Natural Carboxylic Acid-Based Deep Eutectic Solvents: Physi-Cochemical Properties, Biological Activities and Application in Flavonoids Extraction from Walnut Green Peel

The development of novel green solvents has become a prominent research focus in the field of green chemistry. Deep eutectic solvents (DESs) have emerged as a promising class of sustainable solvents, offering distinct advantages in terms of cost-effectiveness, tunability, and environmental compatibility. This study presents a systematic investi-gation of key physicochemical properties, including water content, polarity, conductiv-ity, pH, and viscosity, for ten carboxylic acid-based DESs (CADESs). Furthermore, the antimicrobial efficacy of the investigated CADESs was assessed against representative Gram-positive (Staphylococcus aureus and Enterococcus faecalis) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacterial strains. The investigations reveal that CADESs possess broad-spectrum antibacterial activity. The antioxidant properties of the CADESs were systematically evaluated using ABTS and DPPH scavenging assays, and Fe2+ chelating activity. Concurrently, phytotoxicity was assessed through germina-tion and early seedling growth tests in mung beans (Vigna radiata). Finally, CADESs were used to extract flavonoids from walnut peel. The extraction yield of the Choline chloride/levulinic acid system (containing 30% water) was the highest, reaching 112.8 mg RE·g-1 DW. The superior physicochemical properties of CADESs position them as promising green solvents for flavonoids extraction, demonstrating significant potential for widespread application in sustainable technologies.