Solubility Determination and Thermodynamic Model Analysis of Esculetin in Different Solvents from 273.15 to 318.15 K

Esculetin is derived from the leaves of lemons of Rutaceae and the bark of bittersweet ash, belladonna, mandala, and Rehmannia plants. In this research, the solubility of esculetin in ten solvents such as ethyl acetate, isobutyl alcohol, water, methanol, acetonitrile, n-propyl alcohol, isopropyl alcohol, n-butanol, ethanol, and acetone was determined by utilizing static equilibrium-high performance liquid chromatography (HPLC) under standard atmospheric pressure conditions and temperatures in the range of 273.15 to 318.15 K. Among the above ten pure solvents, water displayed the least solubility, while methanol exhibited the highest solubility. At the same temperature (T = 273.15 ∼ 318.15 K), the solubility of esculetin increased with the increase of the molar ratio of soluble solvents. Similarly, when the temperature increases (T = 273.15 ∼ 318.15 K), the solubility of the three binary solvents increases under the condition of a constant molar ratio. The above results suggest that the main factor affecting the solubility of esculetin in the solvent may be the polarization/bipolarity of the solvent. The experimental data was fitted by five thermodynamic models (the Buchowski–Ksiazaczak λh model, the Modified Apelblat model, the Jouyban–Acree model, the SUN model, and the CNIBS/R-K model), and the relative average deviation and root-mean-square deviation of the data were calculated, which proved that the correlation of the solubility data and the five models was relatively good. XRD and DSC were used to detect the crystal form and stability of esculetin during the experiment.