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 ≈ 2900), and PVP-K15 (average M_r≈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 PPGPVP-2000PEG-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)₂SO₄3PO₄. 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.