Probing Conformational Heterogeneity at the Ionic Liquid-Vacuum Interface by Reactive-Atom Scattering.

The atomic-level description of liquid interfaces has lagged behind that of solid crystalline surfaces because existing experimental techniques have been limited in their capability to report molecular structure in a fluctuating liquid interfacial layer. We have moved toward a more detailed experimental description of the gas-liquid interface by studying the F-atom scattering dynamics on a common ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. When given contrast by deuterium labeling, the yield and dynamical behavior of reactively scattered HF isotopologues can resolve distinct signatures from the cation butyl, methyl, and ring groups, which help to quantify the relative populations of cation conformations at the liquid-vacuum interface. These results demonstrate the importance of molecular organization in driving site-specific reactions at the extreme outer regions of the gas-liquid interface.