A Critical Review of Surfactant-Mediated Phase Separations (Cloud-Point Extractions): Theory and Applications

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Abstract
The general concept of using the unique phase separation behavior of some surfactant micelle solutions as a means for extractive separation is outlined and described. Next, the specific micellar parameters and phase separation behavior of nonionic and zwitterionic charge-type surfactant solutions are summarized. In addition, the phase behavior of some derivatized B-cyclodextrin solutions is briefly described. The specific applications of such systems and their phase behavior for the extractive preconcentration, separation, and/or purification of metal chelates, biomaterials, and organic compounds are summarized and discussed. The potential use of affinity ligands in conjunction with the cloud-point approach for selective bioextractions is also mentioned. The experimental protocols, advantages, and limitations of the different cloud-point extraction techniques are outlined. The use of zwitterionic as opposed to nonionic surfactant media in such separations is compared and contrasted. In addition, extractions based on use of aqueous two-phase polymer systems are compared and contrasted to extractions employing the aqueous surfactant systems. Some areas for further work are identified. The use of such phase separation behavior in environmental cleanup applications is described. Last, some general experimental considerations with respect to surfactant purity and analyte recovery in such systems are presented.