Calorimetric determination of enthalpy changes for the proton ionization of 3-[N-morpholino]propanesulfonic acid (MOPS), 4-[N-morpholino]butanesulfonic acid (MOBS) and 3-[N-morpholino]-2-hydroxypropanesulfonic acid (MOPSO) in water–methanol mixtures

Proton ionization enthalpies for three structurally related biological buffers, each with two ionizable protons, were determined using solution calorimetry. These buffers are 3-[N-morpholino]propanesulfonic acid (MOPS), 4-[N-morpholino]butanesulfonic acid (MOBS) and 3-[N-morpholino]-2-hydroxypropanesulfonic acid (MOPSO). Enthalpies were obtained in water–methanol mixtures with methanol mole fraction (Xm) from 0 to 0.360. The first ionization enthalpy (ΔH1) of all buffers was small (±1.2 kJ mol−1) at all solvent compositions. The second ionization enthalpy (ΔH2) increased steadily with Xm, rising from 20.7 to 25.6 kJ mol−1 for MOPS, 26.3–31.0 kJ mol−1 for MOBS and 23.5–32.2 kJ mol−1 for MOPSO. The results were compared to those for related biological buffers. The observed variations were interpreted in terms of solvent–solvent and solvent–solute interactions.