Chemical Reactivity as a Probe of Ionic-Liquid Surfaces

摘要

This report results from a contract tasking Heriot-Watt University as follows: The work will be similar to that which the group has pioneered in related studies of the reactions of O(3P) atoms with hydrocarbon liquids. It is based on laser-photolysis of a low pressure of a suitable O(3P) precursor (NO2 at 355 nm) above the surface of the liquid. This produces O(3P) atoms with a relatively broad, superthermal kinetic energy distribution, centred on 16 kJmol- 1 with a FWHM of 26 kJmol-1. The project will measure the relative reactivity of the series 1-alkyl-3-methylimidazolium (Rmim), where R = Me, Et, Bu, Hex, Oct, Dec, and Im salts by detecting relative OH yields by laser-induced fluorescence (LIF). The work will also compare the reactivity of one of the simpler members, Emim, with the NO3-counter-ion, because this is the combination most commonly selected for theoretical studies. All reactivities will be calibrated against the 'benchmark' hydrocarbon liquid, squalane. Guided by the results of the relative reactivity measurements, preliminary studies of the OH product internal and translational energy distributions for a selection of the ionic liquids will be carried out. The experiments are uniquely capable of providing this information, which helps to distinguish between the two limiting mechanisms seen in essentially all previous studies of reactive and inelastic scattering from hydrocarbons and some other liquids. One component is direct, dynamically controlled scattering at the outer layers of the liquid. The other is trapping-desorption, in which products are accommodated at the surface and take on near-thermal translational and rotational energy distributions. The balance between them is affected by the surface structure, in particular its penetrability and molecular roughness, whose dependence on the molecular structure of the Rmim will therefore be tested by measurement.