3 research outputs found
Collision-Energy Dependence of the Uptake of Hydroxyl Radicals at Atmospherically Relevant Liquid Surfaces
A new
experimental approach to the study of collisions of hydroxyl
radicals with liquid surfaces is described, incorporating a molecular-beam
source of OH (or, in practice, OD, for technical reasons) radicals.
This allowed the collision-energy dependence of the scattering to
be examined. The incident and scattered OD molecules were detected
by laser-induced fluorescence. The representative branched, long-chain
alkane, squalane (2,6,10,15,19,23-hexamethyltetracosane), and
its partially unsaturated analogue, squalene (2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene),
were compared with perfluoropolyether as an inert reference liquid.
Dynamical aspects of the scattering necessary to quantify the OD survival
probability, and hence its complement, the reactive sticking coefficient,
were determined. Results were obtained at average laboratory-frame
kinetic energies of 7.2 and 29.5 kJ mol<sup>–1</sup>; they
are compared with previous independent measurements using a photolytic
source of OH with an average kinetic energy of 54 kJ mol<sup>–1</sup>. At lower collision energies, the survival probability is significantly
lower on squalene than on squalane but increases significantly with
collision energy. This is consistent with a negatively activated contribution
to loss of hydroxyl through addition to double-bond sites at the squalene
surface. In contrast, survival on squalane surface is found to be
approximately independent of collision energy across the range examined.
This is surprising because it does not reflect the positively activated
behavior typical of gas-phase OH + alkane reactions. We suggest that
this may be explained by a higher probability of trapping dynamics
at lower collision energies, enhancing the probability of reaction
following migration to more reactive sites. The results have implications
for the modeling of OH uptake on atmospheric aerosol surfaces as a
function of chemical composition and temperature
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