Lyotropic liquid crystals phase diagrams

Figure 8.6 Cj2E08-water lyotropic liquid-crystal phase diagram. Reproduced with permission from [63], Copyright (1983) Royal Society of Chemistry...

Figure 8.7 The lyotropic liquid crystal phase diagram for CTAB-water system...

SBA-1 and SBA-6 were synthesized by using different surfactants and from acidic and basic synthesis media, respectively. They have the same structure and show the similar XRD patterns. Their structure is similar with cubic Ij phase, spherical micelles packed in Pm3n symmetry, in lyotropic liquid-crystal phase diagram for surfactant-water systems. 

Other surfactants show the similar lyotropic liquid-crystal phase behavior and follow the same succession of phases, but not all of the phases are always present. Figure 8.7 shows a phase diagram for the CTAB-water binary system. CMC can also be classified CMC1 (spherical micelle) and CMC2 (rod-shaped micelle). 

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. 

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...

Figure 6 shows the phase diagrams plotting temperature T vs c for PHIC-toluene systems with different Mw or N [64], indicating c( and cA to be insensitive to T, as is generally the case with lyotropic polymer liquid crystal systems. This feature reflects that the phase equilibrium behavior in such systems is mainly governed by the hard-core repulsion of the polymers. The weak temperature dependence in Fig. 6 may be associated with the temperature variation of chain stiffness [64]. We assume in the following theoretical treatment that liquid crystalline polymer chains in solution interact only by hardcore repulsion. The isotropic-liquid crystal phase equilibrium in such a solution is then the balance between S and Sor, as explained in the last part of Sect. 2.2. 

The phase behavior of surfactant systems is particularly complex because of the existence of numerous lyotropic (solvent-induced) liquid crystal phases (3). These phases, like liquids and crystals, are discrete states of matter. They are fluids, but their x-ray patterns display sharp lines signifying the existence of considerable structure. They are often extremely viscous because of their high viscosities and for other reasons they are difficult to study using conventional methods. This is evident from the fact that serious errors in the presumably well-established classical aqueous phase diagrams of soaps, sodium alkyl sulfates, monoglycerides, and... 

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 formation of lyotropic liquid-crystal mesophase depends on the structure and properties of surfactant, solvent, and reaction conditions. Although studies on lyotropical liquid crystals have been carried out for many years, the structure and properties of some mesophases are still not very clear. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is difficult to analyse their structures and properties, the boundary in the phase diagram may be not accurate and the minor phase may be missed. 

The common structure models for Iniim symmetry are shown in Figure 8.28. Among the well known lyotropic liquid-crystal mesophases, these are at least two mesostructures with Im3m symmetry one locates near the Ii region in the phase diagram, with a possible spherical micelle packed structure. Another one is close to the Vi region, and its most probable structure can be described by a P surface. 

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. 

FIG. 3.14 Binary phase diagram of a water-ethoxylated nonionic amphiphile system, including lyotropic liquid crystal domains. (From Kalhweit, M. and Strey, R., Angew. Chem. Int. Ed. Engl., 24, 654, 1985. With permission.)... 

As for low molecular weight surfactants, the superstructures are assumed to be formed by micellar aggregates [126], But it seems that the formation of lyotropic liquid crystals is supported by the additional presence of thermotropic mesogens [87,122-124,126], Lamellar, hexagonal, cubic and even nematic and cholesteric mesophases were reported for binary systems, the latter being exceptional. Lyotropic mesophases were also observed in non-aqueous solvents [240,400,401,405], If polymerizable surfactants are studied, not only the phase diagram but also the types of mesophases observed for the monomer and the polymer may be different. 

A quantitative evaluation of the effects of the degree of polymerization on the properties of polysoaps was undertaken for the phase diagrams of polymeric lyotropic liquid crystals. Here the property shifts level off within a degree of 10-15 (Fig. 44) [126,451], This value may serve as a first approximation, but, as additional studies, e.g. on viscosity, surface tension or solubilization are missing, more studies are needed. 

The phase transitions of liquid crystals in solutions occur normally through two mechanisms, i.e. lyotropic and thermotropic transitions. The lyotropic liquid crystal occurs upon addition of solvent into the crystalline phase, while the thermotropic liquid crystal occurs upon heating the crystalline phase, as illustrated by the two arrows in Fig. 10.3, respectively. The phase diagrams for the transition from the homogeneous solution to the liquid crystal are formed by two almost parallel curves, reflecting the concentration gap between the two coexisting phases. 

For lyotropic liquid crystals, the temperature plays a secondary role in the formation of the individual mesophases. The primary influence on the phase sequence is exerted by the solvent concentration. The solvent concentration is directly connected to the packing parameter and thus to the micellar shape (cf. Sect. 3.1), which largely determines the mesophase type. At low solvent concentrations lamellar phases are usually formed. By increasing the solvent concentration, columnar and nematic phases appear. At very high solvent concentrations an isotropic micellar solution dominates. An illustration of this phase behavior is shown in the theoretical phase diagram depicted in Fig. 3.9 (c [14]). The individual phases in Fig. 3.9 are separated by biphase regions. 

Fig. 3.9 Theoretical phase diagram of a lyotropic liquid crystal. The phase transition from one lyotropic liquid crystalline phase into another mainly depends on the solvent concentration...

In addition to the staled aims of this thesis, the phase diagram of the selected surfactant mixed with iV-methylformamide as solvent was investigated to show that no lyotropic SmC analog phase occurs with a solvent that does not form a three-dimensional hydrogen bond network. However, two other interesting phases appear by the addition of this solvent. The first phase is a rare example of a re-entrant cholesteric phase and the second is a solvent-induced twist grain boundary phase, the first observation of this phase in a lyotropic liquid crystal. 

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