Lyotropic columnar phases

The first reports of metallomesogens capable of forming lyotropic columnar phases were by Gaspard, who described mesomorphism in peripherally carboxylated copper(II) phthalocyanines [203], although their phase behaviour was never properly... 

However, more remarkable was the discovery that in alkane solvents, large, orrfiometallated macrocyclic complexes of palladium shown in Figure 92 would form lyotropic columnar phases [171], These remarkable materials have been shown to form columnar hexagonal phases and, in suitable solvents, lyotropic nematic phases derived from columnar organization. Further, in certain non-mesomorphic examples, mesophases can be induced by the addition of an electron acceptor such as trinitrofluorenone chiral phases are introduced when the acceptor is resolved 2 -(2,4,5,7-tetranitro-9-fluorenylideneaminoxy)propionic acid (know as TAPA). 

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... 

The first reports of metallomesogens capable of forming lyotropic columnar phases were reported by Gaspard, who described mesomorphism in peripherally carboxylated copper(II) phthalocyani-nes, although their phase behavior was never properly defined ((320) M = Cu, X, Z = H Y = C02 ). He later reported other derivatives ((320) M = Co, Ni, Pd and Pt) which were not mesomorphic. ... 

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Discotic LC are formed by disk-like molecules with aromatic cores and side chains that are either hydrophobic (i.e., thermotropic) or hydrophilic (i.e., lyotropic). The discotic nematic (No) phase behaves like a normal nematic phase formed by rod-like molecules, and the disk-like molecules are oriented with their short molecular axes parallel to the director but show no positional order. More ordered columnar phases are commonly formed by thermotropic discotics. The two-dimensional structure can pack the columns into a hexagonal or rectangular columnar phase, while within the columns, disks can be... 

Kumar et al. nevertheless succeeded in creating stable suspensions of hexanethiol-capped gold nanoparticles (1.6 nm core diameter) and gold nanoparticles capped with a 10CB thiol (similar to nanoparticle 3 in Fig. 5) in a lyotropic hexagonal columnar phase (HI phase formed by a 42 58 w/w Triton X-100/water system) as well as an inverse hexagonal columnar phase (H2 phase formed by AOT). Both types of nanoparticles were shown to stabilize the HI phase... 

However, the area was further studied by Usol tseva [205] who synthesized several peripherally carboxylated phthalocyanines and successfully characterized their phase behaviour as lyotropic columnar. The complexes in Figure 111 with M = 2H, Cu, Zn or Co(II) and X = H, Y = COOH and Z H or COOH were found to show columnar nematic and hexagonal phases in aqueous ammonia, but this was suppressed when the phthalocyanine ring was substituted according to X = COOH and Y = Z = H or when the central metal ion was Al(III). In this latter case, suppression of the mesomorphism was due to the formation of p-oxo dimers ([PcAl-O-AlPc]). 

In addition, these complexes, except 49a and 50a, form lyotropic columnar (oblique) and nematic phases when dissolved in linear, apolar organic solvents (alkanes) over wide temperature and concentration ranges. Interestingly, for some of them, 49b-c, an unexpected transition between two lyotropic nematic phases has been observed, for which a model has recently be proposed [93]. As for 48, formation of lyotropic nematic and columnar mesophases is also extended by n-n interactions with electron-acceptors, such as TNF, in apolar solvents (pentadecane). Induction of chiral nematic phases by charge transfer interactions, in a ternary mixture (49b/alkane/TAPA TAPA is 2-(2,4,5,7-tetranitro-9-fluorenylideneaminooxy)-propionic acid and is used (and is available commercially) enantiomerically pure), has recently been demonstrated for the first time [94], and opens new perspective for producing chiral nematic phase of disc-like compounds. 

The dipalladium organyls 51, derived from 49 (M = Pd, X = C1) by ligand exchange reaction between the bridging group and acetylacetonate, are not mesomorphic in their pure state, but form mesomorphic charge transfer systems with the electron acceptor TNF [95]. The identity of the induced mesophase is still unknown, but seems very likely to be columnar [96]. Furthermore, lyotropic nematic phases were obtained in the ternary mixture 51/TNF/linear alkane (the binary mixture 51/alkane did not yield mesomorp-hism). The nematic phase in this system is though to have a columnar nature, namely a nematic columnar phase. 

The first liquid crystals of disc-shaped molecules, now generally referred to as discotic liquid crystals, were prepared and identified in 1977. Since then a large number of discotic compounds have been synthesized and a variety of mesophases discovered. Structurally, most of them fall into two distinct categories, the columnar and the nematic. The columnar phase in its simplest form consists of discs stacked one on top of the other aperiodically to form liquid-like columns, the different columns constituting a two-dimensional lattice (fig. 1.1.8 (a)). The structure is somewhat similar to that of the hexagonal phase of soap-water and other lyotropic... 

Lyotropic cellulosics mostly exhibit chiral nematic phases, although columnar phases have also been observed. The molecules in the thermotropic state also form chiral nematic order, but it is sometimes possible to align them in such a way that a helicoidal structure of a chiral nematic is excluded. Upon relaxation they show banded textures. Overviews on lyotropic LC cellulosics are... 

At higher concentrations, a further anisotropic phase termed the columnar phase has been observed for some lyotropic LC cellu-... 

A basic understanding of the structure and behavior of liquid-crystalline cellulosics has yet to evolve. From a conceptual point of view, the chirality of the cellulosic chain is most sensitively expressed in the super-molecular structure of the cholesteric phase, which may be described by the twisting power or the pitch. At present, no information is available about domains or domain sizes (correlation lengths) of supermo-lecular structures. The chirality in the columnar phases has not been addressed at all. The principal problem, i.e., how does chirality on a molecular or conformational level promote chirality on the supermolecular level, has not been solved. If this correlation were known, it would enable the determination of the conformation of cellulosic chains in the mesomorphic phase and the development of models for the polymer-solvent interactions for lyotropic systems. On the other hand, direct probing of this interaction would provide a big leap towards an understanding of lyotropic phases. 

In addition to this thermotropic mesomorphism, the complexes formed lyotropic mesophases when dissolved in linear, lipophilic solvents such alkanes, and in chloroform, benzene, octanol, octadecanol, and stearic acid." " In alkanes, the mesophases were stable over wide ranges of temperature and concentration. In general, at high complex concentration, a columnar phase was... 

It is now certain that metal carboxylates were the first metal-containing liquid crystals, reported in 1855 with Heinz s work on magnesium tetradecanoate. Then, many other mesomorphic mono-, di-, and tri-valent carboxylate complexes, with the general formula [M(02CC H2 +i)J (x=l, 2, 3) or [M2(02CC H2 +i)4] were prepared. Some of them were described in 1910 by Vorlander." These materials may show thermotropic nematic, smectic, cubic, and columnar mesophases, but also, when dissolved in water or alkanes, lyotropic mesophases. While not all of the compounds described in this section show columnar phases, it was decided to keep these materials together. 

The lyotropic L.C.s are mainly based on amphiphilic molecules, those dissolved in either a polar or apolar solvents can form different nanostructures depending on their volume balances between the hydrophilic part and hydrophobic part. At quite low concentration, amphiphilic molecules are dispersed in the solvent without any ordering. As the concentration is raised they spontaneously assemble into micelles or vesicles, and then at higher concentration, the assemblies will become ordered. A typical phase is a hexagonal columnar phase, where the amphiphiles form long cylinders... 

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