2 research outputs found
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<sub><i>n</i></sub>mim] and the tetrafluoroborate
anion [BF<sub>4</sub>] 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 H<sub>2</sub>O, enabling product
flux distributions to be obtained as a function of final translational
energy and scattering angle, <i>P</i>(<i>E</i><sub>T</sub>,θ<sub>f</sub>). 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: [C<sub>12</sub>mim][BF<sub>4</sub>] 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 ([C<sub>12</sub>mim][BF<sub>4</sub>]) in the smectic
A (SmA) and isotropic phases. O atoms with two distinct incident translational
energies were scattered from the surface of [C<sub>12</sub>mim][BF<sub>4</sub>]. 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 [C<sub>12</sub>mim][BF<sub>4</sub>] 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