Solubility and Phase Separation of 2-(N-Morpholino)ethanesulfonic Acid (MES) and 4-(N-Morpholino)butanesulfonic Acid (MOBS) in Aqueous 1,4-Dioxane and Ethanol Solutions

2-(N-Morpholino)ethanesulfonic acid (MES) and 4-(N-morpholino)butanesulfonic acid (MOBS) are useful for pH control as standard buffers in the physiological range of 5.5 to 6.7 for MES and 6.9 to 8.3 for MOBS, respectively. The densities of MES and MOBS in water and in aqueous 1,4-dioxane and ethanol solutions were measured with a vibrating-tube digital densimeter at 298.15 K and concentrations up to saturated conditions. The solubilities of MES and MOBS at 298.15 K were determined in the mixed solvents of (1,4-dioxane + water) and (ethanol + water) using the experimental results of density measurements. MOBS shows liquid-liquid phase separation over the concentrations of 1,4-dioxane from 0.4 to 0.9 in mass fraction. The phase diagram of 1,4-dioxane + MOBS + water system was also determined experimentally at 298.15 K, including tie-lines in the two-liquid phase region. Binodal data were fitted to an empirical equation and the effective excluded volume (EEV) model. The reliability of the experimental tie-line data was confirmed by using the Othmer-Tobias and the Bancroft equations. Apparent free energies of transfer (Delta G'(tr)) of zwitterionic buffers from water to aqueous 1,4-dioxane and ethanol solutions have been calculated from the solubility data. The contribution of transfer free energies (Delta g'(tr)) of the (-CH (2) -CH (2) -) group from water to aqueous 1,4-dioxane and ethanol solutions was estimated from the AG'(tr) results.