Interfacial properties of charge asymmetric ionic liquids

We report molecular dynamics simulations of the coexistence and interfacial properties of ionic liquids as a function of cation/anion, (z+:z-) = (2:-1), (4:-1), charge asymmetry. Our results correct previous computations of the coexistence curve of (2:-1) charge asymmetric systems, obtained via the fine-lattice discretisation method. In agreement with previous computations we report a reduction in the critical temperature and an increase in the critical density with charge asymmetry. We have quantified the interphase potential resulting from the charge asymmetry, by analysing the charge density across the liquid-vapour interface. We show that the Debye-Huckel theory predicts reasonably well the magnitude and temperature dependence of the interphase potential for moderate charge asymmetries (2:-1) but it underestimates, by about 50, the electrostatic potential of ionic liquids with high charge asymmetries (4:-1).