Micellar aggregates shape

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Micelles are extremely dynamic aggregates. Ultrasonic, temperature and pressure jump techniques have been employed to study various equilibrium constants. Rates of uptake of monomers into micellar aggregates are close to diffusion-controlled306. The residence times of the individual surfactant molecules in the aggregate are typically in the order of 1-10 microseconds307, whereas the lifetime of the micellar entity is about 1-100 miliseconds307. Factors that lower the critical micelle concentration usually increase the lifetimes of the micelles as well as the residence times of the surfactant molecules in the micelle. Due to these dynamics, the size and shape of micelles are subject to appreciable structural fluctuations. 

Arborol Dendrimers. The spherical topology of dendrimers resembles the size and shape of the Hartley model (1936) of a micellar aggregate formed by surfactant molecules (Fig. 11.2 Tomalia et al. 1990). Micellar stmctuies have a dynamic, spherical structure possessing a close-packed, solvent-incompatible core surrounded by an open, solvent-compatible layer. 

Since their effective diffusivities are of the same magnitude as those of micellar solutions, the hquid crystalUne phases, though viscous, do not significantly hinder surfactant dissolution for these rather hydrophihc surfactants. Indeed, a drop of Ci2(EO)6 having Ro = 78 pm dissolved completely in only 16 s at 30 °C. Rapid dissolution is favored because free energy decreases as the surfactant is transferred from the Hquid surfactant phase L2 to liquid crystals) to aqueous micellar solution and the aggregate shape approaches that of a dilute Li phase, where its free energy is minimized at this temperature. 

What is meant by the optimal head group area of a surfactant What is the packing parameter (P Explain how packing considerations can be used to determine the possible shapes of the micellar aggregates. 

In aqueous solution, amphiphilic molecules aggregate into micelles above the critical micelle concentration. Such solutions have been the object of research for many years, with special interest in shape and size of these micellar aggregates [37]. Size and shape (spherical, wormlike, or disklike micelles) depend strongly on the molecular structure of the amphiphilic molecule. 

Phase diagrams of many Cm-(EO)n systems were found to demonstrate the complex influence of hydrophilic-hydrophobic balance on miscibility gap and mesophases [37]. An unambiguous description of the solution structure is difficult because size and shape of micellar aggregates can change with temperature and concentration. 

In contrast to the evaluation of critical micelle concentrations the determination of the size of micellar aggregates utilizing the fluorescence depolarization is reasonable as long as the solubilization does not affect the (average) degree of association of the particles and their shapes. Use is made of Perrin s relation164 l96) between the volume V of the micelle and the polarization of light emitted by an immobilized dye molecule p0 and that emitted by the dye solubilized in a micelle p, i.e.,... 

Micellar aggregates are considered in chapter 3 and a critical concentration is defined on the basis of a change in the shape of the size distribution of aggregates. This is followed by the examination, via a second order perturbation theory, of the phase behavior of a sterically stabilized non-aqueous colloidal dispersion containing free polymer molecules. This chapter is also concerned with the thermodynamic stability of microemulsions, which is treated via a new thermodynamic formalism. In addition, a molecular thermodynamics approach is suggested, which can predict the structural and compositional characteristics of microemulsions. Thermodynamic approaches similar to that used for microemulsions are applied to the phase transition in monolayers of insoluble surfactants and to lamellar liquid crystals. 

When neutron scattering of a sample is combined with contrast variation, information can be obtained not only about the shape and size of the micelle but also about its detailed (internal) molecular architecture. Because of the unique level of information about micellar systems that can be obtained from SANS experiments, the technique is now an extremely well-established tool for investigating the shape, size, and, to a lesser extent, the internal structure of micellar aggregates with several hundreds of papers being published since the 1970s when micelles were the first colloidal systems to be studied using SANS. 

Micellar shape can be affected by changes in temperature, concentration and the presence of added electrolyte to the liquid phase. Changes in any of these factors may affect micellar size, shape, and aggregation... 

Schematic models for the expanded structure of bile acid-phosphatidylcholine mixed micelles are shown in Fig. 2B. The original model was proposed by Small in 1967 (S36). In this model the mixed micelle consisted of a phospholipid bilayer disk surrounded on its perimeter by bile acid molecules, which were oriented with their hydrophilic surhices in contact with aqueous solvent and their hydrophobic sur ces interacting with the hydrocarbon chains of the phosphohpid molecules. This model has recently been revised, based on further studies of mixed micelles using quasi-elastic light scattering spectroscopy (M20). In a new model for the molecular structure of bile acid-phospholipid mixed micelles. Mazer et al. (M20) propose a mixed disk, in which bile acids are found not only on the perimeter of phospholipid bilayers, but also incorporated within their interior in high concentrations (Fig. 2B). The size of these mixed micelles was estimated to be as high as 200 to 400 A in radius in some solutions, and disk-shaped particles in this size range were observed by transmission electron microscopy (M20). Micellar aggregates similar in size and structure to those found in model bile solutions have been demonstrated in dog bile (M22).

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