Molecular dynamics simulation of the interfacial structure of[Cnmim][PF6] adsorbed on a graphite surface: effects of temperature and alkyl chain length

Abstract

The structures and diffusion behaviors of a series of ionic liquids[Cnmim][PF6] (n = 1, 4, 8 and 12) on a graphite surface have been investigated by means of moleculardynamics simulation. It was found that three or four ordering layers of ionicliquids were formed near the graphite surface, and this layering structure wasstable over the temperature range investigated. At the liquid/vacuum interface,the ionic liquid with a butyl chain had a monolayer ordering surface, while[C8mim][PF6] and[C12mim][PF6] exhibited a bilayer ordering with a polar domain sandwiched betweentwo orientational nonpolar domains. More impressively, the simulatedresults showed that for the ionic liquids with alkyl chains longer thanC4, the adjacent alkyl chains in the whole film tended to be parallel to each other, with theimidazolium rings packed closely together. This indicated that the ionic liquids have abetter regulated short-range structure than was previously expected. It was also found thatboth in the bottom layer and in the bulk region, the diffusion of the alkyl chains was muchfaster than that of the polar groups. However, as the alkyl chain length increased, thecharge delocalization in the cation and the enhanced van der Waals interaction between thenonpolar groups contributed by reducing this difference in the diffusivity of major groups.