Anatomy of a deep eutectic solvent: structural properties of choline chloride : sesamol 1 : 3 compared to reline

Abstract

The structural properties of the deep eutectic solvent (DES) formed by choline chloride (ChCl) and sesamol in 1 : 3 ratio have been investigated and compared to those of reline (ChCl : urea 1 : 2). An integrated approach combining small and wide angle X-ray scattering with molecular dynamics simulations has been employed and the simulation protocol has been validated against the experimental data. In the ChCl : sesamol DES, strong hydrogen bonds (HBs) are formed between the chloride anion and the hydroxyl groups of the choline and of sesamol molecules. Conversely, choline–choline, choline–sesamol and sesamol–sesamol interactions are negligible. A more extended interplay between the constituents is observed in reline where, besides the HBs involving the chloride anion, the eutectic formation is favored also by strong choline–urea and urea–urea interactions. The three-dimensional arrangement around the individual components shows that, in the ChCl : sesamol DES, the cholinium cations and the sesamol molecules are packed in such a way to maximize the interactions with the chlorine anion. This structural arrangement may favor the π–π interactions between the sesamol molecules and the aromatic species mediated by the chloride ions, providing an interpretation for the high separation rates previously observed for phenolic DESs towards aromatic compounds.