Solute partitioning in aqueous biphasic systems composed of polyethylene glycol and salt: The partitioning of small neutral organic species

Aqueous biphasic systems (ABSs) represent wholly aqueous systems that are safe, nontoxic, and nonflammable, and thus, they represent relatively environmentally benign extraction media. Such systems could be employed as alternatives to traditional aqueous-organic systems for the separation of, inter alia, small organic molecules, thus it is important to develop a fundamental understanding of these systems and the variables that govern solute partitioning within them. The partitioning of a series of neutral, substituted benzene compounds and neutral, aliphatic compounds in ABSs composed of different molecular weights of poly(ethylene glycol) (PEG-1000, 2000, and 3400) and formed as a result of the addition of different salt types [K3PO4, K2CPO3, (NH4)(2)SO4, Li2SO4, MnSO4, ZnSO4, and NaOH] has been examined. The results show that the distribution of organic solutes is a function only of the degree of phase divergence of the biphasic system as expressed by the difference in polymer concentration between the phases: A[poly(ethylene glycol)], A[ethylene oxide monomers], or tie line length (APEG, DeltaEO, or TLL, respectively). Solute partitioning depends only on the composition of the phase-forming components, PEG and salt. Using ideas taken from the study of critical-phenomena, it can be shown that the composition of the phases is the result of the salting-out ability of the salt and the number of ethylene oxide monomers comprising the PEG. The salting-out strength of the salt (measured by its lyotropic number or position in the Hofmeister series) is related to its ability to lower the cloud point of the PEG solution. Hence, cosmotropic salts salt-out PEG, producing a series of nearly identical ABSs that, although differing in their overall concentrations of PEG and salt, are identical in terms of their lyotropic properties. This is an extraordinary simplification of a complex array of different ABSs to a single series of ABSs of graded lyotropy. Further comparison of solute partitioning in PEG/salt ABSs to partitioning in 1-octanol/water systems is discussed, and a greater similarity of solute distribution was found between different PEG/salt ABSs than between PEG/salt ABSs and 1-octanol/water.