Effect of water on the structure and dynamics of choline chloride/glycerol eutectic systems

Deep Eutectic Systems (DESs) are an emerging class of green solvents with a myriad of applications, from biotechnology to material science. The high viscosity of most DESs is an obstacle that can be circumvented by the addition of water. But how much water is too much water? There is a need to understand the effect of water in the organizational dynamics of DESs, and to clarify the role of water on the physical properties of the DES, and as a functional additive. In this work, NMR spectroscopy was used to study choline chloride:glycerol (at a 1:2 molar ratio) systems with added water, at water contents ranging from 1 wt% (x_w = 0.06) to 70 wt% (x_w = 0.94). We identified three distinct water behaviour domains. Up to a water content of 11 wt%, water does not disrupt the DES structure; in fact, water has a glue-like effect, acting as a bridge between the DES components. Between water contents of 11 wt% and 35 wt%, the solvation of the DES components starts to occur, but the DES nanostructure is still present. At 35 wt% of water, the DES structure is disrupted, and the system transitions to a DES-in-water solution.