Measurement and correlation of the solubility of kojic acid in pure and binary solvents

The solubility of kojic acid in seven pure solvents (isopropanol, n-butanol, 2-butanol, n-pentanol, acetonitrile, methyl acetate, THF) and two binary solvents (2-methoxyethanol + water, 2-ethoxyethanol + water) were determined from 278.15 K to 323.15 K under 101.3 kPa by the gravimetric method. Besides, the modified Apelblat equation, lambda h model, van't Hoff model, NRTL model, Wilson model and UNIQUAC model were selected to correlate the experimental data in pure solvents, and the modified Apelblat equation, ,1,17 model, van't Hoff model, NRTL model, CNIBS/R-K model and the modified Apelblat-Jouyban-Acree model were applied for binary solvents. The results indicated that in this work the modified Apelblat model had the highest accuracy among those semi-empirical models, while UNIQUAC model was the most representative local component model with the best fitting effect among all models. The solubility of kojic acid in all the tested pure and binary solvent systems monotonously increased with increasing of the temperature. Under the isothermal condition, the solubility value of kojic acid increased first and then decreased with the increase of the mass fraction of 2-methoxyethanol and 2-ethoxyethanol, and reached the maximum value when the mass fraction of 2-methoxyethanol and 2-ethoxyethanol were 0.9 and 0.7, respectively. The solubility parameters of solvents were determined by the improved mixing rule. According to the solubility parameters obtained, the co-solvency in the binary solvent mixtures can be explained. To further explain the observed phenomena, the molecular electrostatic potential surface (MEPS), Hirshfeld surface analysis (HS) and KAT-LSER model were applied. Furthermore, the standard thermodynamic properties of the mixtures, including Gibbs energy, entropy and enthalpy were calculated and discussed, which indicated that the dissolution of the kojic acid in all solvents was spontaneous and entropy-driven. The conductor-like screening model for real solvents (COSMO-RS) was also applied to calculate solvation free energy of kojic acid in the studied solvents. The results showed a reasonable explanation to the dissolution behavior of kojic acid. (C) 2022 Elsevier Ltd.