Stacking chromonics

In addition to those formed by surfactant amphiphiles, two other types of lyotropic mesophases are generally recognized, neither of which exhibits a cmc. The first of these are lyotropic phases of rigid-rod polymers that can form mesophases in both aqueous and non-aqueous solvents " these mesophases are of the nematic or hexagonal type. Examples include polymeric metal acetylide complexes and DNA." The other type is usually formed from flat and largely aromatic molecules which stack to give lyotropic columnar phases, also referred to as chromonic phases." " This latter class is formed from systems with ionic or strongly hydrophilic peripheral functions, and forms mesophases... 

Figure 2. The structure of the chromonic N and M phases The basic structural unit of both phases is the untilted stack of molecules. The N phase is a nematic array in which these stacks lie in a more or less parallel pattern, but where there is no positional ordering. Tlie M phase is a hexagonal array of these columns. The six-fold symmetry is a result of orientational (but not positional) disorder. A schematic diagram of a localized region, as shown in (a) has only orthorhombic symmetry, but, averaged over the whole structure, each column actually lies in a site with sixfold symmetry (b). The restrictions to the possible orientations of the columns are shown in (c). Because of packing considerations, for any particular orientation of a column, as shown on the left, an adjacent column (right) can take up only two of the three possible orientations (i) and (ii). A representation of the orientationally disordered state of the M phase is given in Fig. 9. Note that the molecular columns are shown here in a highly stylized way. They are not necessarily such simple one-molecule-wide stacks.

There is then a sudden drop in the free energy as virtually all of the amphiphile molecules are incorporated into micelles, enabling their hydrophobic alkyl chains to be more or less completely shielded from the aqueous part of the phase. This leads to the familiar abrupt change in physical properties at the cmc. In chromonic systems there is also the aggregation of molecules in dilute solution before mesophase formation, but the pattern of association is different. The hydrophobic surfaces of the molecules cause them to aggregate in stacks like packs of cards. As these stacks grow, the fraction of the total hydrophobic surface area exposed to the aqueous part of the phase steadily falls, but there is no minimum free energy state, no cmc, and there is no structure directly comparable to the micelle [38]. 

Figure 6. The explanation for the tiger skin texture sometimes adopted by chromonic N (and P) phases The banded pattern seen with crossed polars is thought to result firom a twisted rope-like arrangement of the columns. Such a pattern could be a way of accommodating initial misalignment in the sample with a minimum of splay distortion, (a) The postulated arrangement of molecular stacks in two adjacent twisted ropelike assemblies, (b) The gross structure of the sample showing the large-scale organization of the assemblies represented in (a), (c) The banded appearance of a sample when viewed vertically between crossed polars.

Figure 13. The chromonic nature of DNA (a) A stylized sketch of the B form of DNA showing the central stack of base pairs and the two helical sugar-phosphate chains, (b) A chromonic column of unpolymerized molecules, to be compared with the central stack of bases in DNA. (c) The intercalation of a guest molecule in DNA. Note the untwisting of the sugar-phosphate chain to accommodate the host molecule, (d) The intercalation of a guest molecule in an unpolymerized chromonic column.

The ease of intercalation of other chro-monic and potentially chromonic materials. A number of biochemical reagents, such as acridines and ethidium bromide, intercalate readily between the stacked bases in DNA and RNA. Anticancer drugs, such as the square planar platinum complexes, intercalate avidly (Fig. 14) and naturally occurring antibiotics, such as actinomycin, similarly act by intercalation into the stack of bases [67]. They act as tailor-made spanners in the works and prevent the reading and replication of DNA. 

According to Lydon, aggregation of LCLC molecules is a "steady, progressive build-up of chromonic aggregates where the addition of a molecule to a stack is always associated with the same increment of free energy." On... 

There are some specific amphiphile systems which form exceptionally stable pancake-like micelles which, as one might expect, give anisotropic solutions analogous to discotic nematic phases. Conversely, there are other lyotropic systems containing elongated stacks of aromatic molecules, i.e., chromonic systems, which are analogous to thermotropic calamitic nematic phases. 

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