Disk shaped micelles

The minimum in the interfacial free energy predetermines three kinds of geometry in nature spheres, cylinders and planes. Correspondingly, the most stable amphiphile aggregation structures are i) spherical (Hartley) micelles, ii) rod-shaped micelles and anisotropic middle phases, iii) disk-shaped micelles and lamellar mesophases. They exist as aggregates in a water continuum with a hydrocarbon core surrounded by hydrated polar groups (the normal type) and as aggregates in a hydrocarbon continuum (the reverse or inverted type) where water and... 

Similar analyses for surfactants for which 0.33 Pc — 0.5 predict that cylindrical or disk-shaped micelles will result. 

A lyotropic nematic phase formed by disk-shaped micelles resulting from the addition of small amounts of lauric acid to a micellar solution of sodium dodecyl sulfate and cocamidopropyl betaine has been studied using Na NMR quadrupolar studies, H pulsed field gradient NMR and other... 

Boden et al. [172] showed that a thermodynamically independent nematic mesophase exists in the cesium perfluorooctanoate (CsPFO)-water system between 37% and 87% (w/w) H2O and 11-75°C. The nematic phase is intermediate to an isotropic micellar solution at higher temperatures and a smectic lamellar mesophase at lower temperatures. The isotropic phase consists of disk-shaped micelles. The lamellar phase has been described by a structure in which continuous lamellae of the surfactant are broken by irregular water-filled defects without interlayer correlations [182]. In the nematic phase, the aggregates make the transition from discrete disks to continuous lamellae [160,182]. Both positional and oriental order increase when the temperature is lowered. The nematic phase of CsPFO-water is stable over a wide range of concentrations without needing a cosurfactant or salt as a stabilizer. The disk-shaped micelles of the nematic phase orient with their unique axis parallel to the direction of an applied magnetic field. 

For a nonionic surfactant of the polyoxyethylene class, the situation is more complex. In a relatively pure sample of such a surfactant one may encounter not only the usual hexagonal and lamellar phases but also one or more isotropic liquid phases. It may be that such phases contain disk-shaped micelles resulting from dismption of the extended lamellar phase by the large steric requirements of the hydrated POE head group. Similar effects have been reported for anionic surfactants at sufficiently high electrolyte concentrations or in the presence of oil-soluble alcohols. In those cases, some form of association phenomena (e.g., ion binding and/or some form of molecular complex formation) might be involved. 

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).

The structure of this type of L2 phase has been analyzed by electron microscopy and by X-ray diffraction [6]. The results indicate that flexible disk-shaped water micelles occur, separated by lipid bilayers. The X-ray data are in agreement with electron micrographs of freeze-etched freeze-fractured samples. An analysis of the X-ray scattering results at a weight ratio of monoglyceride to water of 8 2 gave these results ... 

Although the classic picture of a micelle is that of a sphere, most evidence suggests that spherical micelles are not the rule and may in fact be the exception. Due to geometric packing requirements (to be discussed below) ellipsoidal, disk-shaped, and rodlike structures may be the more commonly encountered micellar shapes (Fig. 15.8). However, from the standpoint of providing... 

Experiments have shown that in most cases, such as for SDS, the initial spherical-shaped micelles may grow under some influence into larger aggregates (disk-like, cylindrical, lamellar vesicle) (Figure 1.29). The spherical micelle has a radius of 17 A. The extended length of the SDS molecule is about 17 A. However, larger micelles (as found in 0.6 mol/L NaCl solution) have dimensions of 17 and 25 A, radii of an eUipse. 

Both models are sketched in Figure 14.18. A rod-like micelle can be imagined to have a shape like a corkskrew, for a disk-like micelle it is not as obvious how it can be twisted without disruption. 

The decanoyl derivative forms micelles up to around 15 wt % surfactant, while the stearoyl and oleyl only form micelles at less than 2 wt % surfactant. In addition, the decanoyl derivative displays a significantly depressed Krafft temperature boundary, around 30°C compared to more than 40°C for the other two. The micelles formed by the decanoyl derivative were shown by NMR to be spherical in the dilute region but become more disk-shaped as the phase boundary is approached. This work provides data that complements that of Soderberg et al. [27] in that it examines the systematic change in hydrophobe rather than the systematic change in headgroup structure as performed by Soderberg et al. 

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.)... 

In many cases, under changing experimental conditions, water-containing reversed micelles evolve, exhibiting a wide range of shapes such as disks, rods, lamellas, and reverse-vesicular aggregates [15,107,108], Nickel and copper bis(2-ethylhexyl) sulfosucci-nate and sodium bis(2-ethylhexyl) phosphate, for example, form rod-shaped droplets at low water contents that convert to more spherical aggregates as the water content is increased [23,92,109,110],... 

Miller et al have reported that in bile salt solutions in the presence of EL, if the EL concentration is less thcin half of that of the bile salts, the mixed micelle shape beccmes spherical, but, otherwise, the shape is a disk as shown in Figure 2. All solutions used here inclixie 32 mM lecithin and 100 mM total bile salts, therefore the micelle shape in all systems here must be spherical. Edward et al... 

Block copolymers self-assemble to form nanoscale organized structures in a selective solvent. The most common structures are spheres, with the insoluble core surrounded by a solvent-swollen corona. In some instances, disk- or worm-like micelles form, and are of particular interest, since the control of their association can lead to a broad range of new applications [1,2]. An important subset of block copolymer micelles are those which contain metal atoms, through covalent attachment or by complexa-tion [3], These structures are interesting because they take advantage of the intrinsic properties of their components, such as the mechanical properties of the polymer micelles and the optical and magnetic characteristics of the metal atoms. Moreover, the assembly permits the control of the uniformity in size and shape of the nanoparticles, and it stabilizes them. 

A spherical micelle should be rather considered as idealized picture, and non-spherical aggregates are often formed due to geometric packing requirements. Very high surfactant concentrations induce transition of spherical micelles into other shapes (rods, disks, vesicles, lamellar structures). 

---