Do Deep Eutectic Solvents Form Uniform Mixtures Beyond Molecular Microheterogeneities?

Area of Science:

• Physical Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Deep eutectic solvents (DESs) are a versatile class of solvents with tunable properties.
• Understanding the molecular organization and phase behavior of DESs is crucial for their application.
• Concentration fluctuations and long-range order influence the macroscopic properties of DESs.

Purpose of the Study:

• To investigate the structure and molecular correlations in different types of deep eutectic solvents (DESs) and their aqueous solutions.
• To elucidate the role of solvent composition and interactions on mesoscopic segregation and mixing.
• To utilize isotopic labeling and small-angle neutron scattering (SANS) for detailed structural analysis.

Main Methods:

• Small-angle neutron scattering (SANS) was employed over a momentum transfer range of 0.05 < Q < 0.5 Å⁻¹.
• Isotopic labeling strategies were used to selectively probe spatial correlations between different solvent components.
• Analysis included examining low-Q peaks and partial structure factors to understand mesoscopic organization.

Main Results:

• Ethaline (choline chloride/ethylene glycol) DESs and aqueous solutions showed mesoscopic segregation of excess or water-substituted ethylene glycol.
• Nonionic hydrophilic DESs (glycerol/lactic acid) exhibited homogeneous mixing due to balanced interactions.
• Hydrophobic DESs (menthol/decanoic acid) displayed characteristics of non-interacting spherical domains, with no evidence of large-scale demixing.

Conclusions:

• The molecular interactions and stoichiometry significantly dictate the mixing behavior and structural organization in DESs.
• Ethaline DESs can exhibit segregation of non-solvating ethylene glycol, particularly at higher dilutions or excesses.
• Nonionic and hydrophobic DESs demonstrate distinct phase behaviors, highlighting the diversity in DES structural properties.