Influence of polymer molecular weight and temperature on phase composition in aqueous two-phase systems

Abstract In an effort to understand the molecular weight and temperature dependence of protein partition coefficients, we have conducted experiments to determine how the binodal curve and the difference in polymer concentrations between the top and bottom phases (▵ PEG  [PEG] T — [PEG] B and Δ Dx  [Dx] B — [Dx] T ); depend on polymer molecular weight and temperature. We report the equilibrium compositions at 4°, 25° and 40°C for 16 different aqueous two-phase PEG-Dx systems which consist of all possible combinations of the four PEG molecular weights (4000, 6000, 10 000, and 20 000) and the four Dx molecular weights (10 000, 40 000, 110 000 and 500 000). Four tie lines were measured for each system. We confirm that increasing polymer molecular weight increases the tie line length and depresses the binodals to lower polymer concentrations, and that this effect will be greater when the ratio of this molecular weight increase to the molecular weight of the other species is large. The increase in the tie line length and in Δ Dx and Δ PEG with increasing polymer molecular weight tends to level off at high polymer molecular weight and at high total polymer concentrations. It is shown that the effect of temperature is not symmetric, i.e. changes in temperature produce a change in Δ Dx while Δ PEG remains nearly constant. The effect of polymer molecular weight on both Δ Dx and Δ PEG is magnified with increasing temperature. The results found here suggest a series of experiments which could be used to isolate (or highlight) the effect of the second virial coefficient on the partition coefficient.