Extraction from micelles

Recent development of the use of reversed micelles (aqueous surfactant aggregates in organic solvents) to solubilize significant quantities of nonpolar materials within their polar cores can be exploited in the development of new concepts for the continuous selective concentration and recovery of heavy metal ions from dilute aqueous streams. The ability of reversed micelle solutions to extract proteins and amino acids selectively from aqueous media has been recently demonstrated the results indicate that strong electrostatic interactions are the primary basis for selectivity. The high charge-to-surface ratio of the valuable heavy metal ions suggests that they too should be extractable from dilute aqueous solutions. 

Entrapment of enzymes within reversed micelles can be achieved simply by dissolving the biopolymer, pure or solubilized in an appropriate solvent, in a solution of reversed micelles or by extraction from an immiscible liquid phase [13,165,166]. 

The Winsor II microemulsion is the configuration that has attracted most attention in solvent extraction from aqueous feeds, as it does not affect the structure of the aqueous phase the organic extracting phase, on the other hand, is now a W/0 microemulsion instead of a single phase. The main reason for the interest in W/0 microemulsions is that the presence of the aqueous microphase in the extracting phase may enhance the extraction of hydrophilic solutes by solubilizing them in the reverse micellar cores. However, this is not always the case and it seems to vary with the characteristics of the system and the type of solute. Furthermore, in many instances the mechanism of extraction enhancement is not simply solubilization into the reverse micellar cores. Four solubilization sites are possible in a reverse micelle, as illustrated in Fig. 15.6 [19]. An important point is that the term solubilization does not apply only to solute transfer into the reverse micelle cores, but also to insertion into the micellar boundary region called the palisade. The problem faced by researchers is that the exact location of the solute in the microemulsion phase is difficult to determine with most of the available analytical tools, and thus it has to be inferred. 

Reverse micelles of CTAB in octane with hexanol as cosurfactant were reported to be able to lyse whole cells quickly and accommodate the liberated enzyme rapidly into the water pool of surfactant aggregates [50,51]. In another case a periplasmic enzyme, cytochrome c553, was extracted from the periplasmic fraction using reverse micelles [52]. The purity achieved in one separation step was very close to that achieved with extensive column chromatography. These results show that reverse micelles can be used for the extraction of intracellular proteins. 

Work Is presently under way to extend the above model so as to extract from the experimental data the relevant parameters from a least-squares analysis (13). This model should be applicable to non-lonlc and Ionic systems. In the latter case, an extra term Is required to account for the shift In the CMC of solute 2 due to the sal-tlng-out of the monomers of 2 by solute 3 (7 ). The model In Its present form can still be used to estimate the thermodynamic properties of solute 3 In the micelle of surfactant 2 by adjusting the parameters to get a good fit with the experimental data. 

In the same area, some authors [13,14] have shown that a new selective extraction from a fermentation broth in supercritical CO2 can be driven by adding surfactants and molecular micelles. Promising preliminary results indicate new possibilities for treating water-soluble compounds with CO2. 

FICU RE 7.6 Schematic view of DMDBTDM A reverse micelle. The polar core is composed of extracted ion pairs, coextracted water, and polar heads of malonamide, surrounded by the hydrophobic chains of the extractant. (From F. Testard, P. Bauduin, L. Martinet, B. Abecassis, L. Berthon, and C. Madic, Radiochim. Acta, 96 1-8, 2008. With permission.)... 

By such an adsorption of 3 at the membrane it is intended to prevent an establishment of cancer cells in the healthy tissue. A somewhat different interpretation of the action of such detergents has been given by Calvin (31), who supposes that the enzymes are extracted from the cell membrane by the formation of detergent micels or that inhibitors are stabilized in such away that they cannot interact anymore with the enzyme. 

Normal aqueous micellar media can also be employed to extract and purify components from solid matrices. Proteins have been extracted from wheat kernals using aqueous NaLS (399). This same surfactant system has been employed in an improved method for the extraction of filth from cheese (417). In another application, aqueous solutions of Brij-35 micelles have been employed to extract components (i.e. vanillin and ethylvanillin) from smoking tobacco (106). In a similar manner, various phenolic compounds have been extracted from herbal/plant leaves using nonionic Triton X-100, Brij-35, or octyl glucoside (0G) (393). In both of these latter examples, the indicated compounds could be identified and quantitated by reversed phase HPLC using as mobile phase the same micellar solutions (refer... 

Another type of self-assembly mode is based on looser molecular interactions, where one of the main binding forces comes from hydrophobic interactions in aqueous media. Amphiphihc molecules (amphiphiles) that have a hydrophihc part and a hydrophobic part form various assembhes in water and on water. The simplest example of this kind of assembly is a micelle, where amphiphiles seh-assemble in order to expose their hydrophilic part to water and shield the other part from water due to hydrophobic interactions. A similar mechanism also leads to the formation of other assembhes, such as hpid bilayers. These molecules form spherical assembhes and/or two-dimensional membranes that are composed of countless numbers of molecules. These assembhes are usually very flexible. When external signals are applied to them, they respond flexibly while maintaining their fundamental organization and shape. This research held was initiated by the work of Bangham in 1964. It was found that dispersions of hpid molecules extracted from cells in water spontaneously form cell-like assembhes (liposomes). In 1977, Kunitake and Okahata demonstrated the formation of similar assembhes from various arti-flcial amphiphiles. The latter finding showed that natural lipids and artificial amphiphiles are not fundamentally different. 

The purpose of this research is to compare micelle-like polymeric particles with a surfactant with respect to the sorption to soil and the extraction of an organic soil contaminant. Nano-sized polyurethane (APU) particles synthesized from amphiphilic urethane acrylate anionomers were used as model micelle-like polymeric particles. Employing APU particles with various degrees of hydrophobicity, the effects of hydrophobicity on the soil sorption and the phenanthrene extraction from soil of polymeric particles were studied. Sodium Dodecyl Sulfate (SDS) was used as a model conventional surfactant. Phenanthrene was used as a model soil contaminant. 

Pure water at a high pressure and temperature was the solvent used as extractant in most applications. However, the addition of a modifier [157,173] or a co-extractant [47] can dramatically improve the extraction of some substances. Such is the case with the extraction of nonylphenol polyethoxy carboxylates from industrial and municipal sludges, where recovery was increased by more than 30% in the presence of 30% (v/v) ethanol in the water used as leaching agent [157]. Because of the hydrophobic nature of PAHs, the increased dielectric constant of water at a high temperature did not suffice to ensure quantitative extraction from soil. However, as can be seen from Fig. 6.14, the addition of a co-extractant (viz. sodium dodecyl sulphate, SDS, which forms charged micelles) dramatically improved the extraction of these hydrophobic compounds also, it substantially reduced the extraction time and enabled the quantitative recovery of benzo(a)-acenaphthene [47]. 

Micelle-water partition coefficients are extracted by micelle chromatography (high performance liquid chromatography, HPLC) using micelle aqueous solution as mobile phase. For determination of retention times are measured using a usual HPLC system at various concentrations of micelle in the aqueous mobile phase and then estimated from the following equation ... 

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