Structural Properties and Hydrogen-Bonding Interactions in Binary Mixtures Containing a Deep-Eutectic Solvent and Acetonitrile.

Abstract

Deep-eutectic solvents (DESs) are a new class of green solvents. Here, we report the hydrogen bonding and structural properties of the archetypal DES ethaline, a mixture of choline chloride (ChCl) and ethylene glycol (EG) of a 1:2 molar ratio, and its pseudo-binary mixtures with acetonitrile. The investigations were carried out employing Fourier-transform infrared (FTIR) spectroscopy combined with quantum chemical calculations. Excess and two-dimensional (2D)-correlation spectroscopies were used to identify favorable species in the solutions and to explore the heterogeneity. The results show that the mixing process is the transformation from ethaline and CH3CN dimer to the complexes of ethaline-1CH3CN and ethaline-2CH3CN, together with the increased percentages of the EG dimer, EG trimer, and CH3CN monomer with respect to their total amounts in the mixtures. Theoretical calculations show that, for ChCl, the positive charge is located at the methyl groups and methylenes, rendering their ability to form hydrogen bonds. Adding CH3CN to ethaline can hardly break apart the doubly ionic hydrogen bonds between Ch+ and Cl-. The cosolvent molecules mainly surround the core structure of ethaline, forming noncovalent hydrogen bonds with hydroxyl groups of EG/Ch+ but not Cl-. These in-depth studies on the properties of ethaline and CH3CN/CD3CN mixed solvents may shed light on exploring their applications.