Effects of increasing polymer hydrophobicity on distribution ratios of TcO4- in polyethylene/poly(propylene glycol)-based aqueous biphasic systems.

The partitioning behavior of the pertechnetate anion was studied in aqueous biphasic systems (ABS) formed from (NH4)2SO4 and four types of polymers-poly)ethylene glycol) PEG, poly(propylene glycol) (PPG), Pluronic (a PEG/PPG block copolymer), and polyvinylpyrrolidone (PVP). Phase diagrams are reported for five (NH4)2SO4-polymer ABS systems including the polymers PEG-2000, PEG-3400, PEG-12,000, Pluronic-L64 (average molecular mass approximately 2900), and PVP-K15 (average M(r) approximately 10,000). Distribution ratios for the TcO4- anion in each of these ABS were investigated as a function of increasing salt concentration. In addition, the water-insoluble polymer PPG-2000 was studied. Pertechnetate partitions nearly quantitatively to the polymer-rich phase in each ABS, however, distribution ratios of near one were found for the PPG system. The relative ordering of the distribution ratios is PPG << PVP << PEG-2000 < PEG-3400 approximately PEG-12,000 < Pluronic-L64 for the three PEG-ABS systems, distribution ratios of the sulfate anion decrease in the order PEG-2000 > PEG-3400 > PEG-12,000, exhibiting the expected increase in phase incompatibility with increasing polymer M(r). Investigation of pertechnetate partitioning in two additional ABS based on K3PO4 and NaOH with Pluronic-L64 revealed trends similar to those reported for PEG-2000; the distribution ratio (D) values increase in the order NaOH < (NH4)2SO4 < K3PO4. Despite the higher distribution ratios from Pluronic-L64 at lower concentrations than found for PEG, the limited useable range of salt concentrations available may limit the practical utilization of this polymer in ABS separations.