Effects of increasing polymer hydrophobicity on distribution ratios of TcO 4− in polyethylene/poly(propylene glycol)-based aqueous biphasic systems

Abstract

The partitioning behavior of the pertechnetate anion was studied in aqueous biphasic systems (ABS) formed from (NH 4) 2SO 4 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 (NH 4) 2SO 4-polymer ABS systems including the polymers PEG-2000, PEG-3400, PEG-12 000, Pluronic-L64 (average molecular mass ≈ 2900), and PVP-K15 (average M r≈10 000). Distribution ratios for the TcO 4 − 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⪡-2000<PEG-3400≈PEG-12 000<Pluronic-L64. For the three PEG-ABS systems, distribution ratios of the sulfate 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 K 3PO 4 and NaOH with Pluronic-L64 revealed trends similar to those reported for PEG-2000; the distribution ratio ( D) values increase in the order NaOH<(NH 4) 2SO 4<K 3PO 4. 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.