High-energy aluminium atoms as reactive-atom probes of Ionic-Liquid surface structure

Paul LANE¹, Thomas GSTIR², Simon PURCELL¹, Naomi ELSTONE³, Duncan BRUCE³, John SLATTERY³, Matthew COSTEN¹, Kenneth MCKENDRICK^1
1Heriot-Watt University, Edinburgh, United Kingdom
²Universität Innsbruck, Innsbruck, Austria
³University of York, York, United Kingdom

The structure of the extreme outer layer of ionic liquids (ILs) is crucial to their function in a range of applications, particularly those such as multiphase catalysis that involve the uptake or transport of gas-phase molecules through a gas-ionic liquid interface. A number of techniques have been used to attempt to measure the composition of liquid surfaces, but they are all subject to their own limitations in terms of chemical specificity or the depth to which they probe. Previously, we have successfully introduced a novel approach, termed reactive-atom scattering (RAS), in which gas-phase projectiles impinging on the surface react selectively with only one component of the liquid. The yield of the corresponding gas-phase product immediately above the surface is then a measure of the surface exposure of the relevant functional group. We have demonstrated that quantitative information on alkyl-chain exposure in a range of pure ionic liquids and their mixtures can be determined via RAS using oxygen atoms as the projectile, with OH radicals as the gas-phase product. In an effort to broaden the scope of the RAS technique, we have developed a new experiment which uses a source of high-energy aluminium atoms to abstract fluorine atoms from the surface of the liquid. The aluminium monofluoride (AlF) products are then probed in the gas phase using-laser induced fluorescence. We are using the new experiment to study the 1-alkyl-3-methylimidazolium family of ILs and their analogues bearing semiperfluorinated alkyl chains, with both fluorine and non fluorine-containing anions. We have successfully detected AlF from both cationic and anionic components, and are exploring the correlation between the yields from different liquids and the expected relative abundances of fluorine atoms at their surfaces. The experimental work is complemented by molecular dynamics simulations of the surface structure computed using the OPLS-AA force fields in GROMACS.