Surface Enrichment in Equimolar Mixtures of Non‐Functionalized and Functionalized Imidazolium‐Based Ionic Liquids

Abstract

For equimolar mixtures of ionic liquids with imidazolium‐based cations of very different electronic structure, we observe very pronounced surface enrichment effects by angle‐resolved X‐ray photoelectron spectroscopy (XPS). For a mixture with the same anion, that is, 1‐methyl‐3‐octylimidazolium hexafluorophosphate+1,3‐di(methoxy)imidazolium hexafluorophosphate ([C8C1Im][PF6]+[(MeO)2Im][PF6]), we find a strong enrichment of the octyl chain‐containing [C8C1Im]+ cation and a corresponding depletion of the [(MeO)2Im]+ cation in the topmost layer. For a mixture with different cations and anions, that is, [C8C1Im][Tf2N]+[(MeO)2Im][PF6], we find both surface enrichment of the [C8C1Im]+ cation and the [Tf2N]− (bis[(trifluoromethyl)sulfonyl]imide) anion, while [(MeO)2Im]+ and [PF6]− are depleted from the surface. We propose that the observed behavior in these mixtures is due to a lowering of the surface tension by the enriched components. Interestingly, we observe pronounced differences in the chemical shifts of the imidazolium ring signals of the [(MeO)2Im]+ cations as compared to the non‐functionalized cations. Calculations of the electronic structure and the intramolecular partial charge distribution of the cations contribute to interpreting these shifts for the two different cations.

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.…”

“…As a general practice, AR‐XPS has been employed to measure only at two angles, commonly 0° and 80°, experimentally to inspect the molar composition of the constituents. This is sufficient for obtaining an estimate of the cation‐anion ratio at a specific point near the surface but not for acquiring a composition profile with respect to liquid depth [4a,b,12a,16a,b,17] …”

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 …”

Anion Exchange at the Liquid/Solid Interface of Ultrathin Ionic Liquid Films on Ag(111)

“…[C 8 C 1 Im][Tf 2 N] was synthesised under ultrapure conditions according to previous publications . For comparing the thin film results with bulk mixtures, bulk amounts of [C 8 C 1 Im][Tf 2 N]/[C 8 C 1 Im][PF 6 ] mixtures were prepared with various compositions using acetonitrile (Sigma‐Aldrich, purity 99.8 %) as a co‐solvent to ensure proper mixing of the respective ILs; the IL mixture was spread as macroscopic film (about 0.1 mm thickness) on polycrystalline Ag foil. After careful degassing of the liquid film, the sample was introduced into the vacuum chamber for ARXPS.…”