Loose micelles

ELP-based triblock copolypeptides have also been used to produce stimulus-responsive micelles, and Chaikof and coworkers envisioned the possible application of these micelles as controlled drug delivery vehicles. These amphiphilic triblock copolymers were constructed from two identical hydrophobic ELP endblocks and a hydrophilic ELP midblock. Below the transition temperature, loose and monodispersed micelles were formed that reversibly contracted upon heating, leading to more compact micelles with a reduced size [90]. 

First, micelles have very loose, mobile, structures and there are considerable entropy costs in a concerted reaction. These costs are much less serious in enzymic systems where conformation at the active site is tailor-made to fit the transition state. Secondly, the sites of micellar reactions are very wet and omnipresent water molecules are available to transfer protons. 

An interesting extension of aqueous solution radiolysis involved solutions of sodium dodecyl sulphate in the presence of MNP. Spin adducts of secondary alkyl radicals were detected provided that the critical micelle concentration of the surfactant was exceeded. Whilst it was rather loosely concluded that there is a marked catalytic effect of micelles on the rates of reaction of radicals with nitroso spin traps , no single origin of this effect could be clearly identified (Bakalik and Thomas, 1977). 

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. ... 

PS-PI-PS Mw = 165, 27% Heptane Observation of loose branched structures rather than micelles SLS, DLS, SANS, viscometry Raspaud el al. (1994)... 

The aggregation numbers Nagg is determined as 27 for C1-(EO)53-C4-VB and 38 for Cr(EO)53-C7-VB micelles by analysis of fluorescence curves. A micelle formation mechanism is proposed for nonionic polymeric surfactants with weakly hydrophobic groups. At low concentrations of PEO macromonomers, large loosely aggregated structures involving the PEO chains are formed. At higher concentrations normal micelles form. These are star-shaped, with a hydrophobic core surrounded by a corona of PEO chains. 

At very low concentrations, the polymers only exist as single chains. As the concentration increases to reach a critical value, called the critical micelle concentration (CMC), polymer chains start to associate to form micelles in such a way that the hydrophobic segment of the copolymer will avoid contact with the aqueous media in that the polymer is diluted. At the CMC, a signiLcant amount of solvent may still be found inside the micellar core and micelles are described as loose aggregates that exhibit largersize than micelles formed at higher concentrations (Gao and Eisenberg,... 

It is also possible to determine simultaneously by GPC the micelle molecular weight and aggregation number. It is important to ensure that the integrity of polymeric micelles during their elution through the size exclusion column is maintained. Adsorption of the polymer on the column may prove to be a problem (Yokoyama et al., 1993), especially at concentrations close to the CMC, where micelles consist of large loose aggregates (Jones and Leroux, 1999). 

Surfactant micelles Above the stoichiometric line, loose necklaces are formed where all the excess length of polymer Is In the bridges (19-21). Below the stoichiometric line, the necklaces are overloaded with bound spheres, and excess spheres are rejected they coexist In the solution with the tight necklaces. No phase separation Is observed. 

Micelles are loose aggregates of amphiphiles in water or organic solvents which form above a certain temperature (Krafft point) and concentration (critical micellar concentration, cmc). Below the Krafft temperature, clear micellar solutions become turbid and the amphiphile forms three-dimensional hydrated crystals. Below the cmc, micelles dissociate into monomers and small aggregates. Above the cmc, the micelles of an aggregation number n are formed n then remains stable over a wide concentration range . Table 1 gives some typical cmcs and three Krafft point values. 

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