Ion Extraction in Polyethylene Glycol Based-Aqueous Biphasic Systems

Stock solutions are typically prepared on a weight percent (PEGs or salts), molar (salts), or molal (salts) concentration basis. Equal aliquots by volume of each stock solution are combined and mixed to yield an ABS. The total system composition can be calculated and the phase compositions taken from the appropriate phase diagram and tie line data. This method of reporting allows the results to be more readily compared with solvent extraction data. 

When extractants or matrix ions are used in an ABS, they are prepared by diluting the molecule or salt to a known concentration in either the salt or PEG stock solution [8]. This approach results in a net decrease in the concentration of phase-forming salt present in the uptake experiment. The effect is trivial at low extractant concentrations but potentially significant at higher extractant concentrations. Unless otherwise noted, the compositions referred to in this chapter are for preequilibrium stock solution concentrations. 

Rogers et al. [8,23,32,41,51 showed that the pertechnetate anion partitions quantitatively to the PEG-rich phase in a variety of PEG-ABSs. The partitioning behavior of this ion is affected by the numerous variables which affect system composition in an ABS identity of the cation or anion in the phase-forming salt, salt concentration, type and molecular weight of the polymer, polymer concentration, temperature, pH, and the presence of other matrix ions in the ABS. 

The literature suggests that increasing the phase incompatibility between the PEG-rich phase and salt-rich phase increases the affinity of a solute for a particular phase [3,53—55]. Since TcO-f prefers the PEG-rich phase, the distribution ratios increase as the phase incompatibility increases (which results in an increase in the difference in PEG, or salt, concentration in each phase). A linear relationship between log D for neutral solutes and the difference in PEG concentration between the phases has been noted previously in the literature [54,55]. 

Increasing the polymer molecular weight or hydrophobicity also increases the incompatibility between the phases in a given ABS. Distribution ratios for pertechnetate in ABSs formed with (NH4)2S04 and five different polymers are shown in Fig. 9 [41]. Because of its limited solubility range, Pluronic-L64 was studied at a different weight percent concentration. PPG-2000 is hydrophobic and thus does not form an ABS. Distribution ratios between the salt solutions and PPG-2000 are near 1. 

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