Aggregation chromonics

Jahn-Teller type smectic A-C transitions 453 Jelley aggregates, chromonics 988... 

The experimental observations of nDNA LC have also renewed the interest in theoretically modeling systems where reversible linear aggregation and LC ordering are cooperatively correlated [43]. This is also the case for chromonics, flat hydrophobic molecules, mainly dyes, with soluble moieties at their edges [52]. 

Among vast varieties of lyotropic LCs, chromonic mesophases are the families of new research target starting only about two or three decades ago. The properties are different from those of ordinary lyotropic mesogens of surfactant type. The molecules constitute aromatic(s) in the center, and hydrophilic parts are scattered around the molecules rather than aliphatic structures. Therefore, they are rigid and planar disklike or planklike, rather than micelles of flexible molecular assemblies of ordinary surfactants. The molecules aggregate in solution to form... 

Nanohybridization of B67 with silica network was recently attempted by Hara et al. (2007). The chromonic LC of B67 was mixed with silica precursor of tetraethoxysilane (TEOS) together with an anionic surfactant (EmalC20) and 2-(2-aminoethoxyl)ethanol (AEE). Here AEE plays an important role in stabilizing the columnar structure of B67. This molecule works as the mediator of the interface between the anionic B67 aggregate and silica. Without AEE, the columnar structure is transformed to a lamella structure in the process of drying (Hara et al., 2007). 

A patent application by a group at Eli Lilly Co. reported the use of dilute H2SO4 in EtOH promoted the Simonis reaction and esterified the acid to provide chromone 34 in 55-65% yield. Compound 34 was used as an intermediate toward the preparation of agents for platelet aggregation inhibition. 

The tendency for the molecnles of chromonic phases to aggregate into colnmns is present even in dilute solution where colnmn formation occnrs before a phase is formed. Althongh there may be a threshold concentration before aggregation begins to occur, there is no optimum column length and hence no critical concentration, with a further distinction being the absence of a Krafft temperature. Since the process of mesophase formation does not depend on the presence of flexible alkyl chains, there is no threshold temperature below which mesophases cannot form because the vital flexibility of the molecules has been frozen out. 

Chromonic liquid crystals are a special class of lyotropic liquid crystals. Flat aromatic systems with polar substituents form columnar aggregates by n-n interactions in solution. The mesophases of concentrated DNA solutions or of copper-tetracarboxyphthalocyanine are examples for this. Special types of chiral mesophases have been observed [29]. 

The tendency of chromonic molecules (Fig. 1) to aggregate into columns is present even in dilute solution (just as for amphiphile systems, where micelle formation occurs before a mesophase is formed). How-... 

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. 

The Aggregation of Chromonic Molecules in Dilute Solution and on Substrate Surfaces... 

Figure 3. The contrast between the pattern of aggregation of amphiphiles and chromonic molecules For amphiphiles, the energetically unfavorable contact between the hydrophobic alkyl chains and water molecules is reduced to more or less zero when a closed micelle is formed. In contrast, for chromonic systems, the fraction of the total molecular surface exposed to the solvent decreases as the columns lengthen, but there is no optimum aggregate size directly comparable to a micelle.

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]. 

What is the discotics for the thermotropic liquid crystals that is the lyotropic chromonic liquid crystals (LCLCs) for the lyotropic materials. LCLC molecules are plank-like rather than rod-like, rigid rather than flexible, aromatic rather than aliphatic. Typical LCLC molecules and their aggregation in... 

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... 

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