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Towards understanding the structure and capacitance of electrical double layer in ionic liquids
Maxim V. Fedorov and Alexei A. Kornyshev
In order to understand basic principles of the double layer formation in room temperature ionic liquids, we have performed Molecular Dynamic simulations for a simplified system: dense assembly of charged Lennard-Jones spheres between charged walls. For simplicity, in this first investigation we have considered the cations and anions of the same size. We have calculated the corresponding values of the double layer capacitance as a function of the electrode potential and compared the results with existing theories. We have found that the capacitance curve does not follow the U-shape of the Gouy-Chapman theory, but has a bell-shape in agreement with the mean-field theory that takes into account the effect of limited maximum packing of ions. The wings of capacitance decrease inversely proportional to the square root of electrode potential, as prescribed by the mean-field theory and the charge conservation law that the latter obeys at large electrode polarizations. We have found, however, that the mean-field theory does not quantitatively reproduce the simulation results at small electrode potentials, having detected there remarkable overscreening effects (ionic correlations). The plots for the distributions of ions near the electrode at different electrode charges show that for the considered system the double layer is not one layer thick. The overscreening effects, dominating near the potential at the point of zero charge (p.z.c.), are suppressed by the high electrode polarizations with the onset of the so called ‘lattice saturation effect.’ The maximum of the capacitance bell coincides with the p.z.c., but only for this ‘symmetric’ system: if sizes of cation and anion are different the maximum will be shifted away from the p.z.c.