Solvatochromic studies in polyethylene glycol-salt aqueous biphasic systems

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. All rights reserved.