Oblate micelles

With longer chain derivatives, the forces of attraction increase, the curvature decreases and micelles become oblate or form hexagonal (or columnar) phases. When the zero-curvature is reached, the flat oblate micelles can fold and close spontaneously, thus entrapping a volume of water and form vesicles that may contain one or several bilayers of the amphiphile. 

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]. 

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. 

Further studies by electron microscopy on some of the samples exhibiting the Pm3n cubic phase show the existence of grain bormdaries and stacking faults [118]. These are all consistent with the presence of quasi-spherical assemblies or more precisely to polyhedral-like micelles, and moreover suggest that the supramolecular spheres are deformable, interacting with one another through a relatively soft pair potential [119]. The majority of such quasi-spherical assemblies are thus distorted into an oblate shape. 

In the simplest liquid-crystalline phase, namely the uniaxial nematic, there is at rest a special direction designated by a unit vector n called the director (see Fig. 10-2). In the plane transverse to the director, the fluid is isotropic. The most common nematics are composed of oblong molecules that tend to point in a common direction, which defines the director orientation. Oblate, or disc-like, molecules can also form uniaxial nematics for these discotic nematics, the director is defined by the average orientation of the short axis of the molecule. Lath-like molecules or micelles (shaped like rectangular slabs), in which all three dimensions of the molecule are significantly different from each other, can form biaxial nematics (Praefcke et al. 1991 Chandrasekhar 1992 Fialtkowski 1997). A biaxial... 

Large, flat lamellar micelles (disklike extended oblate spheroids)... 

One of the most widely used indexes for evaluating surfactant activity is the critical micelle concentration (CMC). The CMC is in effect the solubility of a surfactant within an aqueous phase, or the minimum surfactant concentration required for reaching the lowest interfacial or surface tension values y. At concentrations above the CMC, surfactants associate readily to form micelles that can be spherical, oblate, tablet shaped, or rod-like, with a hydrophilic surface and a hydrophobic interior. Such micellar structures usually have hydrodynamic radii, ranging from 200 to 400 A. The CMC value is estimated from the inflection point in the y vs log C curve. 

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. " ... 

Fig. 1 Representation of a prolate ellipsoidal (cigar-like) Janus micelle with an oblate ellipsoidal (disc-like) core. The complex coacervate core is depicted in grey, while the corona is depicted in blue (ethylene oxide monomers) and green (acryl amide monomers)...

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