Imidazolium Ionic Liquid Interfaces with Vapor and Graphite: Interfacial Tension and Capacitance from Coarse-Grained Molecular Simulations

A recently developed coarse-grained model (J. Phys. Chem. B, 2010, 114, 12629-12631), previously validated against experimental data for a number of bulk properties, is used in molecular dynamics simulations of two different interfaces involving the ionic liquid [BMI][PF(6)]. First, simulations of the liquid-vapor interface demonstrate that the model is able to predict the surface tension of the fluid (for which we obtain a value of 39.4 mN.m(-1) at 400 K). Second, simulations were performed at constant potential differences applied between two graphite electrodes. From simulations with different applied potentials, the differential capacitances of the positive and negative electrodes can be calculated. It appears that both capacitances (C(+) = 3.9 mu F.cm(-2) for the positive electrode and C(-) = 4.8 mu F.cm(-2) for the negative electrode) agree very well with simulations results obtained with an all-atom model. The coarse-grained model also accurately reproduces the two-dimensional structure observed at the graphite-ionic liquid interface, namely, a defective hexagonal lattice with a lattice spacing of approximately 10 angstrom.