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

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 ([C C Im][PF ]+[(MeO) Im][PF ]), we find a strong enrichment of the octyl chain-containing [C C Im] cation and a corresponding depletion of the [(MeO) Im] cation in the topmost layer. For a mixture with different cations and anions, that is, [C C Im][Tf N]+[(MeO) Im][PF ], we find both surface enrichment of the [C C Im] cation and the [Tf N] (bis[(trifluoromethyl)sulfonyl]imide) anion, while [(MeO) Im] and [PF ] 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) Im] 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.