Interfacial Tension of Aqueous Two-Phase Systems Containing Poly(ethylene glycol) and Potassium Phosphate Ca ́ ssio Carmo de Oliveira, † Jane Se ́ lia dos Reis Coimbra,* ,† Abraham Damian Giraldo Zuniga, ‡ Joa ̃ o Paulo Martins, § and Antonio Marcos de Oliveira Siqueira ∥ † Department of Food Technology, Federal University of Viç osa, P. H. Rolfs Av., s/n, 36570-000, Viç osa, MG, Brazil ‡ Food Engineering Course, Federal University of Tocantins, NS 15 Av., ALCNO 14, 77123-360, Palmas, TO, Brazil § Federal Institute of South of Minas Gerais, Dr Joã o Beraldo, 242, 37550-000, Pouso Alegre, MG, Brazil ∥ Department of Chemistry, Federal University of Viç osa, P. H. Rolfs Av., s/n, 36570-000, Viç osa, MG, Brazil ABSTRACT: The effect of poly(ethylene glycol) (PEG) molar mass, pH, and temperature on interfacial tension was investigated for aqueous two-phase systems composed of PEG with molar masses of (1500, 4000, 6000, and 8000) g·mol −1 and potassium phosphate. The temperatures tested were (293 and 313) K at the pH values of 6, 7, and 9. Interfacial tension was determined by using a spinning drop tensiometer and the Vonnegut equation. An increase in both the PEG molar mass and the temperature resulted in an increase of interfacial tension values. The interfacial tension varied from (0.05 to 3.11) mN·m −1 for systems composed by PEG 4000 (mass fraction; w = 0.12) + potassium phosphate (w = 0.11) + water (w = 0.77) at pH 6.0 and 293 K and PEG 4000 (w = 0.19) + potassium phosphate (w = 0.16) + water (w = 0.65) at pH 7.0 and 313 K, respectively. ■ INTRODUCTION Aqueous two-phase systems (ATPS's), as an alternative separation technique for biological materials, are of substantial interest to the biotechnology industry. Biopharmaceutical companies faced with increasing product quality standards and stiffening economic competition are reconsidering ATPS's as an alternative to chromatography. These systems are formed using two polymers soluble in water or a water-soluble polymer and a component of low molar mass, like inorganic salts. 1 Due to their low interfacial tension and high water content, the ATPS's provide mild separation conditions to preserve biological activities of labile compounds, such as proteins, cells, or other biological materials. 1 ATPS's have been utilized in the determination of hydrophobic properties of cell membranes and extractive bioconversion of biocompounds, as well as the separation and purification of metals, proteins, enzymes, hormones, organelles, and cells. 1−3 The acquisition of equilibrium data and the properties of ATPS's, such as interfacial tension, are necessary for the design of extraction processes and for the development of models that can predict biomolecule partitioning between phases. The interfacial tension between phases plays a decisive influence on the separation and partition mechanism of biomolecules and cells, as well as participates in dispersion, emulsification, flocculation, and solubilization processes. Interfacial tension influences the shape of fluid interfaces and controls their deformability. Data on interfacial tension and other physical properties allow for the prediction of system behavior, velocity of phase formation, phase separation, and reagent composition for the system. The methods for the determination of interfacial tension can be divided as static (in the pendant drop technique) and dynamic (in the spinning drop technique) which was introduced in 1942 by Vonnegut. 4 The spinning drop method 4 was applied to determine interfacial tension of the ATPS poly(ethylene glycol) (PEG) + dextran (DEX) as reported by Ryden and Albertsson. 5 The interfacial tension between two aqueous rich phases is usually very small, often between (0.001 and 1) mN·m −1 . The use of standard methods for interfacial tension measurements in this range, such as the capillary or ring techniques is not easy; thus, the spinning drop technique appears to be a suitable technique for the determination of very low interfacial tension values. Forciniti et al. 6 used the spinning drop technique to study the interfacial tension of PEG + DEX ATPS's. The authors reported a detailed investigation of interfacial tension as a function of temperature, polymer concentration, and PEG and DEX molar masses. Very low values for interfacial tension were found, between (1.5·10 −3 and 0.35) mN·m −1 , when compared to other organic extraction systems. For instance, systems composed of hexane + water, glycerin + hexane, and toluene + water presented interfacial tension values equal to (48.5, 34.9, and 35.7) mN·m −1 , respectively. 7 Bamberger et al. 8 also studied Received: October 8, 2010 Accepted: May 25, 2012 Published: June 1, 2012 Article pubs.acs.org/jced © 2012 American Chemical Society 1648 dx.doi.org/10.1021/je1010104 | J. Chem. Eng. Data 2012, 57, 1648−1652