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

ABSTRACT: 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.

SUBMITTER: Heller BSJ 

PROVIDER: S-EPMC6175172 | biostudies-other | 2018 Jul

REPOSITORIES: biostudies-other