Micelles surface available

Detergents commonly used to form micelles that are amenable to high-resolution NMR are summarized in Tab. 5.2, and the chemical structures of the most commonly used detergents are presented in Fig. 5.2. Unfortunately, only a few of those needed for use with nonisotopically enriched peptides are commercially available in deuterated form. Most frequently, the zwitterionic DPC or the negatively charged SDS have been used as membrane mimetics. Mixtures of DPC doped with small amounts of SDS may be used to modulate the charge distribution on the micelle surface. It should be emphasized here... 

Secondary Micelles. These micelles are formed only by dihydroxy bile salts in the presence of increased counterions. Secondary micelles are probably formed by the aggregation of primary micelles. Since the surface available for hydrophobic interaction is expended in the formation of the primary micelles, the bonding that takes place is probably between some of the hydrophilic parts of the bile salts. It is suggested (Figure 11c) that in the presence of increased counterion concentrations... 

The surface available in micelles may be easily calculated. Table XI shows the ratio of the interfacial area of the micelles to that of the emulsion droplets. For a drop size 3 or greater, this ratio, designated by B, is clearly in favor of the micelles. Calculation (56) was effected using the data of the standard formula above. 

Fig. 10.9 pH-dependent biotin availability for binding, (a) Above pH 7.0, biotin anchored on the micelle core via pH-sensitive PHis is shielded by PEG shell of the micelle, (b) Biotin is exposed on the micelle surface at acidic conditions (6.5 < pH < 7.0) and can interact with cells, which facilitates biotin-receptor-mediated endocytosis. When the pH is further lowered (pH < 6.5), the micelle destabilizes, which enhances drug release, (modified and reproduced from [154], with permission from American Chemical Society)... 

It can be observed from the results that soil composition has a profound influence on the desorption and solubilization behaviors of phenanthrene. It is much more difficult to desorb and solubilize phenanthrene from glacial till than from kaolin, probably due to the strong binding characteristics of phenanthrene to organic matter. The hydrophilicity and concentration of the surfactant/cosolvent affects phenanthrene desorption and solubilization significantly. As hydrophilic surfactants are less prone to sorption onto soil particle surfaces, more surfactant molecules are available for micellar solubilization, resulting in better performance. In addition, more micelles are available for phenanthrene solubilization at higher surfactant concentrations. 

By contrast the particle size distribution ca the end of Interval I for styrene with a variety of emulsifiers proves to be skewed to small particle sizes [94] consistent with the rate of particle nucleation being an increasing function of time which suggested that the particles observed have been formed by the coalescence of smaller, unstable precursor particles. It has generally been assumed that a saturated monolayer of adsorbed emulsifier would be maintained on the surface of the latex particles so long as a supply of emulsifier from the residual micelles is available during Interval I which would prevent any coalescence of... 

When nonsurfactant solutes (electrolytes, etc.) are added to the micellar reaction mixture, the results can be quite unpredictable. It is often found that the presence of excess surfactant counterions (common ions), when added to a system in which an ionic reactant is involved, retards the catalytic activity of the micelle, with larger ions being more effective in that respect. The effect can probably be attributed to an increase in ion pairing at the micelle surface and a reduction of its attractiveness to charged reactants. In contrast, the addition of neutral electrolyte has been found to enhance micellar catalysis in some instances. It has been proposed that the retardation effect of excess common counterions is due to a competition between the excess ions and the reactive substrate most closely associated with the micelle for the available positions or binding sites on or in the micelle. The enhancing effect, however, has been attributed to the more general effects of added electrolyte on... 

Reactions Catalysed by Simple Cationic Micelles.— There have been important recent developments in our understanding of nucleophilic catalysis in micelles. The main credit in this must go to Romsted, whose PhD thesis " contains the analysis now generally accepted to be the best available description, and which is more readily accessible in a review article. It is a pseudo-phase model, which is superior to earlier approaches because it makes specific allowance for ion-dissociation and ion-exchange at the micelle surface. The derived equation for a second-order reaction is ... 

A detailed analysis of the results suggests that when the ligands are associated with the micelle they are entirely available for reaction at the micelle surface. The kinetic results also indicate that the ligand is not involved in the rate-limiting step for reaction on the micelle surface. The rate-limiting step is, in fact, associated with release of a water molecule from the solvation shell of the metal ion prior to complexation with the ligand. The rate constant for this process is similar to that measured in aqueous solution in the absence of micelles. 

The maximum in the curve in Fig. 11.12 corresponds with the CMC in the system. The decrease in rate has been attributed by Holzwarth et aL [87] to the dilution of the reactants over a greater available surface as the concentration of NaLS is increased the decrease would confirm the notion that the reaction only took place at the micelle surface. Some experimental difficulties centred around batch variation in the surfactant. Sodium lauryl sulphate is notoriously difficult to purify to an extent to guarantee pure surfaces [88] while it will be readily purified sufficiently well for the measurement of bulk properties, reactions at the micellar surface would be sensitive to impurities. 

Ki is Ki just below the collector s critical micelle concentration, Cs . Kj is Ki at some higher collector concentration, C. E is the relative effectiveness, in adsorbing colligend, of surface collector versus micellar collector. Generally, > 1. F, is the surface excess of collector. More about each K is available [Eemlich, Adsubble Methods, in Li (ed.), Beeent Developments in Separation Seienee, vol. 1, CRC Press, Cleveland, 1972, pp. 113-127 Jashnani and Eemlich, Ind. Eng. Chem. Proeess Des. Dev., 12, 312 (1973)]. 

We have been developing methods to prepare and characterize supported attune catalysts nsing readily available commercial snpports. One potential means of depositing amines on oxide surfaces is shown in Scheme 38.1, in which the micelle s role is to space the amines on the snrface. Cnrrent work is directed towards characterizing these samples, particularly applying flnorescence resonance energy transfer (FRET) techniques. 

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