Lyotropic liquid crystals lamellar phase

Figure 1.15. Cross-section of the lamellar 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). 

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... 

The potential for novel phase behaviour in rod-coil block copolymers is illustrated by the recent work of Thomas and co-workers on poly(hexyl iso-cyanate)(PHIC)-PS rod-coil diblock copolymers (Chen etal. 1996). PHIC, which adopts a helical conformation in the solid state, has a long persistence length (50-60 A) (Bur and Fetters 1976) and can form lyotropic liquid crystal phases in solution (Aharoni 1980). The polymer studied by Thomas and co-workers has a short PS block attached to a long PHIC block. A number of morphologies were reported—wavy lamellar, zigzag and arrowhead structures—where the rod block is tilted with respect to the layers, and there are different alternations of tilt between domains (Chen et al. 1996) (Fig. 2.37). These structures are analogous to tilted smectic thermotropic liquid crystalline phases (Chen et al. 1996). 

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. 

The calculated results in the absence of electrolyte will be now compared with the experimental results obtained regarding a lamellar lyotropic liquid crystal SDS (sodium dodecyl sulfate)/pentanol/water/dodecane swollen in a mixture of dodecane and pentanol.24 The weight fraction water/surfactant was 1.552 from the dilution line in the phase diagram, we calculated that the initial concentration of pentanol in the oil-free system was 29 wt % and the concentration of pentanol in the dodecane-based diluant was 8 wt %. The experimental values for the repeat distance were obtained from the X-ray diffraction spectrum (Figure 2 in ref 24) for various dodecane concentrations. 

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. 

To summarize, the lecithin studies provide a qualitative picture of how the solvent affects the phase behavior for a zwitterionic surfactant with a large hydro-phobic moiety. Lecithin forms lamellar lyotropic liquid crystals with a wide variety of solvents a sufficiently hydrophilic solvent — and indeed even amphiphilic compounds with a hydrophilic moiety — stabilizes lamellar phases. 

The liquid crystal phases discussed so far are called thermotropic liquid crystals and the transitions from one phase to another are driven by varying temperature. There is another type of liquid crystals, called lyotropic hquid crystals, exhibited by molecules when they are mixed with a solvent of some kind. The phase transitions from one phase to another phase are driven by varying the solvent concentration. Lyotropic liquid crystals usually consist of amphiphilic molecules that have a hydrophobic group at one end and a hydrophilic group at the other end and the water is the solvent. The common lyotropic liquid crystal phases are micelle phase and lamellar phase. Lyotropic liquid crystals are important in biology. They will not be discussed in this book because the scope of this book is on displays and photonic devices. 

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. 

The lamellar (L ) lyotropic liquid crystal phase structure is illustrated in Figure 7.2 and as... 

Cubic lyotropic hquid crystal phases are not as corrrmon as the lamellar or hexagortal phases. However, crrbic lyotropic phases do occtrr in differerrt regions in phase diagrams. Accordingly, there are probably a range of diEfererrt crrbic lyotropic liquid crystal phases, the exact stmcture of which relates to their position within the phase diagram. 

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... 

The construction of phase diagrams of poly(oxyethylene) materials e.g., compound 7) in water reveals the generation of lyotropic liquid crystal phases. Compound 7 is a non-ionic surfactant where the polar head group comprises the oxyetltylene units and, as usual, the hydrophobic alkyl chain is the non-polar unit. At high concentrations (70-90%) of compound 7 in water the lamellar (L ) phase is generated up to moderate temperatures... 

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. 

Figure 2a shows a schematic phase diagram for lyotropic liquid crystals. This figure shows the formation of micelles, cubic phases, bicontinuous cubic phases, and lamellar phases as the concentration of surfactant increases. Also shown in this figure is a schematic diagram of an ordered bicontinuous cubic phase (Fig. 2b). Another interesting example in...

These structures are extensively described in the current literature (Fanum, 2008 Friberg, 1976 Birdi, 2002 Holmberg, 2004 Somasundaran, 2006). Even within the same phases, their self-assembled structures are tunable by the concentration for example, in lamellar phases, the layer distances increase with the solvent volume. Lamellar structures are found in systems such as the common hand soap, which consists of ca. 0% soap + 20% water. The layers of soap molecules are separated by a region of water (including, salts etc.) as a kind of sandwich. The x-ray diffraction analysis shows this structure very clearly. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liquid crystals. Similar phases and characteristics can be observed in immiscible diblock copolymers. 

Little work seems to have been done on thin oriented layers of lyotropic liquid crystals although there is one recent report of preparation of such a layer of the lecithin-water lamellar phase (JO). As indicated by Brochard and de Gennes (II), theories of the hydrodynamics of thermotropic smectic materials can be adapted to describe oriented layers of lamellar liquid crystal in lyotropic systems. 

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