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.

We investigated the surface tension and surface composition of various mixtures of the two ionic liquids (ILs) 1-methyl-3-octyl-imidazolium hexafluorophosphate [C 8 C 1 Im][PF 6 ] and 1,3- bis (polyethylene glycol)imidazolium iodide [(mPEG 2 ) 2 Im]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 [C 8 C 1 Im][PF 6 ] and the PEG chains of [(mPEG 2 ) 2 Im]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 [C 8 C 1 Im][PF 6 ] 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.