On the composition and isomerism effect in the thermal and structural properties of choline chloride/hydroxyphenol deep eutectic solvents

Abstract

We have carried out a comparative study on three sets of eutectic mixtures based on choline chloride (ChCl) and hydroxyphenol isomers having two hydroxyl groups in the ortho-, meta-, and para-positions of the aromatic ring, namely catechol (Cate), resorcinol (Reso), and hydroquinone (Hydro), respectively. Differential scanning calorimetry highlighted a different thermal behavior of the mixtures depending on the composition and precursor isomerism. These systems behave as deep eutectic solvents (DESs) with the exception of the ChCl/Cate mixture at a 1 : 0.75 molar ratio, the ChCl/Reso mixtures at 1 : 2 and 1 : 3 molar ratios, and the ChCl/Hydro mixture at a 1 : 3 molar ratio. Infrared spectroscopy measurements and molecular dynamics simulations show that the stronger hydrogen-bonding (H-bonding) in the mixed states compared to the pure precursors is key for the formation of a DES. This interaction is mostly reliant on interconnected chloride anion coordination shells thanks to the two hydroxyl groups of the hydroxyphenol molecules, which can bridge between different anions to form an extended H-bond network. This structural arrangement maximizes the interactions and is enhanced by the 1 : 0.75 and 1 : 1 molar ratios, while increasing the hydroxyphenol concentration translates into a lowering of the total number of H-bonds formed in the mixture. This is the basis for the different thermal behavior and points to nearly equimolar compositions between the components, as ideally suited to achieve a DES from these precursors. The obtained insights are able to explain the structure-property relationships for the studied systems and are deemed useful for more conscious development of these inherently tuneable solvents.