Resilience of Malic Acid Natural Deep Eutectic Solvent Nanostructure to Solidification and Hydration

Abstract

Little is presently known about the unique nanostructure of deep eutectic solvents (DES). The order of the liquid-solid phase transition is contended and whether DES-water mixtures are merely aqueous solutions, or have properties dominated by the eutectic pair, is unclear. Here, we unambiguously show the structure of choline chloride-malic acid (malicine) as a liquid, and also in solid and hydrated forms, using neutron total scattering on D/H isotope-substituted samples, and quasi-elastic neutron scattering (QENS). Data were refined using empirical potential structure refinement. We show evidence for a stoichiometric complex ion cluster in the disordered liquid, with strong choline-chloride bonding and a hydrogen bond donor (HBD) contribution. The 1:1 eutectic stoichiometry makes these ionic domains more well-defined, with less HBD clustering than seen previously for reline. There is minimal structural difference for the solidified material, demonstrating that this DES solidification is a glass transition rather than a first order phase change. QENS data support this by showing a gradual change in solvent dynamics rather than a step change. The DES structure is mostly retained upon hydration, with water acting both as a secondary smaller HBD at closer range to choline than malic acid, and forming transient wormlike aggregates. This new understanding of DES structure will aid understanding of the properties of these novel green solvents on the molecular length scale in chemical processes, as well as giving an insight into the apparent role of natural DESs in plant physiology.

Bayesian determination of the effect of a deep eutectic solvent on the structure of lipid monolayers

“…The water content of the DES was determined before and after each experiment by Karl-Fischer titration (Mettler Toledo DL32 Karl-Fischer Coulometer, Aqualine Electrolyte A, Aqualine Catholyte CG A) in order to ensure water presence was kept to a minimum. Those measurements showed that the water content of the solvent was kept below 0.3 wt/% during all the experimental procedures presented here, which we assume to be negligible and have to little impact on the characteristics of the DES [3,4].…”

“…However, the native environment for lipids in-vivo is far from a simple aqueous solution. In fact, it has been suggested [2,4] that DES might form within the crowded cellular environment and could assist in solubilizing biological species in an intermediate environment between that of the hydrophobic phospholipid tails and highly polar water-rich regions, thereby assisting survival under extreme conditions such as freezing temperatures or drought where the water content of cells is restricted. Despite the broad interest in these systems, the presence of stable phospholipid monolayers at the interface between air and an ionic solvent media has not been previously reported, to the best of the authors' knowledge.…”

Carbon and carbon composites obtained using deep eutectic solvents and aqueous dilutions thereof

“…Both the ''DES-in-water'' and the ''water-in-DES'' with low water contents -e.g., one or two moles of water per mole of DESsystems have been widely studied and agreement exists with regard to their respective natures -e.g., a simple aqueous dilution of the individual DES components in the former case 73,74 or water playing the role of a second small HBD in the latter one. [75][76][77] However, less agreement exists with regard to the nature of intermediate dilutions; this is the ''water-in-DES'' system with high water contents where the water fraction is yet too low to promote the DES rupture and dissolve its components. Interestingly, in a recent work dealing with pseudo-concentrated aqueous solutions of DES composed of resorcinol, urea, and ChCl, we demonstrated that water played the role of an additional HBD or HBA and, hence, was interstitially accommodated within the DES-based 3D H-bonded network, thus forming the ''water-in-DES'' system.…”

Influence of Hydration on the Structure and Interactions of Ethaline Deep‐Eutectic Solvent: A Spectroscopic and Computational Study

“…So, making appropriate solutions from the desired DESs and less viscous cosolvents is an easy way out. In this regard, the aqueous solutions of DESs as hybrid green systems have garnered increased attention …”