Nonmicellar phases

When a solute is added to a micellar solution, it distributes itself between the aqueous solvent and the micelles. To quantitate this distribution, the micellar partition coefficient or micellar distribution constant, > is defined as the ratio of the solute concentration in the micelles to the solute concentration in the nonmicellar phase. It is expressed by [30] ... 

A micellar phase is formed in the intestinal lumen when the bile salt concentration exceeds the critical micellar concentration (approximately 3-4 mM). This concentration of bile salts is usually exceeded during normal digestion. Mixed micelles contain bile salts, fatty acids, monoglycerides, cholesterol, and other lipid-soluble molecules (including fat-soluble vitamins) and are considered to be the major route of delivery of the products of fat digestion to the absorptive mucosal cell. Other nonmicellar phases may coexist in the intestinal lumen with the micellar phase these include an oil phase and a viscous isotropic phase. 

Hydrophobic ammonium ions which are phase transfer catalysts such as tri-n-octylalkylammonium ions (C8H17)3NR+X (R = Me, Et, CH2CH2OH X = Cl, Br, MeS03) are surface active but appear to form small nonmicellar aggregates (Okahata et al., 1977 Kunitake et al., 1980). The salts of these ions are only sparingly soluble in water, but they are very effective at speeding reactions of hydrophobic nucleophilic anions. 

FIGURE 3-4 Phase diagram showing location of hexagonal (Hi), normal bicontinuous cubic (V)), and lamellar (La) liquid phases, aqueous nonmicellar solution (W), micellar solution (Li), liquid surfactant containing water (L2), and solid surfactant (S). 

Figure 4.4 CTAB adsorption isotherms at 30°C on five different Hypersil phases. A) nonmicellar mobile phases B) micellar mobile phases.

Surfactant adsorption on the thin layer support cannot be avoided. It is the cause of a micellar gradient concentration between the solvent front and the mobile phase reservoir. Adsorption up to saturation of the sorbent depletes the surfactant concentration in the mobile phase. A double solvent front was observed, the upper one was a dilute non micellar surfactant solution, the lower second front corresponded to the micellar front [22-25, 27]. The problem is that this phenomenon introduces potential error in the identification of the exact Rf parameters of the solutes, inducing accuracy concerns of the K coefficients [28]. This adsorption-induced micellar concentration gradient was used to separate the polar solutes in the nonmicellar region (between the two solvent fronts) from the hydrophobic solutes separated by the slow moving micellar phase [22,27]. 

The foregoing discussions have regarded micelles as a chemical species, whereas many reports have been based on a pseudophase model and on the partition of reactants between the micellar phase and the intermicellar bulk phase. Recently, however, the failure of the pseudophase model has been demonstrated even for studies of micellar catalysts. Fendler and Fendler summarized data on the reaction rates for a wide variety of micellar systems on the basis of the studies performed up to 1975. During the past 15 years, the focus of interest has shifted from organic reactions to photochemistry in micellar systems (Chapter 12). Nevertheless, the effects of counterions and of nonmicellar aggregates of hydrophobic ion on micellar catalysis remain highly instructive as do those on micelle and submicelle formation. 

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