Phase equilibrium phenomena in solutions involving tannins, flavonoids and ionic liquids

This work presents a foremost systematic study on the solubility of phenolic compounds (tannic acid, quercetin and gallic acid) in ionic liquids. A series of 1-butyl-3-methylimiazolium salts of hexafluorophosphate, tetrafluoroborate, trifluoromethanesulfonate and bis(trifluoromethylsulfonyl)amide derivatives were used in this research. The solid-liquid equilibrium measurements in binary systems were carried out by using a dynamic method from 273 K to 413 K. Among all tested ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate was found to be the best solvent for tannic acid. The solubility of tannic acid was lower for [bmim][PF6] and decreased for [bmim][OTf] and [bmim][NTf2]. The solubility of tannic acid in ionic liquids follows the E-T(N) polarity scale of the studied ionic liquids. Furthermore, tetrafluoroborate and hexafluorophosphate ionic liquids confirmed their ability to form hydrofluoric acid at elevated temperatures and in the presence of water traces. Differential scanning calorimetry was used to determine the melting point, the enthalpy of melting and the temperature and enthalpies of the solid-solid phase transitions, for the first time for the investigated valuable phenolic compounds. The obtained results accompanied by the solubility of carbohydrates in ionic liquids can serve for a design of the future extraction of value added compounds from raw biomass.