Effect of Water Content on Physicochemical Properties and Extraction Capacity of Hydrophilic Eutectic Solvents

Deep eutectic solvents (DESs) are increasingly used for the extraction of biologically active compounds because of their high solubilizing ability, biodegradability, and low cost. Water is usually added to hydrophilic DESs to reduce their viscosity and improve extraction efficiency, although excessive dilution may disrupt their molecular structure. In this study, the physicochemical properties of hydrophilic DESs based on choline chloride and including amides, acids, sugars, and alcohols were investigated at water contents ranging from 10 to 70 wt %. It was found that all of the studied systems remain stable under ultrasonic treatment at 80°C for 30 min. Infrared Fourier-transform spectroscopy revealed a correlation between the water content and the extent of hydrogen bonding in DESs. Using Calluna vulgaris as a model matrix, it was shown that the extractivity of biologically active compounds strongly depends on the proportion of water in the solvent. The optimal range of 20–30 wt % water ensured the highest recoveries of phenolic acids, flavonoids, and alkaloids. These findings provide a basis for the rational design of DES compositions for the extraction of plant metabolites and can be applied to scaling up green extraction technologies.