Transitioning from Ionic Liquids to Deep Eutectic Solvents

Ionic liquids (ILs) and deep eutectic solvents (DESs) have been lately the solvents of choice in a number of processes because they offer a valid alternative to conventional solvents. Despite main interactions in ILs differ from those in DESs (e.g., electrostatic-type in the former and H-bond-type in the latter), these two neoteric solvents are more closely related that appeared and can be seen as the two sides, the face and the cross, of the same coin. Herein, we hypothesized about a way for transitioning from one to the other. In particular, we promoted the transition from 1-ethyl-3-methylimidazolium chloride (EMIMCl) to EMIMCl center dot nAcOH-based DESs by the simple addition of stoichiometric amounts of acetic acid (AcOH) to EMIMCl. H-1 NMR spectroscopy and DSC studies confirmed the occurrence of such a transition. Molecular dynamics (MD) simulations revealed the capability of the Cl anion to fully accommodate up to 4 AcOH molecules (e.g., EMIMCl center dot 1AcOH, EMIMCl center dot 2AcOH, EMIMCl center dot 3AcOH, and EMIMCl center dot 4AcOH) without signs of H-bond self-interactions between AcOH molecules. These DESs also exhibited quite different solvent properties, with alpha and beta Kamlet-and-Taft parameters that differed from those of EMIMCl and 1-ethyl-3-methylimidazolium acetate (EMIMOAc). Interestingly, excess molar volume and excess viscosity measurements as well as Brillouin spectroscopic experiments indicated that aqueous dilutions of EMIMCl center dot AcOH-based DESs deviated from ideality as a consequence of the formation of HBs between water molecules and the anion, as observed by H-1 NMR spectroscopy.

Automated summary

Ionic liquids (ILs) and deep eutectic solvents (DESs) have emerged as alternatives to conventional solvents, and despite their different interaction types, they can be considered as two sides of the same coin. In this study, a transition from 1-ethyl-3-methylimidazolium chloride (EMIMCl) to EMIMCl·nAcOH-based DESs was promoted by adding stoichiometric amounts of acetic acid (AcOH) to EMIMCl. The transition was confirmed through various spectroscopic and simulation techniques. The resulting DESs exhibited different solvent properties, and the aqueous dilutions deviated from ideality due to the formation of hydrogen bonds between water molecules and the anion.