Assessment of Green Supramolecular Solvents and Their Application for Extraction of Natural Antioxidants from <i>Olea europaea</i> L. Leaves

Antioxidants found in olive leaves are high-added-value compounds for the pharmaceutical and food industries. This work aims to develop an efficient extraction process to recover such bioactive compounds using green, novel, and designable supramolecular solvents (SUPRAS). The supramolecular solvent/equilibrium solution (SUPRAS/EqS) systems were formed by ethanol, water, and five different amphiphiles: two organic acids (octanoic acid and decanoic acid) and three natural eutectic solvents (NAES) (octanoic acid: decanoic acid [1:1], menthol: octanoic acid [1:1], menthol: decanoic acid [1:1]). The SUPRAS/EqS systems were characterized in terms of the chemical compositions of the equilibrium phases, polarity, phase ratios, and phase diagrams of the ternary systems. Moreover, the unique microstructure of the proposed SUPRAS was demonstrated by using Scanning Electron Microscopy (SEM). Afterward, the extraction of polyphenolic antioxidants using SUPRAS/EqS was evaluated along with pure NAES, water, and ethanolic solutions for comparative purposes. The octanoic acid-based SUPRAS/EqS system achieved the highest extracted polyphenol content among all of the solvents tested. The extraction process was optimized by considering the SUPRAS/EqS composition, the SUPRAS/EqS ratio, the extraction time, the extraction method, and the solid–liquid ratio. The highest concentration of polyphenols in the extracts (68.67 mg polyphenols/g dry sample) was obtained with a SUPRAS/EqS composition of 20% (v/v) octanoic acid, 30% ethanol, and 50% water, at 40%/60% (v/v) SUPRAS/EqS ratio used in the extraction process, 1:15 sample:solvent ratio and 30 s of extraction time in vortex. Lastly, the stability of the polyphenolic extracts was evaluated under different storage conditions of temperature and light to assess the antioxidant degradation over time.