Polyol-based deep eutectic solvents: betaine versus choline chloride.

Area of Science:

• Physical Chemistry
• Materials Science
• Thermodynamics

Background:

• Deep eutectic solvents (DES) are tunable solvent systems with diverse applications.
• Choline chloride is a common hydrogen bond acceptor in polyol-based DES.
• Investigating alternative hydrogen bond acceptors is crucial for expanding DES applications.

Purpose of the Study:

• To evaluate betaine as a potential substitute for choline chloride in polyol-based DES.
• To determine the solid-liquid equilibrium (SLE) phase diagrams of betaine-polyol mixtures.
• To compare the phase behavior and properties of betaine-based DES with those of choline chloride-based DES.

Main Methods:

• Experimental measurement of solid-liquid equilibrium (SLE) phase diagrams for binary mixtures of betaine with various polyols (ethylene glycol, 1,3-propanediol, glycerol, meso-erythritol, xylitol, sorbitol).
• Utilizing the COSMO-RS thermodynamic model to predict phase behavior and deviations from ideality.
• Analysis of thermal degradation, boiling points, and viscosity limitations during experimental measurements.

Main Results:

• Betaine exhibited negative deviations from ideality in mixtures with polyols, while polyols showed near-ideal behavior.
• COSMO-RS successfully predicted the observed deviations and phase behavior.
• Betaine-polyol mixtures resulted in a narrower liquid-phase window compared to choline chloride systems.
• Stronger interactions between betaine and polyols than with choline chloride were inferred, leading to more negative deviations.

Conclusions:

• Betaine can form polyol-based DES, but with limitations in the liquid-phase window for room-temperature applications.
• The thermodynamic model COSMO-RS is a reliable tool for predicting the phase behavior of these systems.
• The observed differences in phase behavior are attributed to the distinct interaction strengths and enthalpies of fusion between betaine and polyols compared to choline chloride.