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

## TL;DR

This study uses a novel method to detect fluorinated anions at ionic liquid surfaces, revealing differences in detection based on anion type and cation structure.

## Contribution

A new reactive-atom scattering technique is applied to quantify fluorinated anion exposure at ionic liquid surfaces.

## Key findings

- AlF signals were detected for [BF4]−, [Tf2N]−, and [OTf]− anions with varying intensities.
- Molecular dynamics simulations predicted F-atom exposure, aligning qualitatively with experimental results for [BF4]− and [Tf2N]−.
- Discrepancies suggest projectile penetration and anion-specific reactions affect AlF yields.

## Abstract

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.