Molecular Dynamic Simulations of Ionic Liquids at Graphite Surface

Abstract

The interface structure between room temperature ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM+/PF6−) and 1-octyl-3-methylimidazolium hexafluorophosphate (OMIM+/PF6−), and the graphite (0001) surface has been studied by classical molecular dynamic simulations. It is found that the density of IL is much enhanced at the interfacial region and the density oscillation extends to ∼15 Å into the bulk with three layers. The results also demonstrate that the polar groups tend to aggregate forming a polar network, while the nonpolar groups fill up the rest of the vacancy. The imidazolium rings and the side chains preferentially lie flat at the graphite surface with the alkyl side chains of the cations elongated at the interfacial region, and the cations are closer to the graphite surface (ca. 3.6−3.7 Å) than the anions. The surface potential drop across the interface is more profound for OMIM+/PF6− than for BMIM+/PF6−, due to relatively larger local density of the anions for OMIM+/PF6− near the graphite surface.