Partitioning of small organic molecules in aqueous biphasic systems

Aqueous biphasic systems (ABS) are suitable for the separation of small organic molecules in industrial and environmental applications and thus, it is important to correlate partitioning behavior of model organic solutes with their structure in order to develop predictive models. The partitioning behavior of five, uncharged, substituted benzenes (benzene, toluene, chlorobenzene, 1,4-dichlorobenzene and 1,2,4-trichlorobenzene) were studied in ABS prepared from stock solutions of 40% (w/w) PEG-2000 and increasing concentrations of four water-structuring salts (K₃PO₄, K₂CO₃, (NH₄)₂SO₄ and NaOH). For a given solute and a defined concentration of salt, the partition coefficients increase as the ΔG_hyd value of the salt anion becomes more negative (e.g., D_benzene increases in the order OH⁻<SO₄²⁻<CO₃²⁻<PO₄³⁻). In a given salt, the distribution ratios increase in the order benzene<toluene<chlorobenzene<1,4-dichlorobenzene<1,2,4-trichlorobenzene. The partitioning behavior of the solutes in PEG–salt ABS was found to be strongly correlated with their partitioning coefficients in 1-octanol–water biphasic systems.