Thermodynamics of the second-stage dissociation of 2-[N-(2-hydroxyethyl)-N-methylaminomethyl]-propenoic acid (HEMPA) in water at different ionic strength and different solvent mixtures

The second stage dissociation constant pK2 of 2-[N-(2-hydroxyethyl)-N-methylaminomethyl]-propenoic acid (HEMPA) has been determined in aqueous solution at different ionic strengths and different temperatures, using pH-metric technique. The thermodynamic quantities (ΔG∘, ΔH∘, and ΔS∘) have been studied and discussed. Evaluation of the effect of organic solvent of the medium on the dissociation processes have also been reported and discussed. The organic solvents used were methanol, dimethylformamide (DMF), dimethylsulfoxide (DMSO), acetone and dioxane. The pK2 for the ionization in water +10, +20, +30, +40 and +50 wt% dioxane has been determined at five different temperatures from T = (288.15 to 308.15) K at intervals of 5 K. The thermodynamic quantities were calculated. The implications of the results with regard to specific (solute + solvent) interactions (particularly stabilization of zwitterionic species) are also discussed.