temperature from 300 K to 400 K induces the decrease of the potential drop across the interface that implies the increase of the capacitance of the electrical double layer.
ABSTRACT
The structure of the electrical double layer in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) near a basal plane of graphite was investigated by molecular dynamics simulation. The calculations were performed both for an uncharged graphite surface and for positively and negatively charged ones. It is found that near an uncharged surface the ionic liquid structure differs from its bulk structure and represents a well-ordered region, extending over approximately 20 A from the surface. Three dense layers of ca 5 A thick are clearly observed at the interface, composed of negative ions and positively charged rings. It is established that in the first adsorption layer the imidazolium ring in the [BMIM]+ cation tends to be arranged in parallel to the graphite surface at a distance of 3.5 A. The [PF6]- anion is oriented in such a way that the phosphorus atom is at a distance of 4.1 A from the surface and triplets of fluorine atoms form two planes parallel to the graphite surface. Ions adsorbed at the uncharged surface are arranged in a highly defective 2D hexagonal lattice and the corresponding lattice spacing is approximately four times larger than that of the graphene substrate. The influence of the electrode potential on the distribution of electrolyte ions and their orientation has also been investigated. Increase in the electrode potential induces broadening of the angle distribution of adsorbed rings and a shift of the most probable tilt angle towards bigger values. It was shown that there are no adsorbed anions on the negatively charged surface (sigma = -8.2 microC cm(-2)), but the surface concentration of adsorbed cations on the positively charged surface (sigma = +8.2 microC cm(-2)) has a nonzero value. In addition, the influence of the surface charge (+/- sigma) on the volume charge density and electric potential profiles in an electrolyte was studied. The differences in the cation and anion structure result in the fact that the integral capacitance of the electrical double layer depends on the electrode polarity and equals C = 4.6 microF cm(-2) at sigma = -8.2 microC cm(-2) and C = 3.7 microF cm(-2) at sigma = +8.2 microC cm(-2).
