Lyotropic liquid crystals transitions

Lyotropic Liquid Crystals, Some molecules in a sulvcni form phases with orientational antl/or positional order. In these systems, the transition from one phase to another can occur due to a change of concentration, so they arc given the name lyotropic liquid cry stals Of course temperature can also cause phase transitions in these systems, so this aspect of thermotropic liquid crystals is shared hy lyotropics. The real distinctiveness of lyotropic-liquid crystals is the fact that at least two very different species of molecules nttisl be present for these structures to form... 

In this review we are mainly concerned with thermotropic materials, i.e. with liquid crystals and LC-glasses which do not contain a solvent. The transitions of the macro-molecular, thermotropic liquid crystals are governed then by temperature, pressure and deformation. In lyotropic liquid crystals and LC-glasses a solvent or dispersing agent is present in addition. The transitions then also become concentration dependent. 

Liquid crystalsare an intermediate state in which the molecules in a crystal can undergo a secondary phase transition to a mesophase, which gives them mobility in 1-2 directions. They are birefringent, but possess low properties like a liquid phase. Lyotropic liquid crystals form on uptake of water into a system that increases its mobility, and thermotropic liquid crystals can be disrupted by heating above a transition temperature. Cromolyn sodium (Cox et al., 1971), the HMG-CoA reductase inhibitor SQ33600 (Brittain et al., 1995), and the leukotriefienffagonist L-660,711 (Vadas et al., 1991) are examples of pharmaceuticals that can form liquid crystals. 

We note that earlier research focused on the similarities of defect interaction and their motion in block copolymers and thermotropic nematics or smectics [181, 182], Thermotropic liquid crystals, however, are one-component homogeneous systems and are characterized by a non-conserved orientational order parameter. In contrast, in block copolymers the local concentration difference between two components is essentially conserved. In this respect, the microphase-separated structures in block copolymers are anticipated to have close similarities to lyotropic systems, which are composed of a polar medium (water) and a non-polar medium (surfactant structure). The phases of the lyotropic systems (such as lamella, cylinder, or micellar phases) are determined by the surfactant concentration. Similarly to lyotropic phases, the morphology in block copolymers is ascertained by the volume fraction of the components and their interaction. Therefore, in lyotropic systems and in block copolymers, the dynamics and annihilation of structural defects require a change in the local concentration difference between components as well as a change in the orientational order. Consequently, if single defect transformations could be monitored in real time and space, block copolymers could be considered as suitable model systems for studying transport mechanisms and phase transitions in 2D fluid materials such as membranes [183], lyotropic liquid crystals [184], and microemulsions [185],... 

This article will, in addition to a short description of the essential features of surfactant systems in general, concentrate on the energy conditions in premicellar aggregates, the transition premicellar aggregates/inverse micellar structures and the direct transition premicellar aggregates/lyotropic liquid crystals. 

The range of polymers which were found to be able to form liquid crystalhne systems has been considerably extended. Poly(Y-benzyl-L-glutamate) and its analogs, as well as para-aromatic polyamides, exhibit this property in solutions, which served the basis for relating them to lyotropic liquid crystals (see Sect. 2.1). Subsequently, the classes of polymers were found which exhibited such a transition during a change of temperature thermotropic liquid crystals). 

If the concentration of surfactant becomes high enough, surfactant structures often develop long-range order, and hence they become liquid crystalline. They are lyotropic liquid crystals, because the transition to the liquid-crystalline state is induced by concentration changes. Surfactant solutions can form nematic and smectic-A liquid-crystalline phases analogous to those discussed in Chapter 10. In addition, hexagonal and cubic phases are common in surfactant solutions. 

Table 1 Possible transitions of lyotropic liquid crystals...

Shin, S. Kumar, S. Finotello, D. Keast, S. Neubert, M. High-precision heat capacity study of phase transitions in a lyotropic liquid crystal. J. Am. Phys. Soc. 1992, 45, 8683-8692. 

Strictly linear macromolecules would be, for examjde I, IX, X, and XIII to XV of Table 6.1. Of these not all have been made or are chemically stable and little is known about their structure. Polyparaphenylene, as one example, was described in Sect. 5.1.4 and linked to the better known oligomers (Sect. 5.1.1-5.1.3). The LC glass state proposed for the polymer would be difficult to dissolve into a lyotropic liquid crystal because of the seeming lack of high temperature solvents that could reduce the glass transition temperature sufficiently. By itself decomposition occurs before the glass transition temperature is reached. 

Similar experiments performed at higher CTAB concentrations near the phase transition from isotropic solution to lyotropic liquid crystals show that the phase transition temperature is affected by the presence of rheologically active compounds (155,161). Figure 13 demonstrates that the phase transition temperature increases when small amounts of 9-anthracene carboxylic acid are solubilized. Irradiation at X = 366 nm, i.e. photodimerization, removes the effect, and reirradiation at X = 254 nm (splitting of the dimers) causes a reincrease of the phase transition temperature. 

Somewhat later, even biological systems were investigated from a liquid crystal research point of view (e.g., the tobacco-mosaic-virus) [26], see also Chap. VIII of this volume. The main progress of lyotropic liquid crystal research in these times is connected to works on soap/water mixtures [27, 28] and the investigation of thermotropic mesophases of soaps [29]. The amphotropic character of such compounds was also studied. In such systems, no continuous transitions were observed between thermotropic and lyotropic mesophases, but always biphasic regions could be seen [30, 31]. Thereafter, the interest in understanding biological cell membranes inspired research-... 

Engaged with the constitution of cholesterol, for cholesteryl benzoate Reinitzer reported two phase transitions in the eourse of melting. Melting starts with the formation of a cloudy liquid, which transforms on further heating into a clear liquid. This type of liquid crystal is addressed as thermotropic. In addition, there are lyotropic liquid crystals that change their behavior as both a function of concentration in a solvent and of temperature. 

The lyotropic liquid crystals have been studied as a separate category of liquid crystals since they are mostly composed of amphiphilic molecules and water. The lyotropic liquid-crystal structures exhibit the characteristic phase sequence from normal micellar cubic (IJ to normal hexagonal (Hi), normal bicontinuous cubic (Vi), lamellar (1 ), reverse bicontinuous cubic (V2), reverse hexagonal (H2), and reverse micellar cubic (I2). These phase transitions can occur, for instance, when increasing the apolar volume fraction [9], or decreasing the polar volume fraction of the amphiphilic molecule, for example, poly(oxyethylene) chain length in nonionic poly(oxyethylene) alkyl (oleyl) or cholesteryl ether-based systems (10, 11). 

R. Mezzenga, M. Grigotov, Z. Zhang, C. Servais, L. Sagalowiez, A.I. Romoscanu, Polysaccharide-induced order-to-order transitions in lyotropic liquid crystals. Langmuir 21, 6165-6169 (2005)... 

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. 

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