Reactive-atom scattering (RAS) has recently been demonstrated to be an interesting addition to the armoury of tools available to analyse the composition of liquid surfaces. The method relies on the selective chemical reaction of a chosen gas-phase probe atom with particular functional groups exposed at the surface of a liquid. Because reactions that take place at the extreme outer layers of the liquid can be distinguished by their dynamical characteristics, RAS has inherently high surface sensitivity combined with chemical specificity. Its application is illustrated in liquids of practical interest, consisting of mixtures of a common family of ionic liquids containing alkyl and fluoroalkyl chains of different lengths. These materials have interesting potential applications as solvents in multiphase catalysis, where the properties of the stationary phase can be tuned controllably through the variation in mole fraction of the base materials. The composition of the outer surface is not known trivially, because of the balance between competing intermolecular forces in these chemically heterogeneous materials. We present experimental results combining O(<sup>3</sup>P) atoms as the RAS probe with detection of the OH product of selective reaction with exposed alkyl chains by laser-induced fluorescence. We compare the OH yields with molecular dynamics (MD) simulations of the structures of the liquid surfaces, and discuss how improved understanding of dynamical factors would further enhance the analytical information that can be extracted from the measurements. We will also consider briefly how the methodology might be generalised to detect a wider range of exposed functional groups.

Dynamical factors in reactive-atom scattering used to probe ionic-liquid surfaces

ACS Spring Meeting 2021

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