Nanoclusters of room temperature ionic liquids: a molecular dynamics simulation study

Nanoscopic clusters of the room temperature ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) with diameters in the range of 2-8 nm have been studied using equilibrium molecular dynamics simulations. The butyl tail groups of the [bmim](+) ion protrude outwards from the surface of the cluster while the ring centres lie beneath, similar to the situation in a planar ionic liquid-vapour interface. The number densities of cation ring centres show a non uniform distribution near the surface in comparison to that of anions. An electrostatic potential drop of -0.17 V has been calculated across the cluster-vapour interface for the largest cluster studied. The effective interaction potential between the clusters has been evaluated and is found to exhibit a short-ranged, strong attractive well. A linear dependence of this well depth on the cluster size is observed, consistent with the predictions of the interpenetration model for inter-micellar interactions.