Buffer interactions: Solubilities and transfer free energies of TRIS, TAPS, TAPSO, and TABS from water to aqueous ethanol solutions

Tris(hydroxymethyl)aminomethane (TRIS), N-[tris(hydroxymethyl)methyl]-3-aminopropanesulfonic acid (TAPS), N-[tris(hydroxymethyl)methyl]-3-amino-2-hydroxypropane sulfonic acid (TAPSO), and N-tris[hydroxymethyl]-4-amino-butanesulfonic acid (TABS), are useful for pH control as standard buffers in the physiological region of pH 7.0–9.5 for TRIS, 7.7–9.1 for TAPS, 7.0–8.2 for TAPSO, and 8.2–9.5 for TABS, respectively. These buffers are structurally related and contain at least TRIS groups. Densities of these buffers in binary ethanol + water solvent mixtures have been measured by a high precision vibrating tube digital densitometer at 298.15 K under atmospheric pressure. From these data, the solubilities of these buffers in water + ethanol mixed solvents have been determined at 298.15 K. The solubility data were further used to calculate the apparent transfer free energy (ΔG′tr) of these buffers from water to a mixed solvent of ethanol and water at 298.15 K. The contributions of the transfer free energies from various functional groups were estimated from (ΔG′tr) of the mixtures containing the buffers with different molecular structures.