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Surface Enrichment in Equimolar Mixtures of Non‐Functionalized and Functionalized Imidazolium‐Based Ionic Liquids
Author(s) -
Heller Bettina S. J.,
Kolbeck Claudia,
Niedermaier Inga,
Dommer Sabine,
Schatz Jürgen,
Hunt Patricia,
Maier Florian,
Steinrück HansPeter
Publication year - 2018
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201800216
Subject(s) - hexafluorophosphate , ionic liquid , chemistry , x ray photoelectron spectroscopy , intramolecular force , sulfonyl , ion , inorganic chemistry , trifluoromethyl , imide , chemical shift , stereochemistry , organic chemistry , alkyl , chemical engineering , engineering , catalysis
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 8 C 1 Im][PF 6 ]+[(MeO) 2 Im][PF 6 ]), we find a strong enrichment of the octyl chain‐containing [C 8 C 1 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, [C 8 C 1 Im][Tf 2 N]+[(MeO) 2 Im][PF 6 ], we find both surface enrichment of the [C 8 C 1 Im] + cation and the [Tf 2 N] − (bis[(trifluoromethyl)sulfonyl]imide) anion, while [(MeO) 2 Im] + and [PF 6 ] − 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.