A Classical Density Functional Study of Clustering in Ionic Liquids at Electrified Interfaces

Abstract

Ion association, leading to the formation of clusters in ionic liquids, is investigated within the framework of classical density functional theory. Clusters are incorporated into a generic coarse-grained model for imidazolium-based ionic liquids confined by planar charged surfaces. We find that the short-ranged structure adjacent to surfaces is remarkably unaffected by the degree of ion association. The physical implications of ion clustering only become apparent in equilibrium properties that depend upon the long-range screening of charge, such as the asymptotic behavior of forces between charged surfaces and the differential capacitance around low surface potentials. Surface forces show a long-range exponential decay, which depends primarily on the concentration of nonassociated ions, while the differential capacitance seems to be a sensitive function of the internal structure of clusters. Furthermore, the size of the ion clusters only slightly influences surface forces, but has a significant effect on...