title: Influence of ionic liquid film thickness on ion pair distributions and orientations at graphene and vacuum interfaces.
abstract: Microscopic structures, orientational preferences together with mass, number and electron density distributions of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) ionic liquid (IL) have been studied on a neutral hydrophobic graphene surface, and at the IL-vacuum interface using atomistic Molecular Dynamics simulations. At the IL-graphene interface, distinct mass, number and electron density distributions are observed oscillating into the bulk region with several compact structural layers. The imidazolium ring of [BMIM] cations lies preferentially flat on the graphene surface, with its methyl and butyl side chains elongated along the graphene surface. At the IL-vacuum interface, however, the distributions of [BMIM][PF6] ion pairs are strongly influenced by the thickness of IL film. With the increase of IL film thickness, the orientations of [BMIM] cations at the IL-vacuum interface change gradually from dominant flat distributions along the graphene surface to orientations where the imidazolium rings are either parallel or perpendicular to the IL-vacuum interface with tilted angles. The outmost layers are populated with alkyl groups and imparted with distinct hydrophobic character. The calculated radial distribution functions suggest that ionic structures of [BMIM][PF6] ion pairs in IL-graphene and IL-vacuum interfacial regions are significantly different from each other and also from that in bulk regions.
authors.affiliation: Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, Stockholm S-106 91, Sweden.
authors.forename: Yong-Lei
authors.initials: YL
authors.lastname: Wang
authors.forename: Aatto
authors.initials: A
authors.lastname: Laaksonen
authors.forename: Zhong-Yuan
authors.initials: ZY
authors.lastname: Lu
mesh_terms: D006108:Graphite / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D052578:Ionic Liquids / Q000737:chemistry*; D011789:Quantum Theory; D014618:Vacuum
chemical_list: C412621:1-butyl-3-methylimidazolium hexafluorophosphate; D007093:Imidazoles; D052578:Ionic Liquids; D006108:Graphite