Choline Acetate/Water Mixtures: Physicochemical Properties and Structural Organization

Area of Science:

• Green Chemistry
• Materials Science
• Physical Chemistry

Background:

• Deep Eutectic Solvents (DESs) offer sustainable, biodegradable, and tunable alternatives to conventional organic solvents.
• Water-based DESs (wb-DESs) utilize water as a key component, enhancing their green credentials.

Purpose of the Study:

• To experimentally characterize choline acetate/water mixtures (wb-DESs) across various molar ratios.
• To investigate the physicochemical properties and structural dynamics of these wb-DESs.
• To clarify the solvation structure and challenge existing models of aggregation in wb-DESs.

Main Methods:

• Differential Scanning Calorimetry (DSC) for thermal transitions.
• Physicochemical measurements (density, viscosity, conductivity, refractive index).
• Nuclear Magnetic Resonance (NMR) spectroscopy for dynamics and solvation.
• Small and Wide-Angle X-ray Scattering (S-WAXS) and ab initio molecular dynamics (AIMD) simulations.

Main Results:

• Choline acetate/water mixtures were classified as Low Transition-Temperature Mixtures (LTTMs) with glass transitions between 150-180 K.
• NMR data indicated a strong hydrogen-bonding network at a water:choline acetate molar ratio of n=2.
• S-WAXS, AIMD, and NMR diffusion studies refuted claims of aggregation, revealing a stable, cooperative hydrogen-bonding network stabilizing the choline cation.

Conclusions:

• Integrated physicochemical and computational studies are crucial for understanding and developing sustainable solvent systems like wb-DESs.
• The findings provide a clearer picture of the structure and dynamics in choline acetate/water mixtures.
• This research contributes to the rational design of novel, eco-friendly solvent systems.