Aggregates disc-like

Probably the best example of the manifestation of self-assembly processes in dendritic systems via H-bonds is seen in the recent work of Zimmerman et al. [156]. Dendritic wedges possessing tetraacid moieties (67) self-assembling into a hexameric, disc-like framework (68) was confirmed by SEC and -NMR studies. The tetraacid unit (69) is known to form cyclic as well as linear aggregates in solution via carboxylic acid dimerization (Fig. 30). However, with incorporation of larger dendritic wedges on 69, the hexamer form is preferred. 

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 3.11 Different types of micellar aggregates (a) spherical (b) disc-like (c) cylindrical (d) lamellar (e) vesicular (schematic). 

The aggregates created by amphiphiles are usually spherical (as in the case of micelles), but may also be disc-like (bicelles), rodlike, or biaxial (all three micelle axes are distinct) (Zana, 2008). These anisotropic self-assembled nanostructures can then order themselves in much the same way as liquid crystals do, forming large-scale versions of all the thermotropic phases (such as a nematic phase of rod-shaped micelles). 

Just as chiral induction can be realised in discotic liquid crystals, it can also be realised in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical trisamides (Fig. 15), which actually exhibit discotic liquid crystalline phases, also form chiral columnar stacks through it-it interactions when dissolved in apolar solvents, which are depicted schematically in Fig. 15 [121]. An achiral compound of this type (15) exhibits no optical activity in dodecane, but when the compound is dissolved in the chiral CR)-(-)-2,6-dimelhyloctanc significant Cotton effects (only slightly less intense than those observed in a chiral derivative) are detected. The chiral disc-like trisamide 16 can also be used as a dopant at concentrations as low as 2.5% to induce supramolecular chirality in the stacks of achiral compound. In this case, the presence of the additional hydrogen... 

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

The solution behavior of low molecular weight amphiphilic molecules has been intensively investigated in the past (12-16) with respect to the formation of liquid crystalline phases. In very dilute aqueous solutions, the amphiphiles are molecularly dispersed dissolved. Above the critical micelle concentration (CMC), the amphiphiles associate and form micelles (Figure 4) of spherical, cylindrical or disc-like shape. The shape and dimension of the micelles, as a function of concentration and temperature, are determined by the "hydrophilic-hydrophobic" balance of the amphiphilic molecules. The formation of spherical aggregates is preferred with increasing volume fraction of the hydrophilic head group of the amphiphile, because the... 

It is of interest to note here that the behaviour of lipid aggregates for which the optimal shape is a spherical vesicle or an infinite bilayer will be quite different from that of rod-shaped micelles. One might anticipate by analogy with rods that finite disc-like bilayers could form with cylindrical rims. For such a system the free energy would take the form = n +akT/N. From section 2, for large a, the system will assemble spontaneously into infinite bilayers. Repetition of an analysis similar to that for rods leads to values of a sufficiently large so that this process always occurs. 

Thermotropic liquid crystals can then be furflier subdivided into high molecular mass, main and side-chain polymers [10] and low molecular mass, the latter class of compounds being one of the areas of this review. The phases exhibited by the low molecular mass molecules are then properly described with reference to the symmetry and/or supramolecular geometry of the phases, which are briefly introduced here and are discussed in more detail further below. Thus, the most disordered mesophase is the nematic (N), which is found for calamitic molecules (N), discoidal molecules (Nq) and columnar aggregates (Nc), among others. The more ordered lamellar or smectic phases (S) [11, 12] are commonly shown by calamitic molecules, and there exists a variety of such phases distinguished by a subscripted letter (e. g. Sa, Sb)- Columnar phases (often, if incorrectly, referred to as discotic phases) may be formed from stacks of disc-like molecules, or from... 

The peculiarities of the aggregation behaviour of surfactant molecules in non-aqueous media. It is admitted that the aggregates exhibit disc-like structures with small aggregation numbos (<20) [39,40]. Such aggregates would not swell with monomer solutions, but on the other hand, would be able to capture initiator radicals, thus decreasing the overall radical efficiency. 

Just as chiral induction can be realized in discotic liquid crystals, so it can in assemblies of disc-like molecules or disc-like aggregates. As far as molecules are concerned, C3-symmetrical fm-amides (Fig. 6), which exhibit discotic liquid-crystalline phases, also form chiral columnar stacks through n-n interactions when dissolved in apolar solvents,which are depicted schematically in... 

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. 

Figure 2-6. 2-dimensional periodic structures formed by local disc like aggregation of bent-core molecules (a) or due to antiferroelectric in-plane ordering (b)... 

Fig. 2.2 Typical liquid crysUilline molecules and their self-oiganized aggregates. Rod-like molecules form (a) the nematic phase and/or (b) smectic mesophases. Liquid crystal molecules orient in a given direction but have no positional order in the nematic phase, and they are arranged in layers in the smectic phases. Disc-hke molecules form (c) the nematic phase and (d) ate self-organized into columns in the columnar phases...

Fig. 1.9. Columnar hexagonal disordered structure held together by hydrogen bonds. In the central column we see six non-discotic tri-forked diols which aggregate via hydrogen bonding forming a disc-like shape. (After Ref. 37.)...

Fig. 5.17. Schematic representation of the aggregation of disc-like amphiphiles into cylindrical micelles.

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