Micelles surface

In otlier words, tire micelle surface is not densely packed witli headgroups, but also comprises intennediate and end of chain segments of tire tailgroups. Such segments reasonably interact witli water, consistent witli dynamical measurements. Given tliat tire lifetime of individual surfactants in micelles is of tire order of microseconds and tliat of micelles is of tire order of milliseconds, it is clear tliat tire dynamical equilibria associated witli micellar stmctures is one tliat brings most segments of surfactant into contact witli water. The core of nonnal micelles probably remains fairly dry , however. 

At the end of the 1960s, Subba Rao et al. examined the influence of the interface on the CMC values [56]. They found a decrease in the CMC at the oil-water interface compared with the air-water interface. The CMC decreased by about 10% in the presence of heptane and by about 30-40% in the presence of benzene. The solubilization of the hydrocarbon in the micelle interior results in an increase in the micelle size and a slight change in the curvature of the micelle surface. The electrical potential and hence the electrical work of... 

But this static picture is clearly inadequate, because solutes and surfactant monomers move rapidly from water to micelles, and the surfactant head groups will oscillate about some mean position at the micelle surface (Aniansson, 1978). Non-ionic substrates are not localized within the micelle or its Stern layer and there is no reason to believe that they are distributed uniformly within the Stern layer. 

Calculations based on non-specific coulombic interactions between the micelle and its counterions gave reasonable values of a, which were insensitive to the concentration of added salt (Gunnarsson et al., 1980). Although these calculations do not explain the observed specificity of ion binding, they suggest that such hydrophilic ions as OH- and F- may not in fact enter the Stern layer, as is generally assumed. Instead they may cluster close to the micelle surface in the diffuse layer. 

The kinetics of organic reactions occurring at miceller surfaces formed from relatively simple surfactants have focused the attention of chemists because the reaction kinetics at micellar surfaces is an interface between physical... 

Murakami and Kondo (1975) reported that the cationic micelle is quite effective for the pyridoxal-catalyzed elimination of S-phenylcysteine. The significant rate acceleration was explained by the binding of the Schiff s base to the micelle phase, followed by the efficient proton abstraction by hydroxide ion at the micelle surface. According to Gani et al. (1978), mixed micelles of CTAB and dodecylamine hydrochloride are good models for the site accommodating pyridoxal 5 -phosphate in glycogen phosphorylase, since the micelles can imitate well the formation of SchifT s bases in hydrophobic environments. 

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... 

Firstly, the (negative) values of the NOE for residues of the unstructured N-terminus that do not interact with the DPC micelle surface are larger. This result is most probably due to increased saturation transfer from the water and results from increased exchange of amide protons at the used pH of 6.0 compared to that used in the absence of DPC (pH 3.1). Secondly, the values for residues from the C-terminal pentapeptide are negative in the case of NPY free in solution whereas they are positive in the micelle-bound form. This clearly indicates that the C-terminal pentapeptide is significantly rigidified upon binding to the micelle. The result is supported by the structure calculation that displays rather low RMSD values for that part... 

It is thought that the micelle surface is undulating to some degree, so it is not a perfect entity. However, it is described well by double-layer theory, which requires some order. It is not easy to describe a micelle as nobody has ever seen one it is something that is projected from all its properties. 

As mentioned above, a substantial part of the electrical charge of the micelle surface has been shown to be neutralized by the association of the counter ions with the micelle. In the calculation based on Equation 12, however, the loss in entropy arising from this counter ion association is not taken into account. This is by no means insignificant in comparison to of Equation 12 (4). A major part of the counter ions are condensed on the ionic micelle surface and counteract the electrical energy assigned to the amphiphilic ions on the micellar surface. The minor part of the counter ions,in the diffuse double layer, are also restricted to the vicinity of the micellar surface. 

A pseudophase ion exchange model has been applied to reactions in micellar systems with varying success (1-7). According to this model, the distribution of nucleophile is considered to depend on the ion-exchange equilibrium between the nucleophile and the surfactant counterion at the micelle surface. This leads to a dependence on the ion-exchange constant (K g) as well as on the degree of dissociation (a) of the surfactant counterion. The ion exchange (IE) model has recently been extended to oil in water microemulsions (8). 

Micellar catalytic methods were used to operate a choice between these two mechanisms. When an ion-radical has a charge opposite to that of the micelle surface, it is trapped by the micelle (Okamoto et al. 2001). In the presence of a surface-active compound, the aromatic substrate is nitrated in the very depth of a micelle, and the reaction rate depends on the local concentration of the nitrating agent on phase boundaries between the micelle and solution. A positively charged... 

MICELLAR CATALYSIS. Chemical reactions can be accelerated by concentrating reactants on a micelle surface or by creating a favorable interfacial electrostatic environment that increases reactivity. This phenomenon is generally referred to as micellar catalysis. As pointed out by Bunton, the term micellar catalysis is used loosely because enhancement of reactivity may actually result from a change in the equilibrium constant for a reversible reaction. Because catalysis is strictly viewed as an enhancement of rate without change in a reaction s thermodynamic parameters, one must exercise special care to distinguish between kinetic and equilibrium effects. This is particularly warranted when there is evidence of differential interactions of substrate and product with the micelle. Micelles composed of optically active detergent molecules can also display stereochemical action on substrates. ... 

When present in micelles, ester quats hydrolyze faster than free unimers in the bulk phase. This is due to an increased hydroxyl ion concentration around the micelle, i.e., the local pH in the vicinity of the micelle surface is higher than in the bulk. The phenomenon is referred to as micellar catalysis and is further discussed in the Betaine esters section. 

In a previous publication ( ), results were presented on the micellar properties of binary mixtures of surfactant solutions consisting of alkyldimethylamine oxide (C12 to Cig alkyl chains) and sodium dodecyl sulfate. It was reported that upon mixing, striking alteration in physical properties was observed, most notably in the viscosity, surface tension, and bulk pH values. These changes were attributed to 1) formation of elongated structures, 2) protonation of amine oxide molecules, and 3) adsorption of hydronium ions on the mixed micelle surface. In addition, possible solubilisation of a less soluble 1 1 complex, form between the protonated amine oxide and the long chain sulfate was also considered. 

The use of TMOS instead of TEOS did not lead to any cubic phase. This may be due to the different properties of methanol compared to ethanol. This solvent is highly polar and hydrophobic and does not penetrate the micelle surface [19]. 

Lowering the pH of milk to 4.6 solubilizes colloidal calcium phosphate. This removes its neutralizing effect, allowing electrostatic interactions between micelles. Under these conditions, micelles coagulate and precipitate from solution. Kudo (1980C) showed that release of whey proteins and K-casein from casein micelle surfaces as the pH is increased from 6.2 to 7.2 allows micelles to stick together and precipitate from solution. 

Most current models put K-casein on the outer casein micelle surface (Heth and Swaisgood 1982 McMahon and Brown 1984A Shahani 1974). This allows the possibility that heat-induced coagulation of milk is the result of serum proteins interacting with K-casein on the micelle surface and with each other to interconnect micelles. The observation that chymosin cannot release macropeptides from K-casein in heated milk (Morrissey 1969 Shalabi and Wheelock 1976,1977) suggests that... 

When milk at a pH of less than 6.5 is heated for 20 to 30 min at 100°C, it coagulates to form a gel. Casein micelles isolated from such milk have denatured whey protein attached to micelle surfaces (Creamer et al. 1978). Such micelle surfaces aggregates of whey pro-... 

Electron transfer can be accomplished by quenching of a micelle trapped chromophore by ions capable of ion pairing with the micelle surface. For example, excited N-methylphenothiazine in sodium dodecylsulfate (SDS) micelles can exchange electrons with Cu(II). The photogenerated Cu(I) is rapidly displaced by Cu(II) from the aqueous phase so that intramicellar recombination is averted, Fig. 5 (266). Similarly, the quantum yield for formation of the pyrene radical cation via electron transfer to Cu(II) increases with micellar complexation from 0.25 at 0.05 M SDS to 0.60 at 0.8 M SDS (267). The electron transfer quenching of triplet thionine by aniline is also accelerated in reverse micelles by this mechanism (268). 

Ion pairing with the polar micelle surface can induce pronounced effects on the observed excited state chemistry. 

The same physical interactions described above for micelle surfaces prove to be operative in vesicles, and depending on the arrangement of donors or acceptor among the four possible aggregates shown in Fig. 6, either accelerated electron exchange or stabilization of ion radical pairs can be observed (276). 

The parameter r2 is independent of the initiator type for the emulsion, however, and is slightly higher than that obtained in benzene (r2=1.23) (Table 3). This behavior results from good compatibility of the macromonomer with poly-BzMA. Therefore the reactivity of the macromonomer does not depend so much on the reaction medium type. In contrast, reversed apparent reactivity was observed in heptane in which the clear solution of monomer turned into a polymer suspension upon polymerization. Since BzMA is soluble in the medium, it has been suggested that the polymerization occurs preferentially on the (inverse) micelle surface which is enriched by the macromonomers. 

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