Calorimetric determination of enthalpy changes for the proton ionization of glycine, N,N-bis(2-hyroxyethyl)glycine (“bicine”) and N-tris(hydroxymethyl)methylglycine (“tricine”) in water–methanol mixtures

Enthalpies for the two proton ionizations of glycine, N,N-bis(2-hyroxyethyl)glycine (“bicine”) and N-tris(hydroxymethyl)methylglycine (“tricine”) were obtained in water–methanol mixtures with methanol mole fraction (Xm) from 0 to 0.360. With increasing methanol the ionization enthalpy for the first proton (ΔH1) of glycine increased from 4.4 to 9.4 kJ mol−1 with a minimum of 4.1 kJ mol−1 at Xm = 0.059. The ionization enthalpy of the second proton (ΔH2) for glycine decreased from 46.3 to 38.1 kJ mol−1. ΔH1 of bicine increased from 3.5 to 7.6 kJ mol−1 at Xm = 0.273 before dropping to 4.1 kJ mol−1 at Xm = 0.360. ΔH2 of bicine increased from 24.9 to 29.4 kJ mol−1. For tricine, ΔH1 increased from 6.7 to 9.8 kJ mol−1 at Xm = 0.194 then dropped to 7.4 kJ mol−1 at Xm = 0.360. ΔH2 for tricine first dropped from 30.8 to 28.5 kJ mol−1 at Xm = 0.059 before increasing to 33.3 kJ mol−1 at Xm = 0.273. The solvent composition was selected so as to include the region of maximum structure enhancement of water by methanol. The results were interpreted in terms of solvent–solvent and solvent–solute interactions.