Micellar, aggregates phase

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

Fluorescence quenching studies in micellar systems provide quantitative information not only on the aggregation number but also on counterion binding and on the effect of additives on the micellization process. The solubilizing process (partition coefficients between the aqueous phase and the micellar pseudo-phase, entry and exit rates of solutes) can also be characterized by fluorescence quenching. 

Thus, in the case B, the repulsion between the alkyl chain and water has been removed. Instead, the alkyl-alkyl attraction (B) is the driving force for micelle formation. The surfactant molecule forms a micellar aggregate at a concentration higher than CMC because it moves from the water phase to the micelle phase (lower energy). The micelle reaches an equilibrium after a certain number of monomers have formed a micelle. This means that there are both attractive and opposing forces involved in... 

To this point, only models based on the pseudo—phase separation model have been discussed. Mixed micelle models utilizing the mass action model may be necessary for micelles with small aggregation numbers, as demonstrated by Kamrath and Franses ( ). However, even for large micelles, the fundamental basis for the pseudophase separation model needs to be examined. In micelles, how much solvent or how many counterions (bound or in the electrical double layer) should be included in the micellar pseudo-phase is unclear. The difficulty is normally surmounted by assuming that the pseudo—phase consists of only the surfactant components i.e., solvent or counterions are ignored. The validity of treating the micelle on a surfactant—oniy basis has not been verified. Funasaki and Hada (22) have questioned the thermodynamic consistency of such an approach. 

High-pressure FT-IR spectroscopy has been used to clarify (1) the rotational isomerism of molecules, (2) characteristics of water and the water-head group, and (3) RSO3 Na4- interactions in reverse micellar aggregates in supercritical ethane. This work demonstrates interesting pressure, temperature, and salt effects on an enzyme-catalyzed esterification and/or maintenance of a one-phase microemulsion in supercritical fluids from practical and theoretical points of view (Ikushima, 1997). 

In MEKC, the supporting electrolyte medium contains a surfactant at a concentration above its critical micelle concentration (CMC). The surfactant self-aggregates in the aqueous medium and forms micelles whose hydrophilic head groups and hydrophobic tail groups form a nonpolar core into which the solutes can partition. The micelles are anionic on their surface, and they migrate in the opposite direction to the electroosmotic flow under the applied current. The differential partitioning of neutral molecules between the buffered aqueous mobile phase and the micellar pseudostationary phase is the sole basis for separation as the buffer and micelles form a two-phase system, and the analyte partitions between them (Smyth and McClean 1998). 

Phase diagrams of many Cm-(EO)n systems were found to demonstrate the complex influence of hydrophilic-hydrophobic balance on miscibility gap and mesophases [37]. An unambiguous description of the solution structure is difficult because size and shape of micellar aggregates can change with temperature and concentration. 

Self assembly of spheroidic aggregates leads in most cases to micellar cubic phases (Cub ) [30-35], where closed spheroidic aggregates are organized on a cubic 3D lattice (Fig. 2d,e).2... 

This approach proves that a phase diagram can be modeled when the solution microstructure is known (i.e., aggregation number and micellar aggregate number per unit volume) together with an experimental determination of the potential between aggregates. If the variation of the potential versus various parameters (metal salt in the organic phase) can be obtained experimentally, the limits of the phase separation can be reliably correlated with theory. 

This model which describes a phase transition naturally overemphasizes the co-operativity with respect to the micellization. The surprising monodispersity of various micellar aggregates and the constancy of the monomer activity support the co-operativity concept of the aggregational process. In its simplest form this model does not contain any size limiting step. The latter is principally independent of the coop-erativity which had to be included in a consideration of the formation of size limited aggregates. It is thus seen that this model can only be of restricted value towards an understanding of the formation of small particles, usually encountered in nonpolar solutions. 

Additives are usually amphiphilic in nature, and thus are either ionic or neutral surfactants or even polymers. The role of surfactants in solvent extraction is ambiguous. Usually, they should be avoided as they lower the interfacial tension, which may lead to emulsion formation in an agitated extractor. However, every metal-loaded ion exchanger is amphiphilic, and can adsorb at the interface or aggregate in the bulk phase. This occurrence is well known with sodium or other metals [17], and above a critical surfactant concentration (cmc, critical micelle concentration) micellar aggregates are formed. A dimensionless geometric parameter is decisive for the structure of the associates, according to Fig. 10.6 ... 

Micellar aggregates are considered in chapter 3 and a critical concentration is defined on the basis of a change in the shape of the size distribution of aggregates. This is followed by the examination, via a second order perturbation theory, of the phase behavior of a sterically stabilized non-aqueous colloidal dispersion containing free polymer molecules. This chapter is also concerned with the thermodynamic stability of microemulsions, which is treated via a new thermodynamic formalism. In addition, a molecular thermodynamics approach is suggested, which can predict the structural and compositional characteristics of microemulsions. Thermodynamic approaches similar to that used for microemulsions are applied to the phase transition in monolayers of insoluble surfactants and to lamellar liquid crystals. 

The basis for separation employing micellar mobile phases stems from their ability to differentially solubilize and bind structurally similar solutes. Skeptics view MLC as a fascinating example of the incorporation of secondary equilibria for control or adjustment of retention (101). However, it is the ultimate of secondary equilibria since the types of interactions possible with micellar aggregates cannot be duplicated by any single other equilibrium system, or for that matter, any one or mixture of traditional normal or reversed phase mobile phase systems. This is due to the fact that solutes can interact with the surfactant aggregates via hydrophobic, electrostatic, hydrogen bonding, and/or a combination of these factors. 

A micellar mobile phase can be viewed as being composed of both the surfactant micellar aggregates (pseudophase) and the rest of the... 

Gel Filtration. Micellar solutions have also been utilized in gel permeation (filtration) chromatography ( ] ). In fact, the first example of a separation which used a micellar mobile phase was in this area of exclusion liquid chromatography (ELC) ( 86). The last six entries in Table XI summarize some of the separations/work reported concerning micellar mobile phases in ELC. In most of these applications, the work was conducted with stationary phases of relatively small pore size. With these type phases, the relatively large micellar aggregates are confined to the excluded volume of the column and elute rapidly whereas smaller solute molecules in a mixture... 

The use of ELC to characterize micellar and related aggregates thus appears to be popular and useful. In fact, its use in this manner overshadows the analytical applications of micellar mobile phases to aid ELC separations. However, several recent reports do point out the advantages of micellar mobile phases in ELC (187 189) for the isolation and purification of bacterial and viral proteins. 

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