Solvent property characterization of poly(ethylene glycol)/dextran aqueous biphasic systems using the free energy of transfer of a methylene group and a linear solvation energy relationship

Aqueous biphasic systems (ABSs) composed of poly(ethylene glycol) (PEG) and dextran have long been proposed as useful liquid/liquid extraction systems for biological macromolecules. More recently, they have been proposed as useful partitioning systems for molecular characterization in quantitative structure activity relationships. In this context, the distribution ratios of a wide range of organic solutes differing in structure and functionality were measured in a PEG/dextran ABS and the results compared to the corresponding 1-octanol/water partition coefficients. The relative hydrophobicity of the phases was quantified from the free energy of transfer of a methylene group measured for a homologous series of alcohols. A linear free energy relationship based on Abraham's generalized solvation equation has been derived from the solute partitioning data, which allows a direct comparison to be made between the solvent properties of a PEG/dextran ABS and those of traditional solvent/water systems used, for example, in the determination of log P. A comparison with similar parameters previously determined for ABSs composed of PEG and a salt is also enabled.