Mixed solvent micelles

Yang L, Alexandridis P (2000) Polyoxyalkylene block copolymers in formamide-water mixed solvents micelle formation and structure studied by SANS. Langmuir 16 4819-4829... 

Stop-flow experiments have been performed in order to study the kinetics of micellization, as illustrated by the work of Tuzar and coworkers on PS-PB diblocks and the parent PS-PB-PS triblocks [63]. In these experiments, the block copolymers are initially dissolved as unimers in a nonselective mixed solvent. The composition of the mixed solvent is then changed in order to trigger micellization, and the scattered light intensity is recorded as a function of time. The experiment is repeated in the reverse order, i.e., starting from the block copolymer micelles that are then disassembled by a change in the mixed solvent composition. The analysis of the experimental results revealed two distinct processes assigned as unimer-micelle equilibration at constant micelle concentration (fast process) and association-dissociation equilibration, accompanied by changes in micellar concentration (slow process). 

The spectra of substituted pyridinium iodides are characterized by charge transfer bands involving the interaction of pyridinium and iodide ions. Mukerjee and Rayt showed that this band is shifted about 90 nm toward the red for dodecyl pyridinium iodide, which forms micelles, compared to methyl pyridinium iodide, which does not. They measured max for the micelles in mixed solvents of variable relative dielectric constant and obtained the following results ... 

Secondly, the treatment of inhomogeneous system and clusters is straightforward. So far, the formulation does not assume the system homogeneity and the thermodynamic limit. The application to inhomogeneous and/or finite systems is then possible without modification. The binding of a molecule to such nanoscale structures as protein, micelle, and membrane can be viewed as a solvation in an inhomogeneous and finite, mixed solvent [57], The method of energy representation can thus be a... 

The micellar solution is described using the pseudophase model (1,8,9,10), which implies that the cmc corresponds to a maximum concentration of monomers in the bulk solution above which a second phase (the micelle phase) appears. This is a very crude model which has been discussed extensively in the literature. We use it merely because it is the only one tractable in the complex media we are dealing with (ions + micelles + mixed solvent). We may write in water,... 

Discussion of Thermodynamic Behavior of the Ionic Constituents of Micelles in Mixed Solvents... 

Variation of the Critical Micelle Concentration and Apparent Charge of the Micelles in the Mixed Solvents... 

CMCs of these polyfluorinated surfactants are of the order of 10 5 m.47 Plots of the observed H chemical shifts versus surfactant concentration of cetyl trimethyl ammonium chloride, cetyl pyridinium chloride, cetyl dimethyl phenyl ammonium chloride, cetyl dimethyl benzyl ammonium chloride, cetyl dimethy 1-2-phenyl ethyl ammonium chloride, and cetyl dimethyl-3-phenyl propyl ammonium chloride, are sigmoidal and were fitted to a model based on the mass action. The H chemical shift-based CMC values are in excellent agreement with those determined by the surface tension method.48 The micellization processes of dodecyl trimethyl ammonium halides (chloride and bromide) studied by calorimetric titration show different behaviors at 298 K. However, these disappear at 313 K, while the results measured by the chemical shift versus surfactant concentration do not show this difference.49 The CMC of 3-aminopropyl triethoxy silane in toluene is ca. 0.47m, measured by H and l3C chemical shifts.50 The CMC of optically active potassium A -n-dodecanoyl alaminate measured by H and l3C chemical shifts is lower (11-15 him) in D20 than that in a mixed solvent of 1,4-dioxane and D20 (19mM).-51 The H chemical shift shows that the CMC of resorcinol-type calix[4] phosphoric esters having four alkyl side-chains, [4]Ar 5P-R-n, is insensitive to the length of the side-chains, n.52 The CMC values of a family of surfactants, the sodium cyclohexyl alkanoates, with different lengths of the alkanoate side-chains, were obtained from 13C chemical-shift measurements. The results show that these amphiphiles have high CMCs (0.12-1.02 m).-53... 

The monoacylglycerol 3 is insoluble in water and has to be presented in micelles or lipid complexes (e.g. droplets) to be acceptable as lipase substrate. To generate the substrate, 3 was incorporated into mixed phospholipid micelles. To prepare these micelles, a solution of 3 in Cl I Cl, was added to a solution of phospholipids (phosphocholine, phosphoinositol) in a buffer system. The organic solvent was purged with nitrogen and the organic mixture was sonicated. To our surprise, the development of mixed micelles was accompanied by a color shift from yellow to red (Fig. 7.2, see p. 123). 

FIG. 6.1 Structural information obtained from SANS for an 8 wt% EO37PO58EO37 solution at 60°C plotted as a function of the cosolvent (glycerol, formamide, or ethanol) content in the mixed solvent. First row micellar association number ( association) second row radii of core and core + corona (Wcore and / micelle) third row polymer volume fraction of core and corona (acore and acorona). (From Alexandridis, P. and Yang, L., Macromolecules, 33, 5574, 2000.)... 

Tuzar and coworkers [294] investigated the micellization behavior of styrene-butadiene star-block copolymers with four arms and polybutadiene inner blocks in the mixed solvent tetrahydrofuran/ethanol, selective for polystyrene blocks. 

Spherical micelles with a polybutadiene core were formed for a certain range of compositions of the mixed solvent. The equilibrium between micelles and nonas-sociated macromolecules was found to be consistent with a closed association mechanism. In comparison with a linear triblock copolymer, the star-block sample showed a lower aggregation number. 

The Arriagada and Osseo-Asare chapter presents findings from hydrolysis reactions carried out in mixed solvent systems. Here reverse micelles are used to orchestrate the hydrolysis of tetraethyloithosUicate. 

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