Janus-faced role of water in defining nanostructure of choline chloride/glycerol deep eutectic solvent

Deep eutectic solvents (DESs) have emerged as a set of intrinsically "designer solvents" for many bio-applications such as DNA nanotechnology and biocatalysis. However, the high viscosity of DESs tends to prevent bioactive components from being incorporated into the solvent. Although dilution with water may effectively reduce the viscosity of DES, the effect of water on its cooperative hydrogen-bonding network has not been evaluated systematically. This study conducted a series of molecular dynamics simulations on the DES made of choline chloride and glycerol at different hydration levels. We discovered a Janus-faced role of water in defining the interactive network between choline chloride and glycerol. Chloride played a critical role in bridging choline and glycerol in the anhydrous mixture. But the addition of water results in the decrease in the number of choline-chloride-glycerol supramolecular complexes and the number of hydrogen bonds between choline and glycerol, demonstrating the de-structuring effect of water. Interestingly, we also found that water could link choline to glycerol in place of chloride. The structuring role of water in bridging choline and glycerol reached its maximum in the presence of 35.8 wt% water. The findings in this study will provide valuable guidance to determine the optimal water content that can sufficiently "liquidize" DESs and meanwhile maintain the majority of the eutectic stoichiometry in the DESs, paving the way for tapping the full potential of DESs as the intrinsically "designer solvents".