Solvent micelles with

Diblock copolymers consist of contiguous sequences of two different covalently bound monomer units, arranged in an -A-A-A-B-B-B-B- structure. In an appropriate solvent, the diblock copolymers spontaneously self-assemble into micelles with cores which are essentially pure in one component and a diameter... 

More recently Frechet and Gitsov [130] used a similar approach as above and synthesized a novel series of dendritic copolymers derived from a central penta-erythritol core unit. These hybrid star molecules behaved as unimolecular micelles with different core-shell conformational-structures as a response to the polarity of the solvent used. 

In terms of contrast, Pluronic micelles have a much more higher contrast using SANS (with D20 as a solvent) than with SAXS. It is illustrated in figure 5, where the SANS and SAXS curves in absolute units are compared for the same sample, micelles of P123 in pure D20 at 40°C and at a volume fraction of 2.6 %. The absolute intensity of the SANS curve is about 103 times greater than the SAXS ones. The maximal flux on the D22 experiment is about 10s neutrons/s. Then, in order to perform kinetics experiments with SAXS, with a temporal resolution equal or better than 30 s, the high flux of a synchrotron source (10nphotons/s or more) is needed. 

Hartley showed that micellar effects upon acid-base indicator equilibria could be related to the ability of anionic micelles to attract, and cationic micelles to repel, hydrogen ions. More recently attempts have been made to quantify these ideas in terms of the behavior of a micelle as a submicroscopic solvent, together with an effect due to its surface potential (Fernandez and Fromherz, 1977). 

Micelle formation in standard organic solvents such as toluene or THF is very useful, since fluorinated polymers are usually not soluble in standard solvents micelle formation therefore enables processing of fluorinated products with classical technologies, e.g., for coating applications. 

Micelles from PS-P2VP-PMMA triblock copolymers dissolved in toluene were reported by Tsitsilianis and Sfika [288]. Since the organic solvent was selective for both the PS and PMMA blocks, these authors observed the formation of spherical micelles with a dense P2VP core, surrounded by PS and PMMA chains in the corona. It was shown that Z and the micellar size were strongly influenced by the length of the P2VP middle block. 

It has also been shown [254] that a commercial petroleum sulfonate surfactant which consists of a diverse admixture of monomers does not exhibit behavior typically associated with micelle formation (i.e., a sharp inflection of solvent properties as the concentration of surfactant reaches CMC). These surfactants exhibit gradual change in solvent behavior with added surfactant. This gradual solubility enhancement indicates that micelle formation is a gradual process instead of a single event (i. e., CMC does not exist as a unique point, rather it is a continuous function of molecular properties). This type of surfactant can represent humic material in water, and may indicate that DHS form molecular aggregates in solution, which comprise an important third phase in the aqueous environment. This phase can affect an increase in the apparent solubility of very hydrophobic chemicals. 

Rubredoxin is an electron-transfer protein with an Fe(IlI)/Fe(lI) redox couple at -0.31 V (SCE) in water (20). Our peptide model, [Fe( Cys-Pro-Leu-Cys-OMe)2] (Z = benzyloxycarbonyl) (21) exhibits its Fe(lll)/Fe(ll) redox couple at -0.50 V (SCE) in Mc2SO (9). This is similar to the value observed for the native protein when the difference of the solvent is taken into account. When the model complex is solubilized in water by formation of micelles with addition of the non-ionic detergent, Triton X-KX), we also observed a quasi-reversible redox couple at -0.37 V (SCE) (5). The Fe(lll) complexes of Cys-X-Y-Cys peptides also exhibit a characteristic MCD band at 350 nm due to ligand-to-metal charge transfer which has also been found in oxidized rubredoxin (4). 

It is well known that AB diblock copolymers form micelles in solvents that are selective for one of the blocks. By varying the nature of the solvent, it is also possible to form micelles with the A block in the core or with the B block in the core. However, we have recently demonstrated that certain hydrophilic AB diblock copolymers can form either A-core micelles or B-core micelles in aqueous media. In the original example, both blocks were based on tertiary amine methacrylates and the diblock copolymer was prepared by group transfer polymerisation, a special type of anionic polymerisation which is particularly... 

In case of lipases, one of the simplest methods to combine an enzyme with an organic solvent is to coat the lipase with a lipid or surfactant layer before lyophilisation. It is estimated that about 150 surfactant molecules are sufficient for encapsulating one lipase molecule. Following this route the surfactant coated lipase forms reverse micelles with a minimum of water concentration. The modified lipases are soluble in most organic solvents, and the reaction rates are increased compared to the suspended hpases due to the interfacial activation [59,60]. 

To this point, only models based on the pseudo—phase separation model have been discussed. Mixed micelle models utilizing the mass action model may be necessary for micelles with small aggregation numbers, as demonstrated by Kamrath and Franses ( ). However, even for large micelles, the fundamental basis for the pseudophase separation model needs to be examined. In micelles, how much solvent or how many counterions (bound or in the electrical double layer) should be included in the micellar pseudo-phase is unclear. The difficulty is normally surmounted by assuming that the pseudo—phase consists of only the surfactant components i.e., solvent or counterions are ignored. The validity of treating the micelle on a surfactant—oniy basis has not been verified. Funasaki and Hada (22) have questioned the thermodynamic consistency of such an approach. 

Computer simulations of a range of properties of block copolymer micelles have been performed by Mattice and co-workers.These simulations have been based on bead models for copolymer chains on a cubic lattice. Types of allowed moves for bead chains are illustrated in Fig. 3.27. The formation of micelles by diblock copolymers under weak segregation conditions was simulated with pairwise interactions between A and B beads and between the A bead and vacant sites occupied by solvent, S (Wang et al. 19936). This leads to the formation of micelles with a B core. The cmc was found to depend strongly on fVB and % = x.w = %AS. In the range 3 < (xlz)N < 6, where z is the lattice constant, the cmc was found to be exponentially dependent onIt was found than in the micelles the insoluble block is slightly collapsed, and that the soluble block becomes stretched as Na increases, with [Pg.178]

Block or graft copolymers in a selective solvent can form structures due to their amphiphilic nature. Above the critical micelle concentration (CMC), the free energy of the system is lower if the block copolymers associate into micelles rather than remain dispersed as single chains. Often the micelles are spherical, with a compact core of insoluble polymer chains surrounded by a corona of soluble chains (blocks) [56]. Addition of a solvent compatible with the insoluble blocks (chains) and immiscible with the continuous phase leads to the formation of swollen micelles or polymeric micro emulsion. The presence of insoluble polymer can be responsible for anomalous micelles. 

FIGURE 13.3 Sketch for the structure of a diblock copolymer micelle with a molten liquid core. The core is free of solvent or tail monomers. (Reproduced from Marques, C. M. iaa7ymuir13 1430-1433. VMth permission from American Chemical Society.)... 

Xing and Mattice (1998) applied Monte Carlo simulation techniques to study the models of BAB triblock copolymeric micelles with solubilizates in a selected solvent. They focused on a microscopic picture regarding the locus of solubilizates in BAB triblock copolymer micelles when the... 

H-nuclear magnetic resonance (NMR) also provides some information on the viscosity of the micellar core (Jones and Leroux, 1999). The copolymers are usually dissolv Dirafiti in a solvent where micelle formation is not expected and where all the peaks proper to the hydrophilic and hydrophobic part of the polymer can be detected (e.g., DI0ID2O, the presence of micelles with a highly inner viscous state result in a restricted motion of the protons within the micellar core as demonstrated by the weak signals associated with the hydrophobic part of the copolymer (Nakamura, et al., 1977 Bahadur et al., 1988). 

Dan, N. and M. Tirrell. 1993. Self-assembly of block copolymers with strongly charged and a hydrophobic block in a selective, polar solvent. Micelles and adsorbed la ifasromolecule 6 4310—4315. 

Nagarajan, R. and K. Ganesh. 1989. Block copolymer self-assembly in selective solvents spherical micelles with segregated cored. Chem. Phys. 90 5843-5856. 

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