Solubilization in Surfactant Micelles

The increased solubility of organic materials in aqueous surfactant solutions is a phenomenon that has found application in many scientific and technological areas. Only relatively recently has a good understanding of the stmcmral requirements for optimum solubilization begun to develop as a result of extensive experimental and theoretical work. Empiricism is slowly giving way to well-thought-out correlations between the requirements of a system and the chemical stmcture of surfactant that will provide the necessary environment to promote the solubilization process. 

When discussing a subject such as micellar solubilization, it is very important to define exactly what is meant by the term. As is often the case, there is some 

For present purposes, solubilization is defined as a spontaneous process leading to a thermodynamically stable, isotropic solution of a substance (the additive) normally insoluble or only slightly soluble in a given solvent produced by the addition of one or more amphiphilic compounds, including polymers, at or above their critical micelle concentration. Using such a definition, we can cover a broad area that includes both dilute and concentrated surfactant solutions, aqueous and nonaqueous solvents, all classes of surfactants and additives, and the effects of complex interactions such as mixed micelle formation. It does not, however, limit the phenomenon to any single mechanism of action. 

The history of solubilization research in the first half of the twentieth century has beeu extensively reviewed, and several pertinent references are listed in the Bibliography. A discussion of some important results can be found in the work of Elworthy, et. al., which also includes a description of many of the experimental techniques that have been developed for investigations into the factors affecting the process. 

Before addressing some of the specific aspects of the influence of surfactant structure on solubilization, it will be useful to understand the geography of solu-bihzation—that is, the possible positions in (or on) the micelle that can serve as host sites for the additive molecules and the factors that determine exactly where solubifization will occur. 

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Kabanov, A.V., et al. 1989. The neuroleptic activity of haloperidol increases after its solubilization in surfactant micelles. Micelles as microcontainers for drug targeting. FEBS Lett 258 343. 

When using an alcohol/surfactant solution to modify the density of DNAPLs, several processes occur simultaneously (Figure 2). Perhaps the most important process is alcohol partitioning, where alcohol which is initially solubilized in surfactant micelles partitions into the DNAPL,... 

When nonionic surfactant is applied to a soil-aqueous system, the surfactant can exist as dissolved monomers, sorbed molecules on the soil, or aggregated groups of molecules called micelles. Molecules of HOCs in such a system can be solubilized in surfactant micelles, dissolved in the surrounding solution, sorbed directly on the soil, or sorbed in association with sorbed surfactant. The presence of nonionic surfactant micelles in the bulk solution of the system results in the partitioning of the HOC between two bulk solution compartments, commonly referred to as pseudophases. The micellar pseudophase consists of the hydrophobic interiors of surfactant micelles, whereas the aqueous pseudophase consists mainly of dissolved surfactant monomers and water. Micelles form when the bulk solution concentration exceeds the surfactant CMC. 

Organic Materials. Qrganic materials that have low water solubihty can be solubilized in micelles to produce systems with substantial organic content where no solubility would occur in the absence of micelles. More details on the phenomenon of solubilization in surfactant micelles will be presented below. In any case it is usually found that immiscible hydrophobic materials will have relatively little effect on cmc, although evidence for shght decreases has been reported. 

The quantity of a substance that can be solubilized in surfactant micelles will depend on many factors, some of which have already been discussed. From the standpoint of the additive itself, such factors as molecular size and shape, polarity, branching, and the electronegativity of constituent atoms have all been found to be of some significance, depending on the exact system. One extensively explored factor relating the chemical structure of the additive to its solubilization is the relationship between the molar volume of the additive and the maximum amount of material that can be incorporated in a given surfactant solution. In general, one finds an inverse relationship between the molecular volume of the additive and the amount of material solubilized. 

Substances that are solubilized in surfactant micelles can be separated by ultrafiltration through membranes whose pores are smaller in diameter than the micelle size. For a membrane molecular weight cutoff from 1 to 50kDa, the rejections are 98%. The stream of water-containing monomeric molecules of surfactant (permeate) flows through the membrane. The remaining solution (retentate) contains solutes solubilized in micelles. The MEUF process is used for the separation of organic substances and various ions, the latter after their previous complexation. 

Differences in micellar uptake would affect the bactericidal effect of the chlorhexidine formulation the choice of salt and surfactant must, therefore, involve a careful analysis of intrinsic solubilities and activities of the salts and their percentage solubilization in surfactant micelles. 

When polymerizable monomers are solubilized in surfactant micelles it should be possible to produce polymerized micelles containing co-solubilized drug. The process has been successfully achieved by Speiser [186]. Fig. 11.29 represents an inverse micelle containing a water-soluble drug in which the monomeric species is located in the outer part of the micelle and is cross-linked to form a "nanocapsule ... 

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