Solubility and phase separation of 4-morpholinepropanesulfonic acid (MOPS), and 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO) in aqueous 1,4-dioxane and ethanol solutions Mohamed Taha, Ming-Jer Lee ⇑ Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan article info Article history: Received 22 March 2011 Received in revised form 6 May 2011 Accepted 31 May 2011 Available online 6 June 2011 Keywords: Density Solubility Zwitterionic buffers Solvent mixtures Phase separation Apparent free energies of transfer 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-diox- ane + water) and (ethanol + water) at T = 298.15 K 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 = 298.15 K. 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 fit- ted to an empirical equation and the effective excluded volume (EEV) model. The apparent free energies of transfer (DG 0 tr ) of MOPS and MOPSO from water to 1,4-dioxane and ethanol solutions have been calcu- lated from the solubility data. These DG 0 tr 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 (Dg 0 tr ) of –OH group from water to 1,4-dioxane and ethanol solutions. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Buffers are of immense importance in biological and chemical systems. Good et al. [1] proposed ten new buffers, four of which were zwitterions aliphatic amines derived from taurine or glycine using bromoethanesulfonate. After that, Ferguson et al. [2] also introduced five similar new buffers. Sigma Chemical Company developed four new buffers (MOBS, TABS, HEPBS, and CABS) as well. These aminosulfonic acid buffers form a family of about 20 buffers each with a known pK a . Such synthetic compounds have been used extensively in laboratories as precise buffers in studies involving enzyme dynamics and virus, bacterial and cell tissue cultures. There are several characteristics we need to know about a buf- fer: pK a value; variation of pK a with temperature and ionic strength; anionic, cationic, or multiple charges on buffer species; interaction with other components, e.g., metal ions; expense; UV absorption; solubility. The buffer should have high aqueous solu- bility and minimum solubility in all other organic solvents such that it has low possibility to accumulate in biological systems or traverse biological membranes. The ratio of the solubility in water to those in relatively non-polar solvents is important, since this determines the distribution of buffer between the aqueous medium and the biological phase in particulate systems [1]. Solid solubility data for (MOPS + water) and (MOPS + methanol) were measured by our research group [3]. To our knowledge, there are no solubility data of MOPSO in water or in organic solvents. Hence, through our extended studies of the solubility of biological buffers in various solvents [4–7], we have now determined the solubility of MOPS and MOPSO by density measurements at T= 298.15 K in water, (water + 1,4-dioxane), and (water + ethanol) mixtures con- taining different concentration of the organic solvents at intervals of 10% (w/w) from 0% to 90% (w/w). Ethanol was chosen as a representative of amphiprotic hydrogen bond acceptor–donor (HBA-D) solvents. We have chosen 1,4-dioxane as hydrogen bond acceptor (HBA) solvent. In addition, these solvents are of frequent use in biochemical studies. MOPSO is similar to MOPS in molecular structures except that MOPSO has an –OH group replacing a hydro- gen atom on the b carbon nearby the sulfonic group, as illustrated in scheme 1. MOPS and MOPSO have been used for many years for biological reactions [8–15]. A new phase separation phenomenon triggered by addition of a MOPS into an 1,4-dioxane–water solution is reported firstly in this article. The phase diagram of 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.05.036 ⇑ Corresponding author. Tel.: +886 2 2737 6626; fax: +886 2 2737 6644. E-mail address: mjlee@mail.ntust.edu.tw (M.-J. Lee). J. Chem. Thermodynamics 43 (2011) 1723–1730 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct