Binary Ionic Liquids with a Common Cation: Insight into Nanoscopic Mixing by Infrared Spectroscopy

Mixing of ionic liquids (ILs) can expand the range of properties and versatility of this new class of solvents. Exploiting this potential requires a fundamental understanding of the mixing behavior of ILs. In this study, we have investigated binary IL mixtures involving a common cation (1-butyl-3-methylimidazolium) with two commonly used anions (chloride, bromide, tetrafluoroborate, hexafluorophosphate or bis(trifluoromethanesulfonyl)imide ([NTf₂]⁻)) by means of IR spectroscopy. Upon mixing the ILs, significant changes in the anion IR bands are induced, indicating mixing at the molecular level for highly symmetric anions. Furthermore, density functional theory (DFT) calculations show water to be a suitable probe molecule: IR bands of anion-1···H–O–H···anion-2, anion-1···H–O–H···anion-1, and anion-2···H–O–H···anion-2 are well suitable for inspection and quantification of the coexistence of each type of cluster. In contrast to some previous spectroscopic investigations, this work suggests good molecular mixing and the absence of nanosegregation in these binary ILs.