Application of CO2 expanded liquids as green solvents for the extraction of antioxidants

Abstract: Antioxidants are compounds present in plant, fruit, and vegetables, and they are known for their medicinal benefits such as anti-carcinogenic and anti-fungicidal activity. Most commonly used methods for extractions of such compounds are lengthy due to mass transfer limitations. This study proposes the use of carbon-dioxide expanded liquids (CXLs) as solvent to improve these limitations. Such solvents, considered as green solvents because of its inertness, odourless and low toxic properties, present tunable physical properties, such as low viscosity and high diffusivity rates. The solubility of CO2 in ethanol determines the volume expansion and, depends on temperature and pressure. Hence, the acquisition of solubility data of analyte in CXLs and the density of such mixtures will be valuable for an optimum extraction of antioxidant from complex solid samples. Aiming to study these properties, a system able to perform measurement of solubility and phase equilibria was developed. The equipment consists of a high pressure view cell connected in-line to a densitometer and a supercritical fluid chromatograph with a photodiode array detector. Validation of such equipment was performed with curcumin, a polyphenol extracted from Turmeric (Curcuma longa L.), 200 bar and 40 ºC in 92 % CO2 and 8 % ethanol, giving a solubility of 0.37 x 102 (molar fraction) , while in literature the observed value is 0.41 x 102 (molar fraction). Solubility measurements were successfully determined for curcumin at 200 bar, 35 ºC and at different content of ethanol in the mixture– from 50 to 90 %. Densities for such mixtures were also studied. Results have shown that the solubility of curcumin increases with the amount of ethanol as expected. Another polyphenol studies is quercetin, which it was found to be higher in solubility than curcumin. In general terms, the developed equipment has shown to be efficient for the established purpose. Further investigations are needed to confirm the range of applicability.