Solubility of Toltrazuril and the Effect of a Cosolvent

The purpose of this work is to study the dissolution process of toltrazuril and the influence of the solvent on solubility. In pure solvents, the solubility of toltrazuril increases with the increasing temperature, and the order of solubility data is as follows: ethyl acetate (9.640 × 10–3, 298.15 K) > dichloromethane (6.103 × 10–3, 298.15 K) > toluene (3.132 × 10–3, 298.15 K) > ethanol (1.567 × 10–3, 298.15 K) > isopropyl alcohol (0.9795 × 10–3, 298.15 K) > water (0.01929 × 10–3, 298.15 K), meanwhile in binary mixtures, the solubility increases to a maximum with the increasing temperature and isopropyl alcohol, and then decrease with further increase of cosolvent. However, in the isopropyl alcohol + ethyl acetate mixed solvents, the data increase with the increasing ethyl acetate. Four thermodynamic models are used to discuss the dependence of toltrazuril solubility on temperature and solvent composition. The largest value of relative average deviation and (RAD) and root-mean-square deviation (RMSD) between the calculated and experiment value were no larger than 4.5 × 10–2 and 4.1 × 10–4, respectively. The results indicate that these models were all acceptable to express the dissolution process of toltrazuril in the studied solvents. The method of multiple linear regression analysis (MLRA) involving selected solvent parameters can explain the effect of solvents on solubility of toltrazuril in pure solvents.