Salt effect on the (polyethylene glycol 8000 + sodium sulfate) aqueoustwo-phase system : relative hydrophobicity of the equilibrium phases

The relative hydrophobicity of the phases of several {polyethylene glycol (PEG) 8000 + sodium sulfate (Na2SO4)} aqueous two-phase systems (ATPSs), all containing 0.01 mol.L-1 sodium phosphate buffer (NaPB, pH 7.4) and increasing concentration of a salt additive, NaCl or KCl, up to 1.0 mol.L-1, was measured by the free energy of transfer of a methylene group between the phases, DG(CH2). The DG(CH2) of the systems was determined by partitioning of a homologous series of five sodium salts of dinitrophenylated (DNP) – amino acids with aliphatic side chains in three different tie-lines of each biphasic system. The relative hydrophobicity of the phases ranged from -0.125 to -0.183 kcal.mol.L-1, being the NaCl salt the one to provide the more effective changes. The results show that, within each system, there is a linear relationship between the DG(CH2) and the tie-line length (TLL), and biphasic systems with high salt additive concentration present the most negative DG(CH2) values. Therefore, the feasibility of establishing a relationship between the relative hydrophobicity of the phases in a given TLL and the ionic strength of the salt additive was investigated and a satisfactory correlation was found for each salt.Luisa A. Ferreira acknowledges the financial support (SFRH/BPD/47607/2008) from Fundacao para a Ciencia e a Tecnologia (Lisboa, Portugal)