Temperature and solvent dependent thermodynamic behavior of sulfathiazole

The mole fraction solubility of form II of sulfathiazole (STZ) in two binary solvents methanol + acetonitrile and water + acetonitrile was determined by using the gravimetric method under atmospheric pressure at temperatures ranging from 283.15 K to 323.15 K and 288.15 K to 323.15 K, respectively. The results indicated that solubility of form II of STZ increases with increasing temperature. The solubility of STZ reaches maximum when mole fraction ratio of methanol reaches 0.56. In the acetonitrile-water system, the solvent composition corresponding to the maximum point of STZ solubility changes from 0.64 to about 0.50 with the change of temperature. The solvation free energy suggests that the solubility of form II of STZ increases with solute-solvent interaction strength increasing and there exists the strongest solvation free energy at co-solvency composition in binary solvents. Besides, radial distribution function (RDF) shows the tendency to aggregation of STZ and methanol molecules as the molar fraction of methanol in binary solvent increases, which also reaches maximum corresponding the maximum solubility point, implying that the interaction between solute and solvent molecules dominate the co-solvency phenomenon. Furthermore, different thermodynamic model were employed to analyze the dissolution behavior of form II of STZ in binary solvents. In addition, the Van't Hoff model was used to calculate the thermodynamic properties in the binary solvent systems which implies that the dissolution process is endothermic and entropy-driven process.