Molecular dynamics simulation of the electrical double layer in ionic liquids

The structure of the electrical double layer in the strongly coupled ionic liquid l-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) near a basal plane of graphite was investigated by molecular dynamics simulation. 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 ~2 nm from the surface. Interfacial layering is clearly observed at the surface. Ions adsorbed at the uncharged surface form the 2D molecular clusters of two types. In the first cluster type anions are self-assembled in a triangular lattice (containing ~5÷10 ions) while a cation subsystem is disordered. In the second one cations and anions self-assembled in a honeycomb lattice. The behavior of the screening potential in the ionic liquid [BMIM] [PF₆] near the charged graphite surface with the charge density in the range −1.7 ≤ σ ≤ 1.7 μC/cm² was investigated. It was shown that the potential is a nonmonotonic function of distance. Asymmetric behavior of the screening potential at surface charge densities equal in magnitude and opposite in sign was detected. It was shown that the local self-diffusion coefficients of ions in the vicinity of the surface correlate with the local ion density. 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.