Liquid–liquid equilibrium correlation of aqueous two-phase systems composed of polyethylene glycol and nonionic surfactant

• LLE of polymer/surfactant ATPS was investigated by Flory–Huggins model.
• Two hypothesis forms of surfactant molecules were introduced in correlation.
• The fitting results would be slightly better when surfactants are regard as micelles.
• Temperature plays a key role in the interaction including polymer molecules in ATPS.

Phase diagrams of aqueous two-phase systems (ATPS) composed of polyethylene glycol 20000 (PEG 20000) + polyoxyethylene octyl phenyl ether (Triton X-100) or polyoxyethylene sorbitan monooleate (Tween 80) + water are determined at 273.15, 293.15 and 313.15 K, respectively. The liquid–liquid equilibrium (LLE) experimental data are correlated using the Flory–Huggins model derived from the lattice theories, and new interaction parameters (λ12, λ13 and λ23) between any two compositions (the polymer(1) and the nonionic surfactant(2), the solvent(3)) in the ATPS are estimated at various temperature. Two kinds of fitting results are obtained when the surfactant molecules are regarded as single molecules or micellar molecules, respectively. It indicates that the interaction parameters (λ12) between PEG and nonionic surfactant including Triton X-100 and Tween 80 decrease 52.3% and 105% as single molecules, while decrease 421% and 403% as micellar molecules by raising temperature, respectively. Moreover, the slightly better fitting effect can be observed if the surfactant molecules are regarded as the micellar molecules. The standard deviation between prediction values and experimental data of the components weight fraction is lower than 0.1% using the surfactant micellar parameters, showing the good descriptive quality and applicability of the Flory–Huggins model.