The Effect of Water upon Deep Eutectic Solvent Nanostructure: An Unusual Transition from Ionic Mixture to Aqueous...

Area of Science:

• Physical Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Deep eutectic solvents (DES) are tunable solvents with unique properties.
• Understanding the structural behavior of DES with varying water content is crucial for their application.
• Choline chloride/urea/water mixtures are common DES systems.

Purpose of the Study:

• To characterize the nanostructure of choline chloride/urea/water DES across a wide hydration range.
• To determine the critical water content at which DES nanostructure is disrupted.
• To elucidate the role of water-solute interactions in DES structural stability.

Main Methods:

• Neutron total scattering.
• Empirical Potential Structure Refinement (EPSR) modeling.
• Analysis of DES nanostructure as a function of water content (wt% and mol%).

Main Results:

• DES nanostructure is retained up to approximately 42 wt% H2O due to solvophobic sequestration of water around cholinium cations.
• At 51 wt% (83 mol%) H2O, DES nanostructure disruption occurs.
• Above 51 wt% H2O, water-water and DES-water interactions dominate, leading to an aqueous solution behavior.

Conclusions:

• The nanostructure of choline chloride/urea/water DES is remarkably stable at moderate hydration levels.
• Water sequestration into specific domains preserves the DES structure.
• High hydration levels transform the DES into a simple aqueous solution of its components.