| Summary: | This article studies the solubility, Hansen solubility parameters (HSPs), and thermodynamic behavior of a naturally-derived bioactive thymoquinone (TQ) in different binary combinations of isopropanol (IPA) and water (H<sub>2</sub>O). The mole fraction solubilities (<i>x</i><sub>3</sub>) of TQ in various (IPA + H<sub>2</sub>O) compositions are measured at 298.2–318.2 K and 0.1 MPa. The HSPs of TQ, neat IPA, neat H<sub>2</sub>O, and binary (IPA + H<sub>2</sub>O) compositions free of TQ are also determined. The <i>x</i><sub>3</sub> data of TQ are regressed by van’t Hoff, Apelblat, Yalkowsky–Roseman, Buchowski–Ksiazczak <i>λh</i>, Jouyban–Acree, and Jouyban–Acree–van’t Hoff models. The maximum and minimum <i>x</i><sub>3</sub> values of TQ are recorded in neat IPA (7.63 × 10<sup>−2</sup> at 318.2 K) and neat H<sub>2</sub>O (8.25 × 10<sup>−5</sup> at 298.2 K), respectively. The solubility of TQ is recorded as increasing with the rise in temperature and IPA mass fraction in all (IPA + H<sub>2</sub>O) mixtures, including pure IPA and pure H<sub>2</sub>O. The HSP of TQ is similar to that of pure IPA, suggesting the great potential of IPA in TQ solubilization. The maximum molecular solute-solvent interactions are found in TQ-IPA compared to TQ-H<sub>2</sub>O. A thermodynamic study indicates an endothermic and entropy-driven dissolution of TQ in all (IPA + H<sub>2</sub>O) mixtures, including pure IPA and pure H<sub>2</sub>O.
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