Thermodynamics of ionization processes for chloro-substituted benzoic acids in water—dimethysulfoxide mixtures at 25°C

The ionization enthalpies of o-, m-, and p-chloro-benzoic acid were measured calorimetrically at 25°C in water—DMSO mixtures ranging from pure water up to 0.8 DMSO mole fraction. In the same mole fraction range, molar free energies and entropies were calculated for the ionization process.The different results obtained for the three acids were explained by taking into account the different effects displayed on the solvation and ionization phenomena by the chlorine atom in the various positions on the aromativ ring.Using the transfer enthalpy of benzoic acid in the same mixtures as a reference point, a solvation sequence is proposed both for the ions and undissociated molecules.