Lyotropic liquid crystals defined

Liquid crystal polymers (LCP) are polymers that exhibit liquid crystal characteristics either in solution (lyotropic liquid crystal) or in the melt (thermotropic liquid crystal) [Ballauf, 1989 Finkelmann, 1987 Morgan et al., 1987]. We need to define the liquid crystal state before proceeding. Crystalline solids have three-dimensional, long-range ordering of molecules. The molecules are said to be ordered or oriented with respect to their centers of mass and their molecular axes. The physical properties (e.g., refractive index, electrical conductivity, coefficient of thermal expansion) of a wide variety of crystalline substances vary in different directions. Such substances are referred to as anisotropic substances. Substances that have the same properties in all directions are referred to as isotropic substances. For example, liquids that possess no long-range molecular order in any dimension are described as isotropic. 

Some unique mesostructures, somehmes identified as ill-defined or defected lyotropic liquid crystals, such as ribbon (Ri), sponge (L3), and low-viscosity, modified micellar cubic (QJ, and hexagonal (H2) phase, have been detected [29, 30, 33, 35, 36]. 

Lyotropic liquid crystals are formed in mixtures of amphiphiles (e.g., surfactants) and solvents, for example, detergents and water. Consequently, these phases are thermodynamically stable at defined temperatures, pressures, and concentrations. Like thermotropics, a variety of structurally distinct modifications exist, which are collectively known as lyotropic liquid crystals. 

Under certain conditions, larger structures than micelles form, and these generate lyotropic liquid crystal phases. Of course, on adding more water, the lyotropic liquid crystal phase would eventually dissolve to give a micellar solution. Surfactants dissolved in water have a Krafft point, defined as the temperature (Tj ) below which... 

In this chapter, the solubility of surfactant compounds in liquid water (and in selected nonaqueous solvents) will be considered. Surfactants are defined here as amphiphilic molecules (molecules containing both polar and nonpolar structural fragments) whose aqueous phase behavior displays explicitly stated features [3]. The most characteristic feature of surfactants is their ability to interact with water to form lyotropic liquid-crystal phases, but surf tant behavior is also jeflected by the influence of water on the temperature of the crystal solubility boundary relative to the melting point, and by the distinctive shape of liquid-liquid miscibility gaps (when they exist). Solubility is but one aspect of the broader subject of phase behavior—albeit a very important aspect. The aqueous phase behavior of surfactants has recently been treated in considerable detail [3]. 

Simplistically stated, the hydrophobic effect may be defined as the tendency of water to reject any contact with substances of a nonpolar or hydrocarbon nature. The existence of this effect was first recognized in the study of the extremely low solubility of hydrocarbons in water. The principles involved were later successfully applied to the elucidation of the native conformation of protein molecules by Kauz-mann The application of these ideas to the study of membrane structures has been advanced by Singer. Recently, Tanford published an entire book on the hydrophobic effect, including the influence of this interaction on the formation of micelles, lipid bilayers, membranes and other ordered structures. Aside from Singer s and Tanford s" statements on the decisive role of the hydrophobic effect on lyotropics, the lyotropic liquid-crystal literature seems peculiarly unaware of this phenomenon. Winsor s extensive review with its systematic analysis (R-theory) of the many lyotropic phases does not take the hydrophobic effect into account. More recent reviews of lyotropic liquid crystals do not mention the phenomenon. We hope that the present discussion will help to advance the realization of the importance of the hydrophobic effect to lyotropics. The material of the following sections is taken chiefly from Ref. [3] with some assistance from Refs. [2] and [4]. 

More recent work has shown that the distinction between lyotropic and thermotropic polymer liquid crystals need not be so rigidly defined. If the amphiphilic side chains have an element of rigidity built into them, using for example a biphenyl group, then some of the mesophases formed closely resemble those of thermotropic side chain polymers. Furthermore, polymers which produce lyotropic liquid crystals may well form mesophases in the absence of a solvent, should the molecular structure favour them. (Note that many surfactants, particularly ionic amphiphiles, also form thermotropic mesophases.) Some specific examples will be discussed in due course. 

Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. 

An exposition of the physics of liquid crystals involves many disciplines continuum mechanics, optics of anisotropic media, statistical physics, crystallography etc. In covering such a wide field it is difficult to define what precisely the reader is expected to know already. An attempt is made to present as far as possible a self-contained treatment of each of these different aspects of the subject. Naturally, discussion of some topics has had to be curtailed for reasons of space. For example, we have not dealt with lyotropic systems, whose complex structures are only just beginning to be elucidated or the special applications of magnetic resonance techniques, as these have been adequately reviewed elsewhere or the very recent results of neutron scattering experiments. The primary aim of this monograph is to provide an insight into the variety of new phenomena exhibited by these intermediate states of matter. 

Cellulose triacetate-trifluoroacetic acid cholesteric solutions - This kind of lyotropic polymer liquid crystals undergoes a mesomorphic-isotropic phase transition upon heating. The peak is well defined but very small The determination of N for this... 

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