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

Abstract

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 ( X m) from 0 to 0.360. With increasing methanol the ionization enthalpy for the first proton (Δ H 1) of glycine increased from 4.4 to 9.4 kJ mol −1 with a minimum of 4.1 kJ mol −1 at X m = 0.059. The ionization enthalpy of the second proton (Δ H 2) for glycine decreased from 46.3 to 38.1 kJ mol −1. Δ H 1 of bicine increased from 3.5 to 7.6 kJ mol −1 at X m = 0.273 before dropping to 4.1 kJ mol −1 at X m = 0.360. Δ H 2 of bicine increased from 24.9 to 29.4 kJ mol −1. For tricine, Δ H 1 increased from 6.7 to 9.8 kJ mol −1 at X m = 0.194 then dropped to 7.4 kJ mol −1 at X m = 0.360. Δ H 2 for tricine first dropped from 30.8 to 28.5 kJ mol −1 at X m = 0.059 before increasing to 33.3 kJ mol −1 at X m = 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.