Deciphering the structure of deep eutectic solvents: A computational study from the solute's viewpoint

In the last few years, Deep Eutectic Solvents (DESs) have emerged as sustainable alternatives to traditional organic solvents. The high degree of freedom when tailoring DES structures represents, at the same time, their most praised and most complicated feature. Indeed, given the enormous number of possible combinations, the selection of the most suitable DES for a given application cannot be based on a trial-and-error approach; therefore, reliable computational tools are needed to fully exploit DESs potentialities. In this work, we propose the first computational protocol to investigate absorption properties of solutes dissolved in DES. The protocol combines an accurate sampling of the solute-solvent phase-space by means of Molecular Dynamics (MD) and a fully atomistic Quantum Mechanics/Molecular Mechanics (QM/MM) approach to describe intermolecular interactions and spectral properties. In this way, specific interactions such as hydrogen bonding, which characterize DES complex interaction patterns, are properly modelled. The robustness and reliability of the method are proved by comparing computed data with experimental spectra of 2-hydroxymethylfurfural and syringic, vanillic, p-coumaric, gallic, and caffeic acids dissolved in DES and water. These molecules have been chosen because of their relevance in the frame of biomass valorization. Remarkably, the protocol allows getting insights into DES/water mixtures and opens up to the prediction of aqueous DES behaviour of the water threshold which causes the switch from water-in-DES to aqueous electrolyte-like environments.