Surface Structure and Dynamics of Ions at the Liquid-Vapor Interface of Binary Ionic Liquid Mixtures: Molecular Dynamics Studies

The surface structure and dynamics of ions at the liquid-vapor interface of binary mixtures of ionic liquids 1-n-octyl-3-methylimidazolium trifluoromethanesulfonate ([omim]-[TfO]) and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([emim]-[TfO]) of varying composition have been studied using atomistic molecular dynamics simulations. Global definition of the interface and the identification of the truly interfacial molecule method (ITIM) have been used to analyze the structure and dynamics of the ions at the interface. We have seen enhancement in the density of the longer alkyl chain cation ([omim]) at the liquid-vapor interface compared to the bulk. The surface is mainly enriched with the [omim] cation, and it becomes smoother with a decrease in mole fraction of [omim] cations in the mixtures. The [omim] cation shows greater survival probability at the liquid-vapor interface than other ions, and this probability increases with a decrease in mole fraction of [omim] cations. The reorientational correlation function suggests that the [omim] cations prefer to retain their orientation with respect to the interface normal for a longer time. The movement of ions in and out of the interface is facilitated by the out-of-plane rotation of the ions with respect to the interface.