Restructuring a Deep Eutectic Solvent by Water: The Nanostructure of Hydrated Choline Chloride/Urea

Deep eutectic mixtures are a promising sustainable and diverse class of tunable solvents that hold great promise for various green chemical and technological processes. Many deep eutectic solvents (DES) are hygroscopic and find use in applications with varying extents of hydration, hence urging a profound understanding of changes in the nanostructure of DES with water content. Here, we report on molecular dynamics simulations of the quintessential choline chloride-urea mixture, using a newly parametrized force field with scaled charges to account for physical properties of hydrated DES mixtures. These simulations indicate that water changes the nanostructure of solution even at very low hydration. We present a novel approach that uses convex constrained analysis to dissect radial distribution functions into base components representing different modes of local association. Specifically, DES mixtures can be deconvoluted locally into two dominant competing nanostructures, whose relative prevalence (but not their salient structural features) change with added water over a wide concentration range, from dry up to ∼30 wt % hydration. Water is found to be associated strongly with several DES components but remarkably also forms linear bead-on-string clusters with chloride. At high water content (beyond ∼50 wt % of water), the solution changes into an aqueous electrolyte-like mixture. Finally, the structural evolution of the solution at the nanoscale with extent of hydration is echoed in the DES macroscopic material properties. These changes to structure, in turn, should prove important in the way DES acts as a solvent and to its interactions with additive components.