Lyotropic liquid crystals hexagonal phase

Figure 1.14. Cross-section of the hexagonal lyotropic liquid crystal phase.

Lamellar lyotropic liquid crystal phases are less viscous than the hexagonal lyotropic liquid crystal phases despite the fact that they contain less water. This is because the parallel layers slide over each other with relative ease during shear and this is quite easy to visualise (see Figure 7.2). 

Hexagonal lyotropic liquid crystal phases typically contain 30 to 60% water by weight and despite this high water content the phase is very viscous. The viscosity of the hexagonal phase means that it is best avoided in the practical, industrial handling of surfactants. 

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

Stiff rod-like helical polymers are expected to spontaneously form a thermotropic cholesteric liquid crystalline (TChLC) phase under specific conditions as well as a lyotropic liquid crystal phase. A certain rod-like poly(f-glutamate) with long alkyl side chains was recently reported to form a TChLC phase in addition to hexagonal columnar and/or smectic phases [97,98]. These properties have already been observed in other organic polymers such as cellulose and aromatic polymers. 

The phase diagram of sodium dodecyl sulfate-water is representative of many ionic systems (Figure 3.7) [5], In Figure 3.7 Liquid is the aqueous micellar phase Ha is the hexagonal lyotropic liquid crystal, sometimes called the middle phase and La is the lamellar lyotropic liquid crystal, sometimes called the neat phase. On the surfactant-rich side, several hydrated solid phases are present. 

Three different classes of lyotropic liquid crystal phase structures are widely recognised. These are the lamellar, the hexagonal and the cubic phases, and their structures have each been classified by X-ray dififiaction techniques. 

Structurally, the cubic lyotropic liquid crystal phases are not as well-characterised as the lamellar or hexagonal phases. However, two types of cnbic lyotropic liquid crystal phases have been estabhshed and each can be generated in the normal manner (water continuous) or in the reversed manner (non-polar chain continnous), which makes for a total of fottr different phase types. The most well-known cnbic phase consists of a cubic arrangement of molecular aggregates. The molecttlar aggregates are similar to micelles (Ij phase) or reversed micelles (1 phase). The stractrrre of the normal (1 ) cubic... 

In a solvent, block copolymer phase behavior is controlled by the interaction between the segments of the polsrmers and the solvent molecules as well as the interaction between the segments of the two blocks. If the solvent is unfavorable for one block, this can lead to micelle formation in dilute solution. The phase behavior of concentrated solutions can be mapped onto that of block copolymer melts (97). Lamellar, hexagonal-packed cylinder, micellar cubic, and bicontinu-ous cubic structures have all been observed (these are all lyotropic liquid crystal phases, similar to those observed for nonionic surfactants). This is illustrated by representative phase diagrams for Pluronic triblocks in Figure 6. 

The lamellar lyotropic liquid crystal phase is often formed in detergent solutions. When subjected to shear lamellae can, under certain conditions, curve into closed shell structures called vesicles (Section 4.11.4). These are used in pharmaceutical and cosmetic products to deliver molecules packed into the core. Selective solubilization in micelles finds similar applications, although micelles tend to break down more rapidly than vesicles when diluted. Applications for hexagonal and cubic structures may stem from the recent discovery that they can act as templates for inorganic materials such as silica, which can be patterned into an ordered structure with a regular... 

In concentrated solution, DNA fragments can form lyotropic liquid crystal phases. Short fragments behave like rods, and so the formation of liquid crystal phases is possible. On increasing concentration (above 160 mg/ml for 50 nm DNA in physiological salt solutions), cholesteric and hexagonal columnar phases may be observed (see Chapter 5 for a discussion of these structures) Just below the cholesteric phase, a blue phase is sometimes observed. This phase is named for the colour arising from the double twist cylinders that result from the packing of helices onto a cubic lattice. 

Traditionally, mesoporous metals have been elaborated by using mesoporous silica as a hard template. In 1997, Attard s research group first reported that mesoporous platinum can be produced by the chemical or electrochemical reduction of metal salts dissolved in the aqueous domains of a hexagonal lyotropic liquid crystal (LLC) phase architecture. " They have shown that the reduction of platinum salts in this system led to platinum whose nanostructure was a cast of the LLC... 

Here, a paper by Smalyukh et al. stands out, which reports on nanorod alignment using likely more suitable lyotropic liquid crystals [6], The authors demonstrated spontaneous, long-range orientational ordering of CTAB-capped GNRs dispersed in lyotropic nematic and hexagonal columnar liquid crystalline phases formed by... 

In Attard s approach, tetramethylorthosilicate (TMOS) was hydrolyzed and condensed in the aqneons domain of the liqnid crystal phase at pH of abont 2, leading to mesostmctured hexagonal, cubic, or lamellar sihca. Methanol from the hydrolysis of TMOS destroys the long-range order of the liquid crystal however, upon the removal of methanol, the lyotropic liquid crystal is restored and serves as the template phase for the further condensation of silicates. The resnlting pore system replicates the shape of the lyotropic mesophase, so this process is also termed nanocasting . 

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