Lyotropic LC phases

Since their discovery in the nineteenth century [1], hquid crystals have fascinated scientists due to their unusual properties and their wide range of potential apphcations, especially in optoelectronics. LC systems can be divided into two categories thermotropic LC phases and lyotropic LC phases. Thermotropic LC systems result from anisotropic molecules or molecular parts (so called mesogens or mesogenic moieties, respectively), and form hquid crystalline phases between the soHd state and the isotropic hquid state, where they flow like liquids but possess some of the characteristic physical properties of crys-... 

Cellulose and some derivatives form liquid crystals (LC) and represent excellent materials for basic studies of this subject. A variety of different structures are formed, thermotropic and lyotropic LC phases, which exhibit some unusual behavior. Since chirality expresses itself on the configuration level of molecules as well as on the conformation level of helical structures of chain molecules, both elements will influence the twisting of the self-assembled supermolecular helicoidal structure formed in a mesophase. These supermolecular structures of chiral materials exhibit special optical properties as iridescent colors, and... 

Many biological stmctures also exhibit lyotropic LC phases. In particular, biological membranes and cell membranes are the typical examples of lyotropic liquid crystals (Chapter 8.13). Their constituent molecules (e.g., phospholipids) form a bilayer stmcture in which some proteins, polysaccharides, cholesterol, and other vitally important components are incorporated. 

Lyotropic polymer systems differ in that the stability of the LC phase is influenced by the addition of a low-molecular-mass solvent. An important consequence of solvent addition is that LC phase can be obtained with rigid-rod polymers (Figure 4(b)) which would not otherwise show the mesophase at temperatures below that of thermal decomposition. In other words, lyotropic LC phase formation is a route to... 

Both PBO and PBT polymers only form lyotropic LC phases in strong polyphosphoric acid at a concentration of polymer about 5-6%. 

Amphiphilic dye molecules such as sulfonated chromogens can be used in place of PVA iodine for the preparation of thin polarizing films [60]. These molecules are usually water soluble and at a wide range of concentrations, temperatures, and pH values they self-assemble and stack up to form hexagonal complexes. This is usually referred to as the lyotropic LC phase [1]. It can be orientated, by mechanical shear stress, so that the lyotropic liquid crystal (LLC) is orientated parallel to the shear flow direction (Figure 4.21). 

MAS has been applied to a highly viscous cubic phase of a lyotropic LC formed by 1-monooleolyl-rac-glycerol and water in order to obtain liquid-like and 13C spectra.330 Deuterium, sodium, and fluorine NMR spectroscopy have been applied to study the phase behaviour of several dilute lamellar systems formed by low concentrations of an ra-hexadecylpyridinium salt, a sodium salt (e.g., NaBr, NaCl, or sodium trifluoroacetate), 1-hexanol, and D20.331 The 2H, 19F, and 23Na splittings were used to monitor the phase equilibria. The last two studies are motivated by the search of new lyotropic LC for the alignment of biomolecules. 

Thermotropic liquid crystalline (LC) phases or mesophases are usually formed by rod-like (calamitic) or disk-like (discotic) molecules. Spheroidal dendrimers are therefore incapable of forming mesophases unless they are flexible, because this would allow them to deform and subsequently line up in a common orientation. However, poly(ethyleneimine) dendrimers were reported to exhibit lyotropic liquid crystalline properties as early as 1988 [123],... 

Thermotropic LCs exhibit the phase transition into the LC phase as temperature is changed, whereas lyotropic LCs exhibit it as a function of concentration of the... 

As compared to the cholesteric LC, the lyotropic LC consists of two or more components that exhibit liquid-crystalline properties (dependent on concentration, temperature, and pressure). In the lyotropic phases, solvent molecules fill the space around the compounds (such as soaps) to provide fluidity to the system. In contrast to thermotropic liquid crystals, these lyotropics have another degree of freedom of concentration that enables them to induce a variety of different phases. A typical lyotropic liquid crystal is surfactant-water-long-chain alcohol. 

Lyotropic liquid-crystalline nanostructures are abundant in living systems. Accordingly, lyotropic LC have been of much interest in such fields as biomimetic chemistry. In fact, biological membranes and cell membranes are a form of LC. Their constituent rod-like molecules (e.g., phospholipids) are organized perpendicularly to the membrane surface yet, the membrane is fluid and elastic. The constituent molecules can flow in plane quite easily but tend not to leave the membrane, and can flip from one side of the membrane to the other with some difficulty. These LC membrane phases can also host important proteins such as receptors freely floating inside, or partly outside, the membrane. 

Boy et al. [79] used lyotropic liquid crystals (LC) for the immobilization of HfeHNL. The solid LC phase is not used because of the high viscosity. Therefore, the LC is used in a biphasic system consisting of the LC and an organic solvent. Such biphasic liquid crystal systems consist of organic solvent, water, and surfactant, where poorly soluble substrates and products are dissolved in the organic solvent and the liquid crystal matrix, which contains the enzyme, has a protective effect on it. By optimization and by virtue of the immobilization, it is possible to establish an extractive continuous process [79]. 

LC polymers can either be thermotropic, their LC phases or mesophases are formed by heating a solid or cooling a liquid or solid, or lyotropic, the mesophases are formed by association of molecules in solution. The photochemistry and photophysics of molecules in lyotropic phases such as detergent micelles and lipid bilayers has been a very active research area for about 30 years. This extensive literature [12] is not reviewed in this chapter which is necessarily restricted to thermotropic LC polymers. 

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

Liquid crystals are classified into two groups known as thermotropics and lyotropics. Thermotropics are those that are formed in the melting of crystalline solids, and they can remain in the liquid crystal mesophase without decomposition, passing to the isotropic liquid state when subsequently heated. As their mesophases are turbid, the temperature at which the transition to the isotropic liquid phase takes place is called the clearing temperature. Lyotropic LCs form mesophases in concentrated solution when the concentration exceeds a critical value. 

Liquid crystals (LCs) are molecules that have the ability to self-assemble into organized mesophases with properties intermediate between those of crystalline solids and isotropic liquids [1,2]. In LC phases, the molecules are dynamic and collectively behave as a viscous liquid but retain on average a degree of organization reminiscent of an ordered, crystalline solid. Consequently, they can be considered ordered fluids, as a more accurate definition. LCs can be subdivided into two general classes—thermotropic LCs and lyotropic LCs—depending on the environmental and molecular factors that govern how they form ordered fluid phases. 

In contrast, lyotropic LCs (LLCs) are molecules that form ordered fluid phases that are phase-separated mixtures in which the degree of order depends on the proportion of LC mesogen relative to an added immiscible solvent such as water [3,4], In addition to morphology dependence on system composition, LLC phases are also sensitive to other external parameters... 

Fig.4 Structure of a typical lyotropic chromonic LC, and schematic representations of two lyotropic chromonic LC phases. Reproduced with permission from [10]. 2004 by Elsevier...

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... 

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