The refractive index (RI) is an important physiochemical property of deep eutectic solvents (DESs) for application in the optical identification of specific substances or measuring the concentration of solutes in solutions. However, the available data on the RI of DESs is limited. Here, a systematic investigation on the RI of 48 typical DESs and 30 mixtures with water was conducted under atmospheric pressure. The effect of temperature in the range 293.15-338.15 K, hydrogen-bonding donors (HBDs), and hydrogen-bonding acceptors (HBAs) on RI was investigated. Furthermore, the RI of DESs as a function of mass percentage in the range of 20-80% water was also studied. It was found that the RI of DESs and its aqueous binary mixtures decreases linearly with the increase of temperature. HBDs and HBAs had a significant influence on the RI of DESs. Among them, the RI of choline chloride (ChCl)/phenol and ChCl/o-cresol were obviously higher than those of other DESs. It was also found that the addition of water would decrease the RI of DESs, and the RI of DES content in percentage (wt %) of water binary mixtures increases linearly as a function of mass percentage of DESs for 20 DESs. However, for the other 10 DESs, there is no linear relationship between the RI and the DES content.

There are billions of tea drinkers around the world. However, the optimized tea-brewing temperature and time conditions for achieving a higher concentration of antioxidants in tea drinks have not been thoroughly studied. Finding out the optimized brewing conditions can benefit tea drinkers significantly. In this work, we have studied ten antioxidants from seven different popular green, Oolong, black, and scented teas using hot water extraction followed by HPLC analysis. The antioxidant yield was evaluated at 25–100 °C with 5 to 720 min of brewing time. Our results show that the extraction efficiency was enhanced by increasing the water temperature and the highest yield of antioxidants was achieved at 100 °C. The antioxidant yield increased with prolonged brewing time. However, the degradation of antioxidants occurred when tea leaves were extracted for 120 to 720 min. Caffeine was found in all seven tea samples. At 100 °C, the caffein concentration in the tea extract ranged from 7.04 to 20.4 mg/g in Rizhao green tea. Longjing green tea contained the highest concentration of antioxidants (88 mg/g) in the 100 °C extract. Epigallocatechin and caffeine were the most abundant compounds found in all tea samples studied, ranging from 4.77 to 26.88 mg/g. The antioxidant yield was enhanced by increasing the extraction time to up to 60–120 min for all ten compounds studied.

Data on solid + liquid equilibrium of rhein in solvents can provide essential support for the design of extraction, precipitation, or crystallization processes in industry. In this study the solubility of rhein in eight different solvents namely water, 1,2-dichloroethane, glycol, ethanol, methanol, ethyl acetate, 1-propanol, and 1-butanol was measured over a temperature range from 288.15 to 313.15 K under atmospheric pressure using a UV spectrophotometric method. The experimental result shows that 1-butanol, 1-propanol and ethyl acetate could be suitable solvents for the industrial production of rhein. Two commonly used thermodynamic equations, including the modified Apelblat equation, and λh equation were applied to correlate the experimental solubility data. It was found that the two equations can satisfactorily correlate the solubilities of rhein in the eight solvents under different temperatures. The dissolution enthalpy and entropy were obtained from the experimental values. The dissolution process of rhein in eight solvents within the studied temperature range was endothermic, and the driving mechanism is the entropy.

As an alternative to organic solvents, subcritical water was employed for the first time as an effective solvent for the extraction of isoflavones from Puerariae lobata. Optimum experimental conditions for the extraction of the four main isoflavones were established by single factor experiments, and the optimum experimental conditions for total isoflavone extraction were established further by response surface methodology. With an extraction time of 45 min and a liquid/solid ratio of 1 : 20, the extraction yields of puerarin, 3′-methoxypuerarin, and daidzin reached maxima at extraction temperatures of 120 °C, 140 °C and 200 °C, respectively. Moreover, puerarin, 3′-methoxypuerarin and daidzin were degraded and produced various byproducts due to hydrothermal reactions at higher temperatures. The maximum extraction yields of the total isoflavones were obtained by response surface methodology (extraction time of 45 min, solid/liquid ratio of 1 : 15 and extraction temperature of 120 °C). Compared to conventional solvents, subcritical water utilized less solvent and required a shorter extraction time.