Micelle shape threadlike

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. 

Another type of micelle formation has also attracted the attention of researchers, a brief mention of which will be made below. Gravsholt [93], in an early work, reported viscoelasticity in highly diluted aqueous solutions of some cationic surfactants, namely, certain derivatives of the base structure of cetyltrimethylammonium bromide (CTAB), CTA-X (where X=salicylate, m-chlorobenzoate, p-chlorobenzoate). He came to the conclusion that the viscoelastic behavior indicates a micellar shape other than spheres and rods. Later it has been shown [e.g. 94] by fluorescence anisotropy using fluorescent probe molecules that CTAB and sodium p-toluenesulfonate, NapTS (with suitably weak fluorescence) in aqueous solution could produce long, threadlike micelles with network structure in which the cross-points had finite lifetime. Sodium salicylate was another useful additive for synthesis, but the intensity of its fluorescence was found unsuitable for examining the behavior of the probes. 

There are two specific examples of liquids that often show simple MaxweU-like behaviour within the normal measuring range (lO - 10 Hz), with the expected shapes of the G and G" curves and a single relaxation time, viz. associative-thickener-type polymers (see chapter 16 for details) and worm-like surfactant micellar systems, otherwise known as threadlike or rod-like micelles (linear or branched). (The latter are also called living polymers because if they break imder large stresses, they can reform under conditions of rest or low stress (see chapter 18).) The typical response of such a system is shown in figure 14, where a representative relaxation time would be around 1 s, see [10]. 

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