We consider the effects of the addition of poly(ethylene glycol) (PEG) of different molecular weights to aqueous two‐phase systems of PEG 8000 and dextran 500. The first purpose of this study was to determine the molecular weight partitioning of the polymers themselves so that, for example, aqueous two‐phase separations using affinity ligands can be improved. The second purpose was to examine whether this molecular weight partitioning could be predicted by using solution thermodynamic models so that it would be possible to optimize affinity partitioning without extensive laboratory work. Experimentally, we find that, by increasing the PEG concentration of any molecular weight in the feed, the high molecular weight PEG concentration in the dextran‐rich phase is reduced. This observation can be used to reduce the loss of expensive ligated PEG used in affinity partitioning. Further, there is generally good agreement between our experimental data and the predictions of a solution thermodynamic model.
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