Journal of Chromatography B, 743 (2000) 137–149 www.elsevier.com / locate / chromb Solvatochromic studies in polyethylene glycol–salt aqueous biphasic systems a a a,b, * Jonathan G. Huddleston , Heather D. Willauer , Robin D. Rogers a Center for Green Manufacturing, The University of Alabama, Tuscaloosa, AL 35487, USA b Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487, USA Abstract The polarities of the co-existing phases of a polyethylene glycol (PEG)-2000–K PO aqueous biphasic system (ABS) 3 4 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 polarity to rather polar solvents. Data on the free energy of transfer of solvents suggests that this may be due to the failure of the probe to account for the real polarity of the salt-rich phase compared to the polymer-rich phase. Examination of the monophasic region of these systems suggests that the reason for this is that the probe is partitioned to a discreet solvent domain dominated by PEG, even though phase separation of the solution is not observed. The use of linear free energy relationships for the characterization of ABS is briefly discussed.  2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Aqueous two-phase systems; Solvatochromic studies; Linear free energy relationships; Poly(ethylene glycol) 1. Introduction aqueous biphasic extraction chromatography (ABEC) [7]. Aqueous solutions of polymers have excited some It has been suggested that these wholly aqueous recent interest for their ability to solubilize otherwise solvent extraction systems (WASE) represent en- poorly soluble species without the involvement of vironmentally benign alternatives to the conventional organic solvents [1]. Such systems include inter alia, use of organic solvents in liquid–liquid extraction cloud point extraction (CPE) [2], micellar extraction processes [1]. It is, therefore, of interest to be able to (ME) [3], thermoseparating polymer systems (TPS) make comparisons of the observed solubilization and [4] composed of PEO–PPO (polyethylene and poly- distribution properties of solutes between the differ- propylene oxides) and other co-polymer solutions, ent micellar and biphasic systems, and between these aqueous biphasic systems (ABS) [5], and dendrimers systems and the bulk solvent phases of traditional [6]. This ability to enhance the solubility of, or to solvent extraction. Through illumination of the mo- capture, hydrophobic substances appears to be mani- lecular forces involved in these systems a ‘‘tool box’’ fest both in free solution and when the polymer is of polymeric extractants may be made available to bound to a physical support as for example in aid in the replacement of a solvent based process with one based on solubilization using aqueous polymeric solutions. *Corresponding author. Numerous methods are available by which such 0378-4347 / 00 / $ – see front matter  2000 Published by Elsevier Science B.V. All rights reserved. PII: S0378-4347(00)00230-9