The polarities of the co-existing phases of a polyethylene glycol (PEG)-200
0-K3PO4 aqueous biphasic system (ABS) have been examined using Reichardt's
carboxylated pyridinium-N-phenoxybetaine dye as a probe. Using this probe,
the polarities of these phases have been compared to those of conventional
solvent extraction systems and micellar systems using values obtained from
the literature. In general, these extraction systems are comparable in pola
rity to rather polar solvents. Data on the free energy of transfer of solve
nts suggests that this may be due to the failure of the probe to account fo
r the real polarity of the salt-rich phase compared to the polymer-rich pha
se. Examination of the monophasic region of these systems suggests that the
reason for this is that the probe is partitioned to a discreet solvent dom
ain dominated by PEG, even though phase separation of the solution is not o
bserved. The use of linear free energy relationships for the characterizati
on of ABS is briefly discussed. (C) 2000 Published by Elsevier Science B.V.
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