Prolate micelles

The geometric form of the micelles depends on the concentration of the surfactant and on additives. In the absence of additives the micelles are spherical for concentrations ranging from the critical micelle concentration (cmc) to at least ten times the cmc. - - At higher surfactant concentrations or with additives (added salt is the most efficient) rodUke or prolate micelles will form. At high surfactant concentration or with large amounts of additive, the micellar phase becomes unstable, and other phases will be present. In most cases a hexagonal phase or a lamellar phase will be the phases in equilibrium with the aqueous micellar phase. ... 

The result of this calculation, shown in Hgure 1.8 (sohd curve), is in good agreement with the experimental data. We have applied the ECM to calculate the diffusion expected in the case of prolate micelles of different axial ratios using equation 49 of Jonsson et al. [30] and the same parameters described previously. As shown in Figure 1.8,... 

Swelling exponents between 0.33 and 0.45 (depending on the anisotropy of the micelle) are expected for prolate micelles, intermediate to the exponents for spherical and cylindrical columnar micelles. Oblate ellipsoidal micelles exhibit swelling exponents between 0.33 and 1. The upper bound - for very flat tablet-shaped micelles - overlaps with those of mesh and lamellar phases. 

Figure 3.3 Schematic diagrams of one-half of the micelle of Triton X-100 based on geometrical calculations by Robson and Dennis [72] for (a) spherical, (b) oblate, and (c) prolate micelle models. The spherical model necessitates intrusion of the oxyethylene chains into the micellar core.

The anisotropic micelles forming lyotropic liquid crystals are also oriented by surfaces. Both prolate and oblate micelles orient parallel to flat surfaces [114,115] probably due to hard-core interactions [116]. Prolate micelles can also be azimuthally oriented by grooved surfaces, or homeotropically oriented by two-dimensional topographies [117]. 

The characteristics of micelles formed by hydrocarbon-type surfactants have been extensively studied. In dilute solutions, micelles are considered to be spherical or nearly spherical. The minimum space of the micelle interior has been calculated with the assumption that the radius of the interior region is approximately equal to the length of the fully extended hydrophobic chain and the empty space in the micelle interior is absent. The latter assumption is in accord with the liquidlike character of the micelle interior, supported by substantial evidence. However, micelles formed by hydrocarbon-type surfactants are larger than the space required for the hydrocarbon chain. Oblate or prolate micelle models have been proposed. At higher surfactant concentrations, a surfactant may form rodlike, cylindrical, disk-shaped, or lamellar micelles [1] or vesicles, which are lamellar micelles arranged in a spherical shape with water in between the lamellas (see Fig. 6.1). (See Section 7.4.)... 

Small micelles in dilute solution close to the CMC are generally beheved to be spherical. Under other conditions, micellar materials can assume stmctures such as oblate and prolate spheroids, vesicles (double layers), rods, and lamellae (36,37). AH of these stmctures have been demonstrated under certain conditions, and a single surfactant can assume a number of stmctures, depending on surfactant, salt concentration, and temperature. In mixed surfactant solutions, micelles of each species may coexist, but usually mixed micelles are formed. Anionic-nonionic mixtures are of technical importance and their properties have been studied (38,39). 

At relatively low concentrations of surfactant, the micelles are essentially the spherical structures we discussed above in this chapter. As the amount of surfactant and the extent of solubilization increase, these spheres become distorted into prolate or oblate ellipsoids and, eventually, into cylindrical rods or lamellar disks. Figure 8.8 schematically shows (a) spherical, (b) cylindrical, and (c) lamellar micelle structures. The structures shown in the three parts of the figure are called (a) the viscous isotropic phase, (b) the middle phase, and (c) the neat phase. Again, we emphasize that the orientation of the amphipathic molecules in these structures depends on the nature of the continuous and the solubilized components. 

Small micelles in dilute solution close to the CMC are generally believed to be spherical. Under other conditions, micellar materials can assume structures such as oblate and prolate spheroids, vesicles (double layers), rods, and lamellae. 

Formation of the Sic Phase in Binary Aqueous Alkyltrimethylam-monium Halide Solutions. These considerations are well illustrated by the formation of the Slc phase in aqueous alkyltrimethylammonium halide solutions (17) (Figure 7). This phase is apparently composed of Si micelles—probably on balance prolate—arranged in a primitive, cubic lattice and rotating fairly freely at the lattice points. The lattice is formed by dodecyl- and tetradecyltrimethylammonium chlorides but not by the hexadecyl or octadecyl chlorides nor by any of the corresponding bromides. This may be expressed as follows. 

Figure 3 The structure of die prolate spherocylindrical micelle having an aggregation number N. There are N

Elongated cylindrical, rodlike micelles with hemispherical ends (prolate ellipsoids)... 

At the present time, the major types of micelles appear to be (1) relatively small, spherical structures (aggregation number <100), (2) elongated cylindrical, rodlike micelles with hemispherical ends (prolate ellipsoids), (3) large, flat lamellar... 

Polymersomes offer advantages for clinical therapeutic and diagnostic imaging applications. The ratio of the hydrophilic-to-hydrophobic volume fraction is the key in determining the mesoscopic formulations among micelles (spherical, prolate, or oblate) or vesicles (polymersomes) in aqueous solution [33-35]. In general, a proportion of hydrophilic block-to-total polymer from 25 to 45% favors polymersome formation, while block copolymers that have proportions greater than 45% favors micelle formation [36]. 

At a concentration close to the CMC, the micelles are generally spherical or close to spherical (see Fig. 2). As the concentration is increased, the micelles may remain spheroidal or grow and become oblate (disk-like) or prolate (rod-like). The giant worm-like or thread-like micelles represent an extreme case of growth into elongated micelles. The worm-like micelles can be linear or branched (see Fig. 2). The hemispherical end caps of thread-like micelles have a larger diameter than the cylindrical body," a result correctly predicted by theory. Micelles can also be ring-like. " ... 

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