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^–1; they are compared with previous independent measurements using a photolytic source of OH with an average kinetic energy of 54 kJ mol^–1. 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_<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