Enthalpy of Fusion

The solubility of Sulfanilamide in binary mixtures comprising water and five organic solvents was studied at different temperatures using a spectrophotometric method. Additionally, the differential scanning calorimetry (DSC) measurements were performed to characterize the non-dissolved portion of Sulfanilamide obtained after shake-flask experiments. The Buchowski-Ksiazczak solubility model was applied to interpret the obtained experimental results. The model was successful in reproducing experimental data, as it was evidenced by several statistical measures, including the Akaike Information Criterion. The obtained parameters of the λh-equation were used for calculating the values of enthalpy of mixing. It was observed that the minimum value of enthalpy of mixing for each system is associated with those binary solvent compositions which result in the highest solubility of Sulfanilamide. Also the maximum value of the λ parameter corresponds to the composition of the mixture yielding the highest solubility. The application of COSMO-RS methodology revealed the sensitivity of solute activities and solvation Gibbs free energies to solvent-cosolvent composition. Also, there was noticed a strong correlation of these thermodynamic properties. It happened that low values of Sulfanilamide activities and their monotonous variations with the composition of considered binary solutions are associated with monotonous solubility profiles and also the reverse conclusion holds. Besides, differences in solvation of monomers and intermolecular complexes are the very probable origins of the observed trends in excess enthalpies of saturated solutions. The advanced quantum chemistry computations, including electron correlation and extended basis set, were used for quantification of affinities of systems components toward pairs formation. From the obtained values of activity equilibrium constants it was concluded that the highest affinity in all studied conditions should be attributed to Sulfanilamide dimers. It was also observed that pairs formed by Sulfanilamide with water are more thermodynamically stable than the ones with cosolvent molecules with only one exception, namely 1,4-dioxane.