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-bis2-hydroxyethyl]-2-aminoethanesulfonic acid (BES) and N-trishydroxymethyl]methyl-2-aminoethanesulfonic acid (TES) were obtained in water-methanol mixtures in which the methanol mole fraction (X_m) 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 X_m=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.