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Aqueous solutions of certain surfactant micelles exhibit phase separation behavior upon temperature alteration. This phenomenon can be exploited in separation science for the development of extraction, purification, and preconcentration schemes for desired analytes. Since the addition of just a small amount of an appropriate nonionic or zwitterionic surfactant to the aqueous sample solution is required, this approach is convenient and fairly benign, eliminating the need for the use of organic solvents as in conventional liquid−liquid or solid−liquid extraction. The basic features, experimental protocols, and selected recent applications of this alternative extraction approach, termed cloud point extraction (CPE) or micelle-mediated extraction (ME), are briefly reviewed. In addition, the advantages, limitations, and anticipated future directions of this methodology are discussed.

The technique of extracting and concentrating hydrophobic species from water via nonionic surfactant micelle-mediated phase separations was examined. The effect on extraction efficiency, solute distribution coefficient, and phase volume ratio of pertinent experimental parameters such as surfactant hydrophobicity, pH and ionic strength, equilibration time and temperature, centrifuge time, and analyte and surfactant concentration was systematically evaluated. Surfactant hydrophobicity, which was studied by monitoring the extraction parameters observed for two homologous series of polyoxyethylene glycol) monoethers, appeared to be the dominant factor along with solution pH for those solutes possessing either an acidic or a basic functionality. The use of poly (oxyethylene glycol) monooctyl ether (CsEa) in the cloud point procedure led to very good extraction efficiency and concentrating ability for most solutes, achieving extraction recoveries of 88-99% for a series of chlorinated phenols. The general trends observed as a function of changes in the experimental conditions should be applicable to those cloud point extraction systems that utilize other nonionic surfactants. The optimized procedure was also

The zwitterionic surfactants 3-[nonyl- (or decyl-) dimethyl-ammonio]propyl sulfate, (C9-APSO4 or C10-APSO4) were synthesized using Nilsson's procedure, and their phase separation behavior under different experimental conditions was evaluated. The results indicate that such zwitterionic surfactants can be utilized for the extraction/preconcentration of hydro-phobic species in a manner akin to that previously reported for nonionic surfactants. This was demonstrated for several practical applications including the extraction/preconcentration of some steroidal hormones and vitamin E prior to high-performance liquid chromatography analysis. The zwitterionic surfactant mediated phase separation was also applied to the extraction of the hydrophobic membrane protein, bacterio-rhodopsin, from the hydrophilic cytochrome c protein, both originally present in an aqueous phase. The concentration factors for this aqueous two-phase extraction technique using C10-APSO4 ranged from 26 to 35 with recoveries in the range 88 to greater than 96%. Some comparative studies indicate that the use of zwitterionic surfactants in lieu of nonionic surfactants (e.g. polyoxyethylene(7.5) nonyl phenyl ether PONPE-7.5) in such an extraction method offers some significant advantages such as purer, homogeneous surfactant preparation, minimum background absorbance at UV detection wavelengths, the two-phase region occurring at lower temperatures, and greater extraction efficiencies/concentration factors among others.

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