Solubility and phase separation of 4-morpholinepropanesulfonic acid (MOPS), and 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO) in aqueous 1,4-dioxane and ethanol solutions

Abstract

The buffers 4-morpholinepropanesulfonic acid (MOPS) and 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO) are useful biological zwitterionic buffers within the pH range of 6.5 to 7.9 and 6.2 to 7.6, respectively. The solubilities of these buffers were determined in binary mixtures (1,4-dioxane+water) and (ethanol+water) at T=298.15K by using the results of density measurements. It has been observed that MOPS induced liquid–liquid phase splitting for the mixtures of 40% to 90% (w/w) 1,4-dioxane in water. The two-liquid phase formation was visualized with disperse orange 25. The phase equilibrium boundaries, including the regions of one liquid, two liquids, (one liquid+one solid) and (two liquids+one solid), for the (MOPS+water+1,4-dioxane) system have been determined experimentally at T=298.15K. The tie lines of the (liquid+liquid) equilibrium were also measured. The Othmer–Tobias and Bancroft equation were used to evaluate the reliability of the tie-line data. The binodal curve was fitted to an empirical equation and the effective excluded volume (EEV) model. The apparent free energies of transfer (ΔGtr′) of MOPS and MOPSO from water to 1,4-dioxane and ethanol solutions have been calculated from the solubility data. These ΔGtr′ values were compared with those of some related biological buffers (TRIS, TAPS, TAPSO, and TABS). Furthermore, we also calculated the contribution of transfer free energies (Δgtr′) of –OH group from water to 1,4-dioxane and ethanol solutions.