Ultrasonic-assisted hydrophobic deep eutectic solvents for the extraction of seven compounds from Piperis longi fructus-Rhei radix et rhizoma drug pair and their vitro antioxidant evaluation

Deep eutectic solvents have become an alternative to organic solvents due to their unique advantages and are widely used in the extraction of traditional Chinese medicine. In this study, a new green ultrasound-assisted extraction technique based on hydrophobic deep eutectic solvents was developed and applied to the extraction of seven compounds from Piperis longi fructus-Rhei radix et rhizoma drug pair. The key factors affecting the extraction efficiency were systematically investigated by single factor experiments: molar ratio, liquid/solid ratio, vortex time, extraction time and temperature. The parameters of the extraction process after optimization using the response surface method were molar ratio 1:2.4, extraction time 40 min, liquid/solid ratio 22:1 mL/g. In order to gain insight into the superiority of hydrophobic deep eutectic solvents in the extraction, a principal component analysis was performed. The results show that the hydrophobic deep eutectic solvent, consisting of fatty acids and long chain alkanols, has a wide range of extraction polarity and high extraction efficiency compared to methanol. The infrared spectra showed the formation of hydrogen bonds between the deep eutectic solvents, which may be the reason why hydrophobic deep eutectic solvents are potential extraction solvents. Furthermore, the extracts of hydrophobic deep eutectic solvents showed satisfactory free radical scavenging activity. The results obtained from the study proved that the method is efficient and feasible, which could help further the application of hydrophobic deep eutectic solvents for the extraction of weakly polar active compounds in Chinese medicine.