The structural and hydrogen bonding properties of ionic liquid–co-solvent binary mixtures: the distinct behaviors of two anions

Abstract

In practical applications, ionic liquids are often mixed with co-solvents. Understanding their structures and the interactions between them is a prerequisite for expanding their range of applications. In this work, spectroscopic and theoretical methods were employed to explore the structure and hydrogen bonding behaviors of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI)/1-ethyl-3-methylimidazolium thiocyanate (EMIMSCN) and co-solvents. It can be concluded that the hydrogen bonds associated with C2–H and C4,5–H are enhanced with the addition of co-solvents in the EMIMTFSI–DMSO system, while those associated with C4,5–H are weakened in the EMIMSCN–DMSO system. Infrared and excess spectra in the v(imidazolium C–H) range of EMIMSCN–CD₃CN/CD₃COCD₃ systems further indicate that the abnormal change of hydrogen bonds associated with C4,5–H can be attributed to [SCN]⁻. These differences can be explained by the change of the primary interaction site. For EMIMTFSI, the primary interaction site in ion pairs and ion clusters is always C2–H, while for EMIMSCN, the primary interaction site in ion pairs is C2–H, and in ion clusters, it becomes C4,5–H. In the EMIMTFSI–DMSO system, the co-solvent primarily interacts with C4,5–H, while in the EMIMSCN–DMSO/CH₃CN/CH₃COCH₃ systems, it primarily interacts with C2–H. In addition, several complexes are identified through excess infrared spectra and DFT calculations.