Calorimetric determination of enthalpies for the proton ionization of N,N-bis[2-hydroxyethyl]-2-aminoethanesulfonic acid (BES) and N-tris[hydroxymethyl]methyl-2-aminoethanesulfonic acid (TES) in water–methanol mixtures

The enthalpies of proton ionization of the biochemical buffers N,N-bis[2-hydroxyethyl]-2-aminoethanesulfonic acid (BES) and N-tris[hydroxymethyl]methyl-2-aminoethanesulfonic acid (TES) were obtained in water–methanol mixtures in which the methanol mole fraction (Xm) varied in the range 0–0.36. For both buffers, ionization enthalpy for the first proton (ΔH1) was small in all solvent media. However, upon addition of methanol, ΔH2 increased steadily from 22.2 to a maximum of 27.2 kJ mol−1 for BES, whereas for TES it varied from 30.0 to 32.4, with a minimum of 28.6 kJ mol−1 at Xm=0.123. It is noteworthy that this solvent composition lies within the region of maximum structure enhancement of water by methanol. The results were interpreted in terms of methanol–water interactions.