Disc-like micelles

The amphipathic compounds shown in Scheme 2 can form a disc-like micelle(7). The shape of a molecular aggregate depends on the shape of the constituent molecules(8). For instance, conical molecules with large polar head groups prefer to form spherical micelles while cylindrical molecules tend to give flat aggregates. Trans-azobenzene is a rod-like molecule whereas the cis-form is bent. 

Figure 3.14 Schematic of surfactant aggregates (cut open) (a) spherical micelle, (b) rod-like micelle and (c) disc-like micelle.

In lyotropic liquid crystals theses micelles are the mesogens which built up the liquid crystalline phases. Depending on the solvent concentration, different types of micelles are possible. The most common micelles, i.e. rod-like micelles, disc-like micelles and spherical micelles, are depicted in the lower part of Fig. 3.1. Furthermore, the surfactant molecules may also aggregate into lamellas which represent full or partially interdigitated bilayers of the molecules. Those lamellas are, strictly speaking, no micelles as they extend infinitely into two dimensions, but yet the driving force for their formation is the same. 

For values of the packing parameter II smaller than 1/3, spherical micelles can be expected. For values up to 1/2, rod-like micelles are most likely, followed by disc-like micelles at increasing values of II. For values of approximately 1 the formation of lamellas dominates. At very low solvent concentrations or if using apolar solvents the packing parameter may take values larger than 1. Under these conditions inverse micelles are formed. They look similar to the micelles shown in Fig. 3.1, but instead of the alkyl chains, the hydrophilic head groups are located in the centers of the micelles. 

Of all liquid crystalline phases, the nematic phase is the phase with the highest symmetry, i.e. Dooh, and the least order. As shown in Fig. 3.3a, b, the mesogens solely possess orientational order. Positional order of the mass centers does not occur in this phase. Nematic phases are usually built up by either rod-like or disc-like mesogens. For thermotropic liquid crystals these mesogens are therefore calamitic or discotic molecules, respectively. In both cases the phase is simply denoted with the abbreviation N. For lyotropics, the notation typically distinguishes between nematic phases Nc, which are formed by rod-like micelles, and nematic phases Np, which are composed of disc-like micelles. 

Nd Nematic phase composed of disc-like micelles... 

Phase Chirality of Micellar Lyotropic Liquid Crystals 477 MODEL OF INTRA-MICELLAR CHIRALITY Disc-like micelle local orientational order... 

A similar mechanism can be concluded for the assumption of a network of connected or fused threadlike micelles. The cross-links between these micelles can be regarded as disc-like micelles from which the rods extend. In this case, the transient intermediate species in the various bond interchange mechanisms are assumed to be stable. In this situation, all the end-caps could be connected and the resulting network could be in the saturated or unsaturated state. The cross-link points could then slide along the thread-like micelles, and this process represents a one-dimensional diffusion with a concentration-independent diffusion coefficient. A knot can disappear, if two network points meet on their random diffusion paths. If the structural relaxation time is determined by this random movement, a similar equation, i.e. x 1/c, can be derived. 

But cosurfactants, like aliphatic alcohols, with an intermediate chain length support the formation of disc-like micelles and thus of lamellar phases [85]. 

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