Reactive-Atom Scattering from Liquid Crystals at the Liquid-Vacuum Interface : [C12mim][BF4] and 4-Cyano-4′-Octylbiphenyl (8CB)

Two complementary approaches were used to study the liquid-vacuum interface of the liquid-crystalline ionic liquid 1-dodecyl-3-methylimidazolium tetrafluoroborate ([C12mim][BF4]) in the smectic A (SmA) and isotropic phases. O atoms with two distinct incident translational energies were scattered from the surface of [C12mim][BF4]. Angle-dependent time-of-flight distributions and OH yields, respectively, were recorded from high- and low-energy O atoms. There were no significant changes in the measurements using either approach, nor the properties derived from them, accompanying the transition from the SmA to the isotropic phase. This indicates that the surface structure of [C12mim][BF4] remains essentially unchanged across the phase boundary, implying that the bulk order and surface structure are not strongly correlated for this material. This effect is ascribed to the strong propensity for the outer surfaces of ionic liquids to be dominated by alkyl chains, over an underlying layer rich in anions and cation head groups, whether or not the bulk material is a liquid crystal. In a comparative study, the OH yield from the surface of the liquid crystal, 8CB, was found to be affected by the bulk order, showing a surprising step increase at the SmA-nematic transition temperature, whose origin is the subject of speculation.