Influence of pH and acid solutes on the phase behaviour of aqueous solutions containing poly(ethylene glycol) and poly(ethyleneimine)

In this paper, which forms part of a study on aqueous two-phase systems (ATPS) for separation of low-molar-mass organic acids in the biotechnology industry, we examine the phase behaviour of ATPSs containing poly(ethylene glycol) (PEG) and poly(ethyleneimine) (PEI) as a function of pH, the nature of the acid used for titrating PEI, and the addition of lactic acid. We show that increasing the pH leads to contraction of the two-phase regions and that titrating PEI with a higher polyvalent acid results in a larger two-phase region. We propose a mechanism explaining the experimentally observed phase behaviour. Finally, we demonstrate that the lactic acid partition coefficient is very favourable (the acid partitions preferentially to the PEI-rich phase), which confirms that the (PEG + PEI) ATPSs could be used to advantage as a medium for lactic acid bio-transformation.