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

Abstract

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.

Unravelling the nature of citric acid:l-arginine:water mixtures: the bifunctional role of water

“…However, above certain amounts, water can solvate the isolated compounds, disrupting the DES supramolecular complex and forming an aqueous solution [8][9][10][11][12] . More recent studies also support these findings and further suggest the formation of water-based DES, accompanied by a much deeper melting depression upon water addition compared to the water-free systems 13,14 . While these studies are all based on choline chloride-based DES, they highlight the importance of characterizing the water influence on other DES, opening the possibility to include water in the DES preparation as a starting material rather than a solvent.…”

“…To the best of our knowledge, the only difference reported in the literature that differentiates a DES from a regular solution is the structural network organization. Whereas DES are recognized by the supramolecular complexes formed between its constituents, aqueous solutions are considered when solvation of the isolated components occurs [8][9][10][11][12][13][14] . Thus, if distinguishable within the available state-of-the-art tools, this would only be possible by molecular dynamic simulations as activity-coefficient analyses rely on non-local spatially-averaged properties.…”

Meta-analysis of viscosity of aqueous deep eutectic solvents and their components

“…A major experimental challenge is the high hygroscopy of DES and the sensitive dependence of viscosity on the water content, especially at χ w close to 0 [50][51][52] . However, only half of the sources reported the method of desiccation of the DES components prior to experimentation.…”

“…The strongly non-ideal mixing behavior of the viscosity and the highly negative values of E excess η of aqueous reline mixtures are surprising, because the densities of aqueous reline mixtures decrease almost linearly with water content (Supplementary Figure S20) 48 . However, it can be explained by the observation that, in contrast to aliphatic alcohols, the addition of urea to water has a negligible effect on the hydrogen-bond network of water at χ w > 0.8 51,58 . Therefore, despite its higher viscosity, addition of reline to water barely increases the viscosity of the aqueous reline mixture, resulting in the highly negative E excess η .…”

Chemically Stabilized and Functionalized 2D‐MXene with Deep Eutectic Solvents as Versatile Dispersion Medium

“…Herein, we developed a novel and well‐coordinated strategy to introduce long‐term chemical stability to Ti 3 C 2 T x using DESs as an anti‐oxidative dispersion medium and surface passivation materials. The most important characteristic of DES in this strategy is that it has a chemical inertness and can be hydrated in water to stabilize the reactive water molecules; [ 37 ] thus, DES can effectively prevent the penetration of oxygen (in water molecules) that cause oxidation of Ti 3 C 2 T x . Compared to previous results, mediating Ti 3 C 2 T x MXene using DESs as anti‐oxidative media is the most efficient, safest, and easiest method to mitigate the degradation of Ti 3 C 2 T x .…”