Hexagonal mesophase involving

Such molecules exhibit columnar mesophases and smectic C phases. In contrast to the corresponding tetracatenar compounds described above, the columnar phase is, in most of these swallow-tailed compounds, the low temperature phase with respect to the smectic C phase [58]. This inversion of thermotropic sequence with respect to the biforked compounds described just above has not found any explanation in relation with molecular parameters. The symmetry of the columnar two-dimensional lattice is found to be hexagonal, oblique and even rectangular [59] in some cases. For the latter case, the model proposed involves several molecules packed side by side to form the lattice unit, the different moieties being packed alternately in order to form a two-dimensional centred cell as shown in Fig. 11. 

Although liquid crystal (LC) theory predicts the existence of as many as 18 distinct structures for a given molecular composition and structure. Nature appears to have been kind in that only four of those possibilities have been identified in simple, two-component surfactant-water systems. The four LC phases usually associated with surfactants include the lamellar, hexagonal (normal and inverted), and cubic (Fig. 15.3). Of the four, the cubic phase is the most difficult to define and detect. It may have a wide variety of structural variations including a bicontinuous or interpenetrating structure that involve components of the other mesophases. The remaining types are more easily characterized and, as a result, better understood. 

First, simple less homogeneous - and therefore likely to be more prevalent among chemically disperse membranes - lower symmetry variations on the known mesostructures deserve consideration. Consistent reports of other mesophases by experienced researchers of the calibre of Luzzati and Fontell in Sweden cannot be discounted (17). Most of the proposed mesostructures involve simple symmetry-breaking deformations of the more common phases, including hexagonal columnar and discrete micellar mesophases. 

A simple view of the state of a chromonic system is that the molecules are effectively insoluble in one dimension. The energy involved in their aggregation is more or less comparable to kT for each aromatic ring and only at extremely low dilution are there individual, unaggregated molecules. For ionic systems, the columns attract counterions, forming electrical double layers. This results in column-column repulsion, which causes the columns to behave as if they were separated by compressed springs. This effect presumably stabilizes the hexagonal structure of the M phase and causes the columns to move apart equally when the mesophase is diluted. 

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