Natural Carboxylic Acid Deep Eutectic Solvents: Properties, Bioactivities and Walnut Green Peel Flavonoid Extraction

Novel green solvents are a key focus in green chemistry, and deep eutectic solvents (DESs) are promising sustainable solvents. This study systematically examined the physicochemical properties (water content, polarity, conductivity, Kamlet-Taft parameters, and viscosity) of ten carboxylic acid-based DESs (CADESs). It also evaluated their antibacterial activity against representative Gram-positive (Staphylococcus aureus and Enterococcus faecalis) and Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa). The antibacterial activity of CADESs is closely related to low pH values, hydrogen bond donor properties, and the destructive effect on bacterial cell membranes. antioxidant properties via ABTS, DPPH scavenging assays and Fe2+ chelating assays, and phytotoxicity through mung bean tests. When used to extract flavonoids from walnut peel, the choline chloride/levulinic acid system with 30% water achieved the highest yield of 112.8 mg RE·g−1 DW. Therefore, CADESs show great potential as green solvents for flavonoid extraction in sustainable technologies.