Structure-solubility relationships and thermodynamic aspects of solubility of some flavonoids in the solvents modeling biological media

Flavonoids play an important role for our health. Solubility is one of the most important physicochemical properties of bioactive compounds. In order to get more information about physicochemical properties of flavonoids, this paper carried out three aspects of research focused on solubility as following: the relationship between structure and solubility of flavonoid aglycones in water and 1-octanol, the thermodynamic aspects of solubility processes and the thermodynamic functions of transfer processes of the investigated compounds from water to 1-octanol. The solubility of 13 flavonoid aglycones was determined using the isothermal saturation method. The changes of thermodynamic functions were calculated using the Van't Hoff equation and analyzed by a diagram method. The results indicated that an OH or OCH3 substituent on ring B, the ring B connection position, the bond order between C2 and C3 (C2C3 or C2C3), and an OH substituent at 3 position could influence the solubility of flavonoids. The modified Apelblat equation fitted well the experimental solubility data in water and 1-octanol. Dissolution processes of studied compounds in water are mainly enthalpy determined. In 1-octanol, the dissolution processes of chrysin, apigenin, kampferol and morin hydrate are entropy-driven process, while other compounds are enthalpy determined. The transfer processes of the investigated compounds from water to 1-octanol are spontaneous. In this case, the solubility in water is one of the most important factors influencing driving force of the transfer process. These results may provide a better understanding of biopharmaceutical properties of flavonoids and might be important for further studies on the formulation development of flavonoids.