Micelles clustering

Systematic variation in the water temperature, (WW), will produce a profile reflecting this influence. Vary the / b(WW) and J(WW) values in Example 5.3 to simulate different water temperatures. Run the dynamics for these different water temperatures to observe its influence. Note whether this is a linear or nonlinear effect on the cluster size. The structures formed may be quantified by recording the average micelle cluster size. The typical pattern looks like the examples in Figure 5.5. 

The rate of aggregation of fully renneted micelles is very sensitive to temperature. At room temperature it is appreciably less than the diffusional collision rate, which led Payens (1977) to consider the possibility that only a fraction of the surface is reactive (so-called hot spots). The idea of hot spots is consistent with the low fractal dimension of micelle clusters formed during renneting and leads to only a proportion of all encounters between fully renneted micelles being successful. In effect, a statistical prefactor is included in the reaction kernel to reduce the diffusion rate to a level comparable with experiment. However, Payens developed the idea of hot spots only within his theory of the aggregation of fully renneted micelles. 

Clearly, much work remains to be done to fully explain the effects of continuous-phase density and the extent of micelle clustering, on both the rate of polymerization and the molecular weight of the polymer formed via this process. 

Each ion moves as an independent entity - but see discussion below of ion pairing and micelle clustering. 

Micelle clusters in equilibrium with free ions... 

There has been very little work completed in reverse micelles. This may be due to the increased complexity of these systems, where probe migration and micelle clustering occurs on time scales similar to triplet decay. In addition, litde work has been carried out in vesicles or liposomes. With the advent of diffuse reflectance laser flash photolysis, the turbid solutions observed for large vesicles no longer represent a stumbling block [196-198]. Thus, we believe that, in the future, much more work will be available in vesicular systems that will be able to complement the studies already completed in micelles. 

The apparent hydrodynamic diameters of the droplets (or the correlation length), as calculated using the Stokes-Einstein equation for a number of different systems, are given in Table 2. These early findings showed that the micelle sizes measured in near-critical and supercritical solutions were similar to those found for conventional water-in-oil microemulsions in liquid alkane. At lower fluid densities, DLS probes the combined effect of the collective diffusion coefficient of the micelle cluster and that of the individual micelles. 

For those systems near a phase transition, the apparent hydrodynamic diameter of the droplets (or the correlation length), as calculated using the Stokes-Einstein equation, appears to decrease as pressure increases [2,4,39]. For example, the apparent hydrodynamic diameter of a microemulsion droplet (for [surfactant] = 150 mM and 5) in supercritical xenon [2] decreases from 6.5 to 4.5 nm as pressure is increased from 350 to 550 bar (10 bar = 1 MPa). This effect is due to the change in the extent of micelle clustering rather than an actual change in the micelle size. 

Beckman et al. observed an effect of the secondary microemulsion structure on the molecular weight and yield of the polymer. Under conditions where extensive micelle-micelle clustering occurred, at lower fluid density the molecular weight of the polymer was as much as two times higher. Thus, the density of the supercritical phase could be used to control the polymer morphology. Beckman and Smith also completed an extensive study [74] of the effect that acrylamide, surfactant, and water concentrations as well as the pressure and temperature had on the phase stability of the microemulsions. The phase behavior of these systems depends on the choice of operating parameters, and this behavior can be exploited to optimize the properties of the polymer. 

A key enabler often employed in the synthesis of zeolites is the template, often called an organic directing agent. The template type is frequently different for microporous zeolites, mesoporous materials, and macroporous materials. The template can be an individual molecule (e.g., quaternary salts or linear amines), in-situ formed micelle clusters, or preformed structures (e.g., polyethylene spheres). 

Amphiphilic (PsCL)i4(PAA)7 miktoarm star copolymers having a p-cyclodextrine core were prepared by ROP and ATRP techniques. Thermodynamically stable micelles were obtained in aqueous solutions. A variety of stmaures, such as spherical micelles, clusters, and wormlike aggregates, were observed by dynamic LS and AFM measurements. Both the copolymer architecture and the composition affea the morphology and the dimensions of the aggregates. 

X. Z. Qu et ah, Carbohydrate-based micelle clusters which enhance hydrophobic drug bioavailability by up to 1 order of magnitude. Biontacromolecules, 7(12), 3452-3459 (2006). 

One of the rare works on the kinetics of the formation of mixed micelles of Pluronics and surfactants also treats the system L64/SDS [58]. In temperature-jump experiments, the authors identified three different relaxation times for the L64/SDS mixture. The fastest ps) is associated with the binding of additional L64 unimers to the micelles. The two slower relaxation processes are interpreted as structural rearrangement of the mixed micelles and micelle clustering. These findings are qualitatively in agreement with the first kinetic investigation of these mixtures by Hecht and Hoffmann [59]. 

A quantitative analysis of the X-ray intensity scattered by a monodomain of compound lb-2 [10, 25] led to four possible models of the primitive cubic lattice built up by micellar assemblies. Two of them are described by linked spherical micelles clustering around the body-center and face-center positions. In the two other models, the electron density is concentrated in spherical regions at the comer and in the body-center of the cell, as well as in the faces along nonintersecting rods in the [100], [010], and [001] directions. 

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