Intrinsic Structure of the Interface of Partially Miscible Fluids: An Application to Ionic Liquids

We investigate by means of Molecular Dynamics simulations how the intrinsic surface structure of liquid/liquid interfaces involving ionic liquids depends on the opposite phase of varying polarity. We study 1-n-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF6) and 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imid (BMIM NTf2). The opposite phase is either cyclohexane or water, but as a reference, IL-vacuum interfaces are also studied. We combine a distance-based cluster search algorithm with the ITIM intrinsic analyzing method to separate liquid phases showing non-negligible mutual miscibility and to identify atoms residing at the instantaneous surface. In contrast to the well structured surface of IL-vacuum systems, at liquid/liquid interfaces of ILs density correlations, ionic associations, and orientational preferences are all weakened, this effect being much more pronounced when the other species is water. In such systems we observe a drastic reduction in the presence of the cation at the surface and an increase of appearance of polar moieties (of both the cations and anions) leading to decreased apolar character of the interface. Furthermore, cations are mostly found to turn with their butyl chains toward the bulk while having their methyl groups sticking toward water. Anion-cation associations are reduced and partially replaced by water-anion and rarely also water-cation associations. © 2015 American Chemical Society