Solubility of Genistin in Ethanol/Acetone + Water and Daidzein in Ethanol + Water Co-solvent Mixtures Revisited: IBKI Preferential Solvation Method

The preferential solvation parameters (δx1,3) of genistin in ethanol/acetone (1) + water (2) and daidzein in ethanol (1) + water (2) co-solvent mixtures at elevated temperatures were derived from available solubility data using the inverse Kirkwood–Buff integral method. The values of δx1,3 varied non-linearly with the co-solvent (1) proportion in all the aqueous mixtures. For the three co-solvent mixtures, the values of δx1,3 were negative in water-rich mixtures, which indicated that daidzein or genistin was preferentially solvated by water and can act as Lewis bases to establish hydrogen bonds with the proton-donor functional groups of water (1). The same behavior was also observed for daidzein in ethanol (1) + water (2) and acetone (1) + water (2) mixtures with co-solvent-rich composition. For daidzein in ethanol (1) + water (2) mixtures with composition 0.24 < x1 < 1, and genistin in ethanol (1) + water (2) and acetone (1) + water (2) mixtures with intermediate compositions, the local mole fractions of ethanol or acetone were higher than those of the mixtures and therefore the δx1,3 values were positive, which indicated that genistin and daidzein were preferentially solvated by the co-solvent. In these regions, daidzein and genistin were acting as a Lewis acid with ethanol or acetone molecules.