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 [Cnmim] 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 (n = 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, P(ET,θf). 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 n = 8, 12 have smoother surfaces than the ionic liquid with n = 4 and the pure alkane liquid, squalane (2,6,10,15,19,23-hexamethyltetracosane).