Anion Enhancement at the Liquid–Vacuum Interface of an Ionic Liquid Mixture

Abstract

The liquid−vacuum interface was investigated for a ionic liquid (IL) mixture containing 1-ethyl-3-methylimidazolium acetate, [C 2 MIM][OAc], and 1ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C 2 MIM][TFSI]. Herein, we detail a quantitative connection between molecular simulations and angle-resolved X-ray photoemission spectroscopy for an IL−vacuum interface. Results show that for a mixture with a low concentration of [TFSI] − , the anion [OAc] − is slightly depleted from the interface, whereas the [TFSI] − anion is significantly enhanced relative to the bulk. Both experiments and simulations reveal that the mole fraction of [TFSI] − increases significantly from the bulk value in the top 17 Å. Furthermore, simulations show that [TFSI] − has a preferred orientation at the liquid−vacuum interface.

Surface Propensity of Anions in a Binary Ionic‐Liquid Mixture Assessed by Full‐Range Angle‐Resolved X‐ray Photoelectron Spectroscopy and Surface‐Tension Measurements

“…As also depicted in Figure 2(b) the F1s spectra recorded at two take‐off angles for the same mixture reflects clearly ∼20 % depletion of the BF4 − anion at the more surface sensitive observation angle of 60°. Moreover, the observed ratio at the more bulk sensitive take‐off angle of 0° is 1.95±0.1 (see also Figure S4 and Tables S1 and S2) also exhibiting measurable deviations from their expected values of 1.65, which is consistent with reports by Zhang, [15c] and Heller et al [16a,b] . for different IL mixtures.…”

“…are needed for validating the XPS data. Recent works of Steinruck's and Newberg's groups can be given as examples of such successful applications [13b,15c] …”

Temperature‐Dependent Surface Enrichment Effects in Binary Mixtures of Fluorinated and Non‐Fluorinated Ionic Liquids

“…This observation indicates that strong enrichment and depletion effects are not only visible in 80° measurements but also in 0° emission (see also below). Recently, the deviation from the nominal bulk values at 0° emission, and therefore the pronounced enrichment/depletion of one of the species of mixtures, was reported by our group and others …”

“…Therefore, investigations of the topmost layers of IL mixtures are getting more into the focus of research. A variety of studies has been performed, using reactive‐atom scattering with laser‐induced fluorescence detection (RAS‐LIF), neutron scattering, small‐angle X‐ray scattering and X‐ray reflectivity, time of flight secondary ion mass spectrometry (TOF‐SIMS), Rutherford backscattering spectroscopy (RBS), low‐energy ion scattering (LEIS), X‐ray photoelectron spectroscopy (XPS) and molecular dynamics (MD) simulations …”

Molecular Dynamics Simulation of the Ionic Liquid 1‐n‐Butyl‐3‐Methylimidazolium Methylsulfate [Bmim][MeSO4]: Interfacial Properties at the Silica and Vacuum Interfaces

“…In the former region the imidazolium ring orients parallel to, while in the latter it orients perpendicular to the vacuum interface. Further simulations by Heggen et al., by Dragoni et al., and by Yang et al . reveal that the alkyl group of imidazolium ring orients perpendicular to the vacuum interface.…”