Salt effect on the (polyethylene glycol 8000 + sodium sulfate) aqueous two-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, ΔG(CH₂). The ΔG(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^?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 ΔG(CH₂) and the tie-line length (TLL), and biphasic systems with high salt additive concentration present the most negative ΔG(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.