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

Area of Science:

• Materials Science
• Physical Chemistry
• Chemical Engineering

Background:

• The nanostructure and phase transitions of deep eutectic solvents (DES) remain poorly understood.
• The behavior of DES-water mixtures is debated, with uncertainty regarding the dominance of the eutectic pair over aqueous properties.

Purpose of the Study:

• To elucidate the molecular structure of choline chloride-malic acid (malicine) in liquid, solid, and hydrated states.
• To clarify the nature of the liquid-solid phase transition in DES.
• To understand the role of water in hydrated DES.

Main Methods:

• Neutron total scattering on D/H isotope-substituted samples.
• Quasi-elastic neutron scattering (QENS).
• Empirical potential structure refinement.

Main Results:

• Evidence of a stoichiometric complex ion cluster in liquid malicine with strong choline-chloride bonding.
• Minimal structural difference between liquid and solid malicine, indicating a glass transition.
• Water acts as a secondary hydrogen bond donor and forms aggregates in hydrated DES, largely retaining the DES structure.

Conclusions:

• Malicine's solidification is a glass transition, not a first-order phase change, with distinct nanostructure in liquid and solid states.
• The DES structure is largely preserved upon hydration, with water playing a specific role.
• Understanding DES nanostructure is crucial for applications in green chemistry and plant physiology.