Equilibrium Phase Behavior for Ternary Mixtures of Poly(ethylene) Glycol 6000 + Water + Sulfate Salts at Different Temperatures Joa ˜o Paulo Martins, † Fabı ´ola Cristina de Oliveira, ‡ Jane Se ´lia dos Reis Coimbra,* ,‡ Luis Henrique Mendes da Silva, † Maria do Carmo Hespanhol da Silva, † and Ingrid Silva Barberino do Nascimento § Departmento de Quı ´mica and Departmento de Engenharia de Alimentos, Universidade Federal de Vicosa (UFV), P. H. Rolfs Avenue, s/n, Zip Code 36570-000, Vicosa, MG, Brazil, and Universidade Estadual do Sudoeste da Bahia, Prac ¸a da Primavera, 40, 45-700, Itapetinga, BA, Brazil Phase diagrams of aqueous two-phase systems composed of poly(ethylene) glycol (PEG) 6000 g · mol -1 , lithium sulfate, sodium sulfate, and magnesium sulfate or zinc sulfate were determined at (283.15, 298.15, and 313.15) K. The effect of temperature on binodal position was not significant, indicating a small enthalpic contribution associating to the phase segregation. The ability of four sulfate salts to induce the formation of the biphasic system with PEG 6000 is as follows: ZnSO 4 > MgSO 4 > Na 2 SO 4 > Li 2 SO 4 . Introduction Separation processes are applied in various types of industries including chemical, pharmaceutical, and food. Classic methods involving liquid-liquid extraction use an organic solvent and an aqueous solution as the two immiscible phases for the fractionation and purification of molecules. However, these systems are very toxic and present risks to human health. An alternative to substitute organic two-phase systems, based on the beginning of green chemistry, is aqueous biphasic systems (ABS). They are formed by the mixture of a polymer + water + salt or two water-soluble polymers differing in chemical structure. 1,2 This methodology has been widely used to separate metallic ions 3,4 and biological materials, 1,2,5,6 such as viruses, nucleic acids, and proteins. These characteristics are important because an organic solvent is not used in the separation processes. The ABS typically applied in separation or purification processes are constituted of poly(ethylene) glycol (PEG) + dextran + water or PEG + inorganic salt + water. The literature mentions several equilibrium data for different ABS formed of PEGs of different average molar masses and inorganic salts. 1,2,7-12 However, this type of data for systems containing PEG6000 + sulfate salts (Li 2 SO 4 , Na 2 SO 4 , MgSO 4 , ZnSO 4 ) at different temperatures are scarce. In this work, aqueous two-phase systems composed of PEG6000 + sulfate salt + water were determined, and the phase compositions were measured. Equilibrium data at (283.15, 298.15, and 313.15) K were determined for the systems consisting of PEG6000 + lithium sulfate + water, PEG6000 + sodium sulfate + water, PEG6000 + magnesium sulfate + water, and PEG6000 + zinc sulfate + water. The influences of the electrolyte nature and temperature on the phase diagram were also investigated. Experimental Section Materials. The following reagents were used: poly(ethylene) glycol with an average molar mass of 6000 g · mol -1 (Sigma, USA) and the sulfate salts Na 2 SO 4 , Li 2 SO 4 , ZnSO 4 , and MgSO 4 (Vetec, Brazil), all of analytical grade. Millipore (USA) water was used in all experiments (R g 18.2 MΩ · cm -1 ). Aqueous Two-Phase Systems (ATPS). Ternary phase dia- grams were prepared by weighing appropriate quantities of PEG6000, sulfate salts (lithium, sodium, zinc, or magnesium), and water on an analytical balance (Gehaka, AG200, Brazil) accurate to ( 0.0001 g. Liquid-liquid glass tubes were used to carry out phase equilibrium determinations. Typically, 15 g of the system was prepared. After vigorously stirring the system until becoming turbid, it was placed in a temperature-controlled bath (Microquı ´mica, MQBTC 99-20), accurate to ( 0.1 K, for 48 h at (283.15, 298.15, or 313.15) K. The equilibrium state was characterized by the absence of turbidity in both the top and bottom phases. Samples from the top and bottom phases were collected for analysis. QuantitatiWe Analysis. Salt concentration (Na 2 SO 4 , Li 2 SO 4 , MgSO 4 , and ZnSO 4 ) was determined by conductivity (Schott CG853, Germany) of the electrolyte in the range of (10 -4 to 10 -3 ) M. The salt solutions showed the same conductivity in water or the diluted polymer solution [(0.01 to 0.001) %]. The standard deviation of the salt mass percent from this method was ( 0.10 %. A refractometer (Analytic Jena AG Abbe refractometer 09-2001, Germany) was used to measure PEG6000 quantity at 298.15 K. Since the refractive index of the phase depends on the polymer and salt concentrations, PEG6000 content was obtained by subtracting the salt concentration obtained by conductivity from the total solution composition (refractive index). The standard deviation of the PEG6000 mass percent was on the order of 0.03 %. Analytical curves were obtained for the salt and PEG measurements, and water content was determined by mass balance. All analytical measurements were performed in triplicate. Results and Discussion The phase compositions for the aqueous two-phase systems formed of PEG6000 and sulfate salts are shown in Tables 1 to * To whom correspondence should be addressed. E-mail: jcoimbra@ufv.br. Fax: + 55-31-3899-2208. Phone: + 55-31-3899-2229. † Departmento de Quı ´mica. ‡ Departmento de Engenharia de Alimentos. § Universidade Estadual do Sudoeste da Bahia. J. Chem. Eng. Data 2008, 53, 2441–2443 2441 10.1021/je800443p CCC: $40.75  2008 American Chemical Society Published on Web 09/17/2008 Downloaded by UNIV FED DE VICOSA UFV on October 28, 2009 | http://pubs.acs.org Publication Date (Web): September 17, 2008 | doi: 10.1021/je800443p