Micelles micelles

Further addition of fatty acid eventually results in the formation of micelles. Micelles formed from an amphipathic lipid in water position the hydrophobic tails in the center of the lipid aggregation with the polar head groups facing outward. Amphipathic molecules that form micelles are characterized by a unique critical micelle concentration, or CMC. Below the CMC, individual lipid molecules predominate. Nearly all the lipid added above the CMC, however, spontaneously forms micelles. Micelles are the preferred form of aggregation in water for detergents and soaps. Some typical CMC values are listed in Figure 9.3. 

It has been proposed that the overlapping of the surfactant hydrocarbon tails is mainly responsible for the micelle-micelle interactions [247]. However, since tail-tail interactions are of the same order of magnitude as tail-apolar solvent interactions, it seems more reasonable to consider the overlapping of the surfactant hydrocarbon tails as an effect rather than the origin of the micelle-micelle interactions. 

Menger and Portnoy (1967) developed a quantitative treatment which adequately described inhibition of ester saponification by anionic micelles. Micelles bound hydrophobic esters, and anionic micelles excluded hydroxide ion, and so inhibited the reaction, whereas cationic micelles speeded saponification by attracting hydroxide ion (Menger, 1979b). 

Recent progress in novel micellar structures, including micelles containing exotic blocks such as natural or synthetic polypeptides and metal-containing segments, micelles from ABC triblock copolymers, Janus micelles and other noncentrosymmetric micelles, micelles based on interpolyelectrolyte or other noncovalent complexes, and metallosupramolecular micelles, will be discussed in Sect. 7. 

At their critical micelle concentrations, surface active agents (such as sodium dodecyl sulfate, Triton X-100, lysolecithin, and bile salts) self-associate into spherical or rod-shaped structures. Because dilution to below the c.m.c. results in rapid disassembly or dissolution of these detergent micelles, micelles are in dynamic equilibrium with other dissolved detergent molecules in the bulk solution. 

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. ... 

In summary, whether a reaction equilibrium or a phase equilibrium approach is adopted depends on the size of the micelles formed. In aqueous systems the phase equilibrium model is generally used. In Section 8.5 we see that thermodynamic analyses based on either model merge as n increases. Since a degree of approximation is introduced by using the phase equilibrium model to describe micellization, micelles are sometimes called pseudophases. 

This slowing of the solvolysis reaction by the alkyl sulfate requires that 5 be almost completely imprisoned by the micelles, because that part of 5 free in water would hydrolyze rapidly. An important result is in the stereochemistry of the reaction, which changes from 100% inversion with optically active 5 in pure water to only 56% inversion in the micelles. Micelles of the opposite... 

An emulsifier is a molecule that possesses both polar and nonpolar moieties, i.e., it is amphiphilic. In very dilute water solutions, emulsifiers dissolve and exist as monomers, but when their concentration exceeds a certain minimum, the so-called critical micelle concentration (CMC), they associate spontaneously to form aggregates - micelles. Micelles are responsible for many of the processes such as enhancement of the solubility of organic compounds in water, catalysis of many reactions, alteration of reaction pathways, rates and equilibria, reaction loci for the production of polymers, etc. 

Soo et al. (2002) studied tliB vitro release of hydrophobic Luorescent probes from PEO-b PCL micelles. Micelle solutions were placed in dialysis bags (MWCO 50,000) in a stirred water bath with a constant overLow of distilled water. This maintained the release environment at near perfect sink conditions, so the limited solubility ofthe probes in the medium did not affect release kinetics. Release was determined by removing aliquots ofthe dialysis bag contents and measuring Luorescently. Soo et al. found an initial burst release of probe followed by slow diffusional release. For the probes studies, benzopyrene and Cell-Tracker-CM-Dil, diffusion constants were ofthe order 10"15 cnnP/s. 

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. 

The oriented gel consists of micelles, of which the average diameter is about 80 A. Micelle-micelle distance is about 120 A. Area between micelles is filled with water molecules. Micelle interior is packed mostly by 7/1 single helical molecules that are hydrogen-bonded to one another by water molecules. The single helical molecule is also probably hydrated. Some parts of the micelle are occupied by molcules of triple-stranded helix, which are also hydrated to some extent. 

The electrostatic effects are influenced by the micelle concentration. This effect can be viewed as a micelle-micelle interaction mediated by counterions. The most direct way for modelling the finite micelle concentration is to confine the volume per micelle by an outer radius if of finite size298 300). This is called the cell model. 

This micelle is comprised of surfactant molecules consisting of long hydrocarbon tails attached to an anionic lyophilic group. Typically, there are 50-100 molecules in the micelle. Some counterions in the medium are adsorbed on the aggregate, whereas others form the diffuse ionic environment. Some workers believe that there is considerable penetration of the medium into the micelle. Micelles are important for detergent action, with oily dirt particles dissolved in the hydrocarbon interior of the micelle. 

Only a small quantity of an amphiphilic lipid dispersed in water can form a monolayer (unless the water is spread as a very thin film), in which case the bulk of the lipid will form soluble micelles. Micelles can take a variety of forms, each satisfying the hydrophobic effect. Fig. 6-2 shows one such form, representing a spherical micelle, although ellipsoidal, diskoidal, and cylindrical variations are possible. 

Polymeric micelles Micelles consisting of amphiphilic polymers Loading hydrophobic drugs in the core for solubilization, targeted delivery, and controlled release 35,36... 

The rate constants for reactions of all-trans retinal and retinol with have been measured in various solvents. Rate constants increase with dielectric constant, thereby suggesting that charge transfer is involved. Electronically excited triplet species generated during peroxide-catalysed aerobic oxidation of substrates gives rise to fluorescence from acceptors in micelles. Micelle-solubilized chlorophyll is an excellent detector of enzyme-generated triplet carbonyl species, and micelles make photochemistry without light possible for... 

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