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

Angle‐resolved X‐ray photoelectron spectroscopy and contact‐angle measurements guided by a signal attenuation model are utilized to extract molar composition and anion enrichment in the vacuum interface of a binary ionic liquid mixture, having a common quaternary ammonium cation and two different anions. By using the intensity ratio of the F1s peaks belonging to the two different anions recorded at the full electron take‐off angle range, from 0° to 80°, we have determined that only a fractionally covered and anion enriched surface layer can predict the AR‐XPS data, which is also consistent with surface tension measurements. Moreover, the more bulky and non‐spherical anion enrichment is evident even at the conventional and the so assumed bulk sensitive take‐off angle of 0°. This methodology provides a surface enrichment factor of the molecular ions and clearly serves as an experimental evidence for recently debated surface layering and/or island structure in ionic liquid systems. Angle‐resolved X‐ray photoelectron spectroscopy measurements together with contact‐angle data reveal a fractionally covered and TFSI [bis(trifluoromethane)sulfonimide] enriched surface layer.