Thermodynamics of aqueous two-phase systems—the effect of polymer molecular weight on liquid–liquid equilibrium phase diagrams by the modified NRTL model

The modified NRTL (nonrandom two-liquid) model for binary polymer solutions, proposed in the previous paper, is further extended to calculate the liquid–liquid equilibrium of polyethylene glycol (PEG)–dextran (DEX) aqueous two-phase systems (ATPS) and PEG–salt ATPS. The model has six binary adjustable model parameters, and in the calculation the model parameters between PEG and water, DEX and water, and salt and water are correlated with the vapor–liquid equilibrium of the corresponding binary systems, while those between PEG and DEX or PEG and salt are correlated with two tie-line data of one ATPS. The results show that the model can accurately predict a series of the liquid–liquid equilibrium phase diagrams of ATPS at different polymer molecular weights. The effect that the PEG or DEX molecular weights have on the phase diagrams of ATPS, and the influences that the polydispersity of the polymer molecular weight and the correlation between the model parameters have on the model parameters between PEG and DEX or PEG and salt are also discussed in this paper.