Solid–Liquid Equilibrium of 5-Chloro-8-hydroxyquinoline and 5,7-Dichloro-8-hydroxyquinoline in Different Solvents and Mixing Properties of Solutions

The solubility values of 5-chloro-8-hydroxyquinoline and 5,7-dichloro-8-hydroxyquinoline in six pure solvents at temperatures ranging from 283.15 to 323.15 K was determined by the isothermal saturation method. In the investigated temperature range, the solubility values of 5-chloro-8-hydroxyquinoline are larger than those of 5,7-dichloro-8-hydroxyquinoline at all studied temperatures. They decrease according to the sequence N-methyl-2-pyrrolidone > N,N-dimethylformamide > 1,4-dioxane > ethyl acetate > toluene > acetonitrile. The determined solubility data were correlated using the modified Apelblat equation, Buchowski–Ksiazaczak λh equation, and the NRTL model. The results of the correlation indicated that the calculated and experimental values agreed very well. The maximum values of the root-mean-square deviation and the relative average deviation were 14.88 × 10–4 and 1.46% for 5-chloro-8-hydroxyquinoline and 2.53 × 10–4 and 2.56% for 5,7-dichloro-8-hydroxyquinoline, respectively. The calculation results of the thermodynamic properties show that the process of dissolving is apparently not only spontaneous but also entropy-driving.