Size of the micelles

However, in the case of mini- and microemulsions, processing methods reduce the size of the monomer droplets close to the size of the micelle, leading to significant particle nucleation in the monomer droplets (17). Intense agitation, cosurfactant, and dilution are used to reduce monomer droplet size. Additives like cetyl alcohol are used to retard the diffusion of monomer from the droplets to the micelles, in order to further promote monomer droplet nucleation (18). The benefits of miniemulsions include faster reaction rates (19), improved shear stabiHty, and the control of particle size distributions to produce high soHds latices (20). 

The entrapment of a-chymotrypsin, lysozyme, and myehn in AOT-reversed micelles is accompanied by an increase in the micellar water content and in the size of the micelle. As a consequence of the redistribution of water among reversed micelles, the micellar solution results in being constituted by large protein-containing micelles and small unfilled ones [169],... 

Particle size of the micelle was determined by light scattering. Distance between the copper electrodes was maintained at 1 cm. After electrochemical deposition the electrode was removed from the cell and dried at 100° C for 10 min to remove water. Electrochemical deposition was done initially under galvanostatic conditions with the applied current density of 30 - 60 A per square foot, and then changed to potentiostatic conditions after achieving a required film thickness. 

Figure Bll.2.1 shows the normalized autocorrelation functions of various micelles loaded with octadecyl rhodamine B chloride (ODRB) at pH 7 (PBS buffer)3 . The differences in size of the micelles are clearly reflected by the differences in diffusion times td- The translational diffusion coefficients are reported in Table Bll.2.1, together with the hydrodynamic radii and the aggregation numbers.

Usually, activities of enzymes (hydrogenases included) are investigated in solutions with water as the solvent. However, enhancement of enzyme activity is sometimes described for non-aqueous or water-limiting surroundings, particular for hydrophobic (or oily) substrates. Ternary phase systems such as water-in-oil microemulsions are useful tools for investigations in this field. Microemulsions are prepared by dispersion of small amounts of water and surfactant in organic solvents. In these systems, small droplets of water (l-50nm in diameter) are surrounded by a monolayer of surfactant molecules (Fig. 9.15). The water pool inside the so-called reverse micelle represents a combination of properties of aqueous and non-aqueous environments. Enzymes entrapped inside reverse micelles depend in their catalytic activity on the size of the micelle, i.e. the water content of the system (at constant surfactant concentrations). 

Torchilin et al. synthesized an iodine-containing amphiphilic block-copolymer consisting of iodine-substituted poly-L-lysine which is able to form micelles in aqueous solution [37]. The two components of the block-copolymer were methoxy-poly(ethylene glycol) propionic acid (MPEG-PA) with a molecular weight of 12 kDa and poly[ ,M-(2,3,5-triiodobenzoyl)]-L-lysine. The particle size of the micelles was approx. 80 nm, and the iodine concentration was 20 mg mL . Biodistribution studies in rats showed significant and prolonged enhancement of the aorta, the liver and spleen. 

The size of the micelles is significantly increased by the addition of monomer up to a diameter of 4.5-5 nm. However, the size of the monomer droplets is stilt very much larger than that of the micelles (diameters up to 1 pm). In emulsion polymerization, one generally uses 0.5-5 wt% of emulsifier relative to monomer. With the usual oil-in-water emulsions, the water content varies from half to four times the amount of monomer. 

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. 

FIG. 8.5 Illustration of the geometric parameters for a surfactant and how they influence the size of the micelle. (Redrawn from Israelachvili 1991.)... 

Effect of Temperature. All of the bile salts in 0.15JV NaCl at pH 8 to 9 were studied at 20°, 36°, and in some cases at 4°C. (Figure 9). Trihydroxy bile salts which form small micelles (Ag 2 to 9) are unaffected between 4° and 36°C. Further, the small micelles formed by the dihydroxy bile salts in low salt concentrations are not affected by temperature. However, the sizes of the micelles of dihydroxy bile salts having aggregation numbers greater than 10 are decreased as the temperature is increased. 

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]

However, polyphosphazene micelles are known. These are formed by sonication of aqueous suspensions of amphiphilic diblock copolymers—macromolecules in which one block is hydrophilic and the other is hydrophobic. Both blocks may be phosphazene-based, or one block can be a phosphazene and the other an organic polymer. Polymers of this type are assembled via the living cationic polymerization discussed earlier. The polymers shown as 3.95-3.99 illustrate the range of structures that have been studied and the size of the micelles formed.237-240... 

In search of easy dispersible nucleating agents with a high number of nucleants per volume, Spitael et al. [19] investigated the use of nanoscale diblock copolymer micelles on the batch-foaming behavior of PS. The diblock copolymers were composed of a PS block, and either PDMS, PEP, or PMMA as a second block. Several factors were identified as essential for nucleation, e.g., the size of the micelle and the surface tension of the micelle core material. 

Laminar and cylindrical models, in contrast, provide no satisfactory mechanism by which the size of the micelles might be limited. 

It was proven that the carbonate-sulfonate hard-core RMs, but not the calcium carbonate powder, has an effect on acid neutralization and performance. The effective diameter of the micelles is 5 to 10 nm, and it is known that the smaller the size of the micelle, the greater its ability to neutralize acids. In order to develop high performance calcium carbonate in RMs, it is important to understand... 

Fig. 19 Effect of pH on the size of the micelles (PS core as a sphere, P2VP chains in blue, PEO chains in black) [92]...

The basic concept of the present study was to show, other things being equal, that the rate of polymerization is affected by the size of the micelles and not by the total surfactant concentration as expressed by Equation (l). This micellar size effect was believed to be the reason why a nonlinear, i.e., a convex curve, relationship between In Rp and In Cg was obtained with emulsion polymerization systems of changing surfactant... 

Since each of these 8 straight lines of the logarithmic plot of Rp vs Cg were obtained from a constant value of surfactant ratio, r, and thus a constant value of the size of the micelles, this series of experiments indicates that ... 

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