Micelles disklike

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

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

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. 

An interesting similarity of what we discussed here appears if one deals with mixtures of rodlike and disklike micelles. These systems could behave very similarly to a truly biaxial nematic, but show interesting differences to them. Whereas for the usual orthorhombic biaxial nematics both directors are perpendicular to each other by construction, in mixtures there is no need to impose this restriction. Pleiner and Brand [70] investigated how mixtures are influenced by an external field (magnetic field or shear flow) and found that the angle between the two directors exhibits a flow aligning behavior similar to the one studied in [42,43],... 

At an air-water interface, a monolayer forms with heads lying down and tails up (toward air), whereas at an air-hydrocarbon interface the monolayer lies with tails down. By closing on the tail side, the sheetlike structure can be dispersed in aqueous solutions as spherical, rodlike, or disklike micelles (Fig. 3). Closure on the head side forms the corresponding inverted micelles in oil. Oil added to a micellar solution is incorporated into the interior of the micelle to form a swollen micellar solution. Thus, surfactant acts to solubilize substantial amounts of oil into aqueous solution. Similarly, a swollen inverted micellar solution enables significant solubilization of water in oil. 

Changes in temperature, concentration of surfactant, additives in the liquid phase, and structural groups in the surfactant may all cause change in the size, shape, and aggregation number of the micelle, with the structure varying from spherical through rod- or disklike to lamellar in shape (Winsor, 1968). 

Among vast varieties of lyotropic LCs, chromonic mesophases are the families of new research target starting only about two or three decades ago. The properties are different from those of ordinary lyotropic mesogens of surfactant type. The molecules constitute aromatic(s) in the center, and hydrophilic parts are scattered around the molecules rather than aliphatic structures. Therefore, they are rigid and planar disklike or planklike, rather than micelles of flexible molecular assemblies of ordinary surfactants. The molecules aggregate in solution to form... 

A cubic phase of space group Pmln is usually observed in type I systems [164]. Several structures have been suggested for the Pmln phase [168-171]. It is now agreed that it contains two types of micelles [172] two quasi-spherical micelles packed on a body-centered cubic lattice and six slightly asymmetrical micelles arranged in parallel rows on opposite faces of the unit cell. The asymmetrical micelles are assumed to be disklike [173] or rodlike with rotational disorder around one of the short axes [174,175]. In order to pack space completely, each asymmetrical micelle, together with the water that surrounds it, takes the shape of a... 

Apart from a very brief consideration of phase behavior in polymer solutions, the discussion focused on phase separation in polymer blends and block copolymer melts. Another important area is the self-assembly of block copolymers in low-molecular-weight solvents. A detailed discussion of this topic is outside the scope of this chapter. In general, the self-assembly in selective solvents is characterized by the formation of micelles (spherical, disklike, rodlike, with internal stmctures (e.g., lanus type)) or of vesicles. For recent reviews, we refer to References 191, 218, and 219. 

Upon increasing the amphiphile concentration an evolution toward more asymmetric shapes (rodlike or disklike) and decreasing surface/volume ratio is observed. Eventually cylindrical (capped) micelles, bilayers (extended open sheet with rounded edges), and closed vesicles are formed. 

Scheme 1 a Aggregation mode of interlayer micelle-occluded nanocrystals, b Graphic presentation of the formation mechanism of CaCOs mesorings. Gray, calcium carbonate red, hydrophobic block blue, soluble neutral block yellow, charged block. The red arrow indicates the decreasing gradient of occluded polymer within the disklike structure. (Reproduced from [156], 2006, American Chemical Society)... 

Singh et al. ( i) postulated that asphaltenes stabilize w/o emulsions in two steps. First, disklike asphaltenes molecules aggregate into particles or micelles, which are interfadally active. Then, these entities upon adsorbing at the w/o interface aggregate through physical interactions and form an interfacial network. Different modes of action of asphaltenes are represented in Figure 12. 

SANS studies [51,75] of the dichained-diglucamide sugar surfactants [K] (Fig. 1) where C ,H2m+i substitutes CgHis showed that these surfactants form elongated micelles even at fairly low concentration for m = 5-7 and disklike micelles for w = 8. Temperature had little effect on the micelle size. 

As the concentration is increased, the micelles may remain spheroidal or grow and become oblate (disklike) or prolate (or elongated, cylindrical, or rodlike), with the prolate shape much more often encountered than the oblate shape. The micelle shape is determined by the value of the surfactant packing parameter P given by ... 

Various studies suggested the existence of structures intermediate between micelles and vesicles. These structures — perforated vesicles, bilayer fragments, giant wormlike micelles, ringlike micelles, and disklike micelles, depending on the investigated system — were visualized by cryo-TEM.3 > > Some of these structures are shown in Figure 1.10. However, for some systems no intermediate structure... 

Figure 6.13 Disklike micelle D and disjoined disklike micelle D postulated as intermediate states in the micelle-to-vesicle transformation. Reproduced from Reference 97 with permission of lOP Publ.

Vesicle formation in aqueous mixtures of lecithin and bile salt that takes place upon dilution with water has been studied intensively by means of time-resolved scattering methods (static and d5mamic light scattering, SANS), and it has been concluded that this transition takes place via elongated micelles and disklike micelles as intermediate structvu es. ... 

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