The solubility data of diphenoxylate in pure solvents of water, methanol, ethanol, acetone, acetonitrile, 2-propanol were measured within the temperature range of 293.15−318.15 K by a static analytical method under atmospheric pressure. Several commonly used thermodynamic models, including the ideal solution model, the modified Apelblat equation and the Buchowski-Ksiazaczak λh equation, were applied to correlate the experimental solubility data. The experimental results show that the solubility increases with the increasing temperature, and the highest solubility is in methanol, the lowest in pure water. Results of these measurements were well-correlated with the three equations above, but the modified Apelblat equation is more suitable in regressing the solubility data of diphenoxylate. Through the Van't Hoff equation, the enthalpy and the entropy in different solvents can be calculated.

Trimethoprim (TMP) is an important antibacterial synergist in human and veterinary medicine. In this work, the solubilities of TMP in five pure solvents (ethanol, water, ethyl acetate, acetonitrile, and 2-propanol) and two binary mixtures (ethanol + water and 2-propanol + water) were measured by an equilibrium method over the temperature range from (293.15 to 322.65) K under atmospheric pressure. The experimental results show that in the pure solvents the solubility of TMP increases with increasing temperature, and the solubility decreases in the following order: ethanol > acetonitrile > 2-propanol > ethyl acetate > water. In the mixed solvents, the solubility of TMP increases with increasing temperature and mass fraction of the organic solvent. The mole-fraction solubility of TMP was observed to be the highest in the mixed solvent 50% ethanol/2-propanol, and the lowest mole-fraction solubility of TMP was found in water. Three correlating models (the modified Apelblat equation, the λh equation, and the ideal model) were used to correlate the experimental solubility values in the pure solvents, and four models (the modified Apelblat equation, the λh equation, the ideal model, and the CNIBS/R-K model) were applied to correlate the experimental solubilities in the mixed solvents. All of these thermodynamic models gave satisfactory correlation results, with the modified Apelblat equation showing better fitting degree than the other three equations. Furthermore, the standard molar enthalpy of the TMP during the dissolving process (ΔH sol °) was also determined in this work, and the results show that the dissolution process is endothermic.

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