Mesophases and Liquid Crystals

A discussion of surfactant solutions usually involves the solid surfactant phase, dissolved surfactant monomers, and micelles in a solution above the cmc. Actually, a surfactant in a solvent can form several phases, depending on temperature and the surfactant concentration. A complete phase diagram of the surfactant-water system is essential for the understanding of the properties of the surfactant and its solutions [151]. 

Lyotropic liquid crystals result from an interaction between a solid and a liquid. Hence, lyotropic liquid crystals are binary systems. When a solid surfactant is brought into contact with water, the predominantly crystalline solid disintegrates. Molecular and micellar solutions are isotropic. Interactions and ordering in molecular solutions are only short range. However, the dissolution of the surfactant in water does not proceed directly to a micellar solution but involves transitions to intermediate phases. These mesophases (meso = in between) have re- 

Mesophases also form when the concentration of a surfactant in its micellar solution is increased. When the concentration of the surfactant above cmc is increased, the number of micelles and their size increase, in accord with the mass action model. Dilute micellar solutions are isotropic, but at higher surfactant concentrations, intermicellar interactions produce mesophases which are anisotropic and have a one- or two-dimensional ordering. 

The structure of mesophases has been investigated by polarizing microscopy, light scattering, x-ray scattering, and SANS (see Chapter 9). The methods used in the past have been insufficient for the study of surfactant phases [154]. Recent studies have significantly benefited from NMR spectrography. 

The hexagonal phases consist of rodlike micelles packed in an hexagonal assembly (Fig. 6.1). A lamellar phase consists of bilayer surfactant aggregates sep- 

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C tructures with the appearance or properties of mesophases and liquid crystals have been observed in extracts of animal tissue since the end of the 19th century. There is, however, a difference between matter extracted from tissue—usually dead or dying—and matter integrally present in living tissue. Matter in the first category is easily obtainable but liable to be deceptive in appearance for the tissue is altered or killed by the process of extraction. Matter in the second category is extremely difficult, sometimes impossible, to visualize by methods appropriate to identifying ordered systems, but there are now some observations which establish the importance of mesoforms as components of cells and tissue. 

Another type of liquid crystalline polymers are those having rod-shaped side chains. TTius, the backbone may be a random coil, but the side chains are organized into one- or two-dimensional liquid crystals. The stiff backbone types make very high modulus fibers and high temperature plastics. The side chain types are useful for their action in magnetic and electrical fields. (Note the behavior of battery-driven liquid crystalline watches, for example. The liquid crystalline material is held between crossed Nicols the orientation of the molecules is controlled by an electric field.) In the following, the terms mesophase and liquid crystal phase are used interchangably. 

Grint, A. Swietlik, U. Marsh, H. Carbonization and Liquid-crystal (Mesophase) Development-9. The Co-carbonization of Vitralns with Ashland A200 Petroleum Pitch. Fuel, 1979, M,... 

No coherent threadline could be maintained and the extmdate flew off the windup as short, brittle, crystalline lengths. Not until many years later did other workers show that this polymer on cooling exhibits a mesophase transition directly from the isotropic melt to a smectic A phase. Good sources of information on liquid crystals and liquid crystal polymers are available (212—216). 

To improve the effectiveness of the chromatographic separation, a comparison study has been carried out on cyclodextrin and liquid crystal stationary phases Both materials function as "ordered" media with cyclodextrins the inclusion complex formation predominates, whereas the liquid crystals enable interaction of compounds with the ordered structure of the mesophase ... 

The field of soft condensed matter/complex systems spans a seemingly diverse spectrum, including isotropic liquids with short-range order at the one end, and liquid crystals with long-range order in their anisotropic phases, often called mesophases, at the other [2-4]. Anisotropy in molecular shape plays a crucial... 

Ordered (and partially ordered) arrays of metal sites and complexes enable the cooperation of their special electronic, magnetic and optical properties. Such materials have long been sought for their expected physical properties and applications in optics, electrooptics, superconductivity and sensors. The ordering can be by various mechanisms, such as adsorption on surfaces, intercalation into layered structures, formation of mesomorphic structures and liquid crystals, and adoption of specific crystal-packing motifs, all of which are supramolecular phenomena. Organic liquid crystals and their applications are now commonplace, and in recent years the incorporation of metal atoms into mesogenic molecules has demonstrated the occurrence of similar metallo-mesophases [20]. 

Chen, W. (1996) in Liquid-Crystalline Polymer Systems (ed. A.I. Isayev, T. Kyu and S.Z.D. Cheng), American Chemical Society, Washington, chapter 15], and liquid crystals (that is, condensed phases in which molecules exhibit orientational order and varying degrees of positional order [2]). The distinctions among these three mesophase types will be maintained throughout this chapter liquid crystalline systems which are thermotropic (that is, induced by changes in the temperature or pressure of a sample) rather than lyotropic (that is, induced by addition of an isotropic liquid as solvent ) will be emphasized. 

Xenopoulos A, Cheng J, Yasuniva M, Wunderlich B (1992) Mesophases of Alkyl-ammonium Salts. I. First-order Transitions. Molecular Crystals and Liquid Crystals 214 63-79. 

The mesophase state of liquid crystals is normally opaque due to relatively large sizes of ordered domains. Its transition point to the isotropic melt state is called the clear point T,. The DSC scanning curves of liquid crystals can exhibit either enantiotropic or monotropic phenomena. For the thermodynamically stable mesophases of liquid crystals, they occur between the melt and the crystal states during both cooling and heating processes, as illustrated in Fig. 10.5. When both the cooling and heating curves show two symmetric consecutive phase transitions, it is known as the enantiotropic phenomenon. In contrast, for the metastable mesophase... 

The concept of symmetry is equally important for understanding properties of individual molecules, crystals and liquid crystals [1]. The symmetry is of special importance in physics of liquid crystal because it allows us to distinguish numerous liquid crystalline phases from each other. In fact, all properties of mesophases are determined by their symmetry [2], In the first section we consider the so-called point group symmetry very often used for discussion of the most important hquid crystalline phases. A brief discussion of the space group symmetry will be presented in Section 2.2. 

Liquid crystals represent an intermediate state of order (mesophase) between crystals and liquids. Crystals have a three-dimensional long-range order of both position and orientation (Fig. 5.1-la). Liquids, in contrast, do not show any long-range order (Fig. 5.1-lb). In plastic crystals (disordered crystals, Fig.5.1-lc), positional order is maintained, but orientational order is lost. In mesophases, imperfect long-range order is observed, and thus they are situated between crystals and liquids. The reasons for the formation of a mesophase can be the molecular shape or a microphase separation of amphiphilic compounds. 

Xanthan forms lyotropic and isotropic liquid crystals in water at appropriate concentrations (53). The formation of solvent-based liquid crystals is an area of expanding interest in personal care because of their unique ability to form emulsifier-free suspensions, which is discussed in the section describing hydroxypropylcellulose (Section III.C.2.a.ii). Specifically, xanthan forms cholesteric mesophase lyotropic liquid crystals in water at concentrations as low as 3.5 wt%. Formation of the liquid crystal phase is influenced by salt concenfration the higher the salt concentration, the lower the xanthan concentration required to form the liquid crystal phase. 

The superficial similarity between the hexagonal condis crystals of the flexible macromolecules, listed above, and liquid crystals leads frequently to wrong assignments (57). All of the listed flexible polymers are classified as condis crystals based on points 3,4,6, 7, and 8 of Fig. 8 (16,17). It is of particular importance to note that a hexagonal, columnar structure may exist not only for LCs, but also for condis crystals. The X-ray structure of a mesophases is often insufficient to identify the type of mesophase. Most of these condis crystals were earlier called smectic liquid crystals on the basis of X-ray... 

X-ray diffractometry, polarization microscopy and mixture testingt are important in the identification of mesophases of liquid crystals while DSC and DTA are the most valuable aids in revealing and confirming phase transition. Simultaneous polarization microscopy-DSC instruments are now commercially available. 

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