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 ([C₈ C₁ Im][PF₆ ]+[(MeO)₂ Im][PF₆ ]), we find a strong enrichment of the octyl chain-containing [C₈ C₁ Im]⁺ cation and a corresponding depletion of the [(MeO)₂ Im]⁺ cation in the topmost layer. For a mixture with different cations and anions, that is, [C₈ C₁ Im][Tf₂ N]+[(MeO)₂ Im][PF₆ ], we find both surface enrichment of the [C₈ C₁ Im]⁺ cation and the [Tf₂ N]^- (bis[(trifluoromethyl)sulfonyl]imide) anion, while [(MeO)₂ Im]⁺ and [PF₆ ]^- 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)₂ 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.