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

Abstract

We investigated the surface tension and surface composition of various mixtures of the two ionic liquids (ILs) 1-methyl-3-octyl-imidazolium hexafluorophosphate [C8C1Im][PF6] and 1,3-bis(polyethylene glycol)imidazolium iodide [(mPEG2)2Im]I in the temperature range from 230 to 370 K under ultraclean vacuum conditions. The surface tension was measured using a newly developed apparatus, and the surface composition was determined by angle-resolved X-ray photoelectron spectroscopy (ARXPS). In the pure ILs, the alkyl chains of [C8C1Im][PF6] and the PEG chains of [(mPEG2)2Im]I are enriched at the IL/vacuum interface. In the mixtures, a strong selective surface enrichment of the alkyl chains occurs, which is most pronounced at low [C8C1Im][PF6] contents. For the surface tension, strong deviations from an ideal mixing behaviour take place. By applying a simple approach based on the surface composition of the mixtures as deduced from ARXPS, we are able to predict and reproduce the experimentally measured temperature-dependent surface tension values with astonishingly high accuracy.

Evolution of Surface Tension and Hansen Parameters of a Homologous Series of Imidazolium-Based Ionic Liquids

“…Even for the shortest mPEG n chains, the interface is readily saturated. This hypothesis is supported by ARXPS experiments on [(mPEG n ) 2 Im]­[I] ( n = 2, 4, 6). , …”

“…This hypothesis is supported by ARXPS experiments on [(mPEG n ) 2 Im][I] (n = 2, 4, 6). 17,63 In summary, [(mPEG n 2) shows that the magnitudes of δ p and δ h are similar for the ILs with the same anions. This is surprising, as the hydrophilic mPEG n chains with their electronegative oxygen atoms are expected to enable polar interactions compared to the nonfunctionalized alkyl chains.…”

“…The ionic character of ILs leads to characteristic features that are typically not found in conventional organic liquids, including very low vapor pressures, high ionic conductivity, as well as thermal stability over a large temperature range. − The ability to choose different pairs of cations and anions provides enormous flexibility (up to a million binary combinations plus many more ternary combinations are reported) to adjust the physicochemical characteristics and thus tailor their properties for any desired application . Finally, adjusting the interplay of the multiple molecular interactions within ILs provides a large playground to tailor structure formation within their bulk liquid or at their interfaces. ,− The convenient commercial accessibility, in particular of imidazolium-based ILs, further facilitates their applications …”

Understanding the Buoy Effect of Surface‐Enriched Pt Complexes in Ionic Liquids: A Combined ARXPS and Pendant Drop Study**

“…We have recently demonstrated the potential of combining these two experimental methods by correlating the microscopic picture of the IL/gas interface obtained from ARXPS with the macroscopic surface tension using the PD method [15] . By means of the simple Langmuir principle, the composition of the topmost molecular layer was used to predict the surface tension of a binary IL mixture at various temperatures with particularly high accuracy [15a] . In this contribution, we will expand the complementary scope of ARXPS and the PD method to solutions of the Pt complexes in [C 4 C 1 Im][PF 6 ].…”