Micelle shape disklike

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 ... 

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. 

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). 

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... 

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. 

Intermicellar forces are generally of a repulsive nature (i.e., charged amphiphiles) and a reduction of such repulsion accompanies the transformation from spherical to cylindrical micelles. Further increase of concentration results in the formation of linear assemblies and liquid crystalline lyotropic mesophases (cf. Section ni.B). Not only nematic (Nc and Nd from rodlike or disklike shapes, respectively), hexagonal, and smectic phases, but also biaxial (mixtures of Nc and Nd) and complex cubic phases (bicontinuous networks or plastic crystals)... 

Later, Debye and Anacker proposed that micelles are rod-shaped rather than spherical or disklike." The cross section of such a rod would be circular, with the polar heads of the detergent lying on the periphery and the hydrocarbon tails filling the interior. The ends of the rod would almost certainly have to be rounded and polar. In 1956, Hartley s spherical micelle model was established by Reich from the viewpoint of entropy, and the spherical form is now generally accepted as approximating the actual structure (Fig. 4.3). 

Surfactants can self-aggregate in water by exposing their hydrophilic groups to the water and their hydrophobic groups to the aggregate core, which minimizes the free energy of the system and reduces its interfacial tension. Once they reach a critical micelle concentration (CMC), the aggregated surfactants form so-called micelles (Fig. 4) of various shapes (spherical, disklike, cylindrical, sheetlike, etc.). The micelles are termed direct micelles when the lipophilic tails of the surfactants form a hydrophobic core while the continuous phase is hydrophilic (i.e., water). The micelles are termed inverted micelles (or reverse micelles. 

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