Detection of Different Classes of Fluorinated Anions at Ionic-Liquid Surfaces by Reactive-Atom Scattering Using Laser-Ablated Al Projectiles

Reactive-atom scattering (RAS) using laser-ablated aluminum projectiles has been applied to probe the exposure of fluorinated anions at ionic-liquid (IL) surfaces. Gas-phase AlF was detected by laser-induced fluorescence (LIF) following interaction of the Al plume with ILs containing bis-(trifluoromethylsulfonyl)-imide ([Tf₂N]^-), trifluoromethanesulfonate ([OTf]^-), and tetrafluoroborate ([BF₄]^-) anions, paired with 1-ethyl-3-methylimidazolium ([C₂mim]⁺) or 1-octyl-3-methylimidazolium ([C₈mim]⁺) cations. Clear AlF signals were observed for all three fluorinated anions, though yields varied markedly, with relative intensities following the sequence [BF₄]^- > [Tf₂N]^- ≫ [OTf]^-. Molecular dynamics (MD) simulations employing solvent-accessible surface area and a ball-drop algorithm provided quantitative predictions of F-atom outer-surface exposure, defined as the combined surface area of atoms directly accessible to a probe particle of specified radius, which were compared with experimental AlF yields. The reduction in F-atom exposure, qualitatively expected with an increase in cation alkyl-chain length, was predicted by MD for all three anions and observed in the AlF yields from salts with [BF₄]^- and [Tf₂N]^-. However, even for these salts, there were a number of quantitative differences between the predictions of outer-surface exposure and AlF yields, which may partially be explained by penetration of the incident projectiles below the alkyl-chain layer present at the extreme outer surface of the liquids. Discrepancies for [OTf]^- salts were much larger and are most likely evidence for anion-specific competing primary reactions that suppress AlF production, or for secondary processes that prevent it from surviving and escaping into the gas phase. These results provide new insight into the subtlety of the reactions of the species in the Al plume with fluorinated anions and point to the further understanding that is needed to establish Al-ablation RAS-LIF as a quantitative probe of fluorinated species at IL interfaces.