Scattering Dynamics of Oxygen Atoms on Imidazolium Tetrafluoroborate Ionic Liquid Surfaces: Dependence on Alkyl Chain Length

The dynamics of oxygen-atom scattering from the surfaces of a series of room-temperature ionic liquids containing the 1-alkyl-3-methylimidazolium cation [C_<i>n</i>mim] and the tetrafluoroborate anion [BF₄] were studied by reactive-atom scattering with mass spectrometric detection of products (RAS-MS). The length of the alkyl chain was varied (<i>n</i> = 4, 8, 12) in order to investigate the relationship between the scattering dynamics and the density of alkyl chains on the surface and their ordering. RAS-MS uses a beam-surface scattering technique with a hyperthermal O atom beam source and a rotatable mass spectrometer detector. Time-of-flight and angular distributions were collected for inelastically scattered O and reactively scattered OH and H₂O, enabling product flux distributions to be obtained as a function of final translational energy and scattering angle, <i>P</i>(<i>E</i>_T,θ_f). A new analysis technique was used to separate these distributions into three distinct components, corresponding to three dynamical pathways for scattered products: fast and slow impulsive scattering and thermal desorption. The results of these experiments support previous findings that surface alkyl coverage increases with increasing alkyl chain length, but these dynamical studies provide the additional insight that the ionic liquids with alkyl chains of <i>n</i> = 8, 12 have smoother surfaces than the ionic liquid with <i>n</i> = 4 and the pure alkane liquid, squalane (2,6,10,15,19,23-hexamethyltetracosane)

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