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

Abstract

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.

Direct Correlation of Surface Tension and Surface Composition of Ionic Liquid Mixtures—A Combined Vacuum Pendant Drop and Angle-Resolved X-ray Photoelectron Spectroscopy Study

“…In the present study, we focus on the surfaces of IL mixtures, particularly on the resulting surface tension and its relation to the surface composition. While the surfaces and interfaces of pure ILs under the well-defined conditions of ultrahigh vacuum have been studied quite extensively by several groups, including our own [ 6 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ], the investigation on the surface composition of IL mixtures with surface science techniques is a rather new field [ 17 , 25 , 30 , 31 , 32 , 33 , 34 ]. One important quantity related to liquid surfaces is surface tension (ST), which is the macroscopically observable change in free energy per change in unit surface area.…”

Surface tension of binary mixtures containing environmentally friendly ionic liquids: Insights from artificial intelligence

“…Especially, precise information about the ST of these fluids and their mixtures is indispensable in exploitation and designing modern industrial processes such as extraction, absorption, and also distillation involving IL (Carvalho, 2008;Oliveira, 2012). In recent years, many researchers have entered the field of laboratory studies of ionic liquids (Asl, 2020;Mosallanejad et al, 2018;Shaahmadi et al, 2018;Tajikmansori et al, 2020), and a number of them have measured the surface tension of compounds containing these substances (Oz, 2020;Shojaeian, 2019). Clearly, the experimental measurement cannot take the properties of each possible IL and its mixture with other compounds because there are infinite numbers of conceivable systems.…”