Determination of the Solubility of 8-Hydroxyquinoline in Sixteen Pure Solvents: Thermodynamic Correlation and Solvent Effect Analysis

Abstract

The solubility of 8-Hydroxyquinoline (8-HQ) was investigated in 16 pure solvents, including eight alcohols (methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, n-pentanol, isopentanol), four ketones (2-butanone, 2-pentanone, cyclopentanone, cyclohexanone), three esters (ethyl formate, methyl acetate, ethyl acetate) and acetonitrile. The stability of the crystalline form during dissolution was confirmed by X-ray powder diffraction (XRPD), and the melting properties were characterized via differential scanning calorimetry (DSC). Five thermodynamic models, namely the modified Apelblat equation, the λh model, the nonrandom two-liquid (NRTL) model, the Wilson model, and the Yaws model, were employed to correlate the experimental solubility data. The fitting performance of each model was assessed using the coefficient of determination (R2), relative average deviation (RAD), and root-mean-square deviation (RMSD). The results demonstrate that the molar solubility of 8-HQ increases monotonically with temperature, and all five models provide satisfactory correlation of the experimental data. Furthermore, mixing thermodynamic properties (Gibbs free energy, enthalpy, and entropy) calculated from Wilson model parameters reveal that mixing between 8-HQ and every solvent is spontaneous and entropy-driven. This work provides a comprehensive solubility database for 8-HQ, offering essential thermodynamic evidence and model support for the design of high-yield crystallization processes.