Beyond the blend: Unveiling the thermophysical fingerprints of hydrated choline chloride deep eutectic systems with bio-derived and synthetic hydrogen bond donors

Abstract

This study presents a comprehensive thermophysical characterization of hydrated deep eutectic solvents (DESs) composed of choline chloride (ChCl) and four hydrogen bond donors (HBDs): citric acid, malic acid, fructose, and ethylene glycol in equimolar ratios. By introducing 2, 10, and 22 wt% water—spanning key hydration regimes where DESs structure is progressively altered—we systematically quantify the effects of hydration on density, viscosity, electrical conductivity, thermal conductivity, and refractive index over a wide temperature range. Results demonstrate that water addition leads to a dramatic reduction in viscosity, particularly for bio-derived HBDs, enhancing processability and enabling practical applications. The ChCl:citric acid DESs maintains high structural cohesion upon hydration, reflected in persistent cooperative dynamics and high activation energy, whereas the synthetic ethylene glycol system exhibits predictable, tunable behavior, ideal for engineered fluid systems. Electrical conductivity increases non-linearly with water content, accompanied by a transition from fragile to strong liquid behavior. Derived parameters—molecular volume, thermal expansion coefficient, and excess molar volumes—reveal non-ideal mixing behavior and structural reorganization. Our findings define structure–property correlations critical for optimizing DESs formulations, offering a foundation for application-specific solvent engineering in energy, electrochemistry, and separation technologies.