Helical Micellar Fibers

Similar assemblies have been extensively characterized for the ion pair cetyltrimethylammonium-salicylate. In both cases the micellar fibers produce slightly viscous solutions and the effect of viscoelasticity is observed if one rotates such a solution and suddenly stops the rotation, smalt particles in the solution (e.g., air bubbles) bounce back. While the bulk water is still rotating in the nonviscous solutions, the inertia of the high molecular weight threads builds up an elastic wall for the suspended particles and pushes them back. Lithium and sodium ricinolates produce helical micellar fibers of opposing chirality in toluene (Tachibaona, 1970,1978). 

Fig. 55a. In the presence of detergents, e.g. SDS, micellar fibers do not rearrange to crystals, because crystallization nuclei with head-to-tail sheets cannot be formed, b Electron micrograph of a 2-month-old gluconaniide D-28-8 gel, which was kept at 60 °C in the presence of SDS (molar ratio 10 1). Micelles and double helices occur (PTA 2% post-strained, bar = lOOnm). c Electron micrograph of a gel, which was kept at 20 °C and contained more SDS (molar ratio 2.5 1). Vesicles and multiple helices are apparent (PTA 2% poststained, bar — lOOnm) [377]...

The observed structures were explained by a chiral bilayer effect mechanism proposing that only the enantiomerically pure compounds can lead to the formation of helical fibers which in turn slowly rearrange to enantiopolar crystal layers (Scheme 7.1). Within the micellar fibers, the polar head groups are oriented toward the aqueous environment and must therefore go through an energetically unfavourable -slow- dehydration followed by a 180 ° to form the enantiopolar crystals. 

Transmission electron microscopy of such a gel shows micellar fibers, which are several micrometers long and about 8 nm wide. Image analysis reveals regular quadruple helices of molecular bilayer rods or ribbons (Fig. 4.5.8). In cryo-electron microscopy the single helices appear somewhat broader (Fuhrhop et al., 1988 Koning et al., 1993). 

The chiral-bilayer-effect hypothesis has been evoked for the rationalization of the helical fibers formed from enantiomeric or diastereomeric surfactants (Fig. 54) [373], Different packing of the chiral surfactants in the crystals (head-to-tail) and in bilayer or micellar aggregates (tail-to-tail) is the basis for this postulate. Crystallization from aggregates requires an energetically costly, 180°... 

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