Thermodynamics of ionization processes for hydroxy-substitute benzoic acids in water—dimethylsulfoxide mixtures at 25°C

The ionization and solution enthalpies of the hydroxybenzoic acids were measured calorimetrically at 25°C in water—dimethylsulfoxide mixtures ranging from pure water to 0.8 DMSO mole fraction.In the same solution, the ΔG° values for the ionization processes have been determined by potentiometric measurements.The different enthalpy solvation values for the three hydroxy isomers were explained taking into account the prevalence of the resonance or the inductive effect as a function of the solvent composition.The intramolecular hydrogen bond of the undissociated molecules seems to be responsible for the greater degree of ionization of o-hydroxybenzoic acid with respect to the m and p-hydroxy isomers.