Lyotropic liquid crystals amphiphiles

A different kind of mesophase is formed by amphiphilic molecules in lyotropic liquid crystals. Amphiphilic molecules exhibit a hydrophilic (polar) head and a hydrophobic (nonpolar) tail and form micelles, columns, or lamellae consisting of many molecules. These xmits can be arranged in an anisotropic structure, if the concentration of the amphiphilic substance in a solvent is suitable. In contrast to thermotropic liquid crystals, these anisotropic solutions of amphiphilic molecules are called lyotropic liquid crystals. In this book, we consider mainly thermotropic liquid crystals an overview on chiral lyotropic liquid crystals is given in Chapter 14. 

Lyotropic liquid crystals, 15 86, 98-101 amphiphilic molecules in, 25 99-101 Lyotropic mesophases, 20 79 Lyotropic polymer liquid crystals, 25 107-108 Lyral, 2 278 24 486 Lysergic acid, 2 100 Lysergic acid diethylamide (LSD-25),... 

Lyotropic liquid crystals differ from thermotropic liquid crystals. They are formed by mesogens which are not the moleeules themselves but their hydrates or solvates as well as associates of hydrated or solvated molecules. In the presenee of water or a mixture of water and an organie solvent as the most important solvents for drug molecules, the degree of hydration or solvation, respectively, depends on the amphiphilic properties of a drug moleeule. Hydration of the mostly rod-shaped molecule— and the same holds for solvation—results in different geometries, eone or cylinder [5] (Fig. 3). 

Drug molecules with amphiphilic character may form lyotropic mesophases, and amphiphilic excipients in drug formulations also form lyotropic liquid crystals. Especially surfactants, which are commonly used as emulsifiers in dermal formulations, associate to micelles after dissolution in a solvent. With increasing concentration of these micelles the probability of interaction between these micelles increases and thus the formation of liquid crystals. 

Note 1 The essential feature of a lyotropic liquid crystal is the formation of molecular aggregates or micelles as a result of specific interactions involving the molecules of the amphiphilic mesogen and those of the solvent. 

A compound that has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an amphiphilic molecule (as mentioned earlier). Many amphiphilic molecules show lyotropic liquid-crystalline phase sequences, depending on the volume balances between the hydrophilic part and the hydrophobic part. These structures are formed through the microphase segregation of two incompatible components on a nanometer scale. Hand soap is an everyday example of a lyotropic liquid crystal (80% soap + 20% water). 

Liquid crystals can broadly be classified into lyotropic and thermotropic. Lyotropic liquid crystals are formed by amphiphilic substances in the presence of a solvent. The... 

Moreover, there are amphiphilic molecules that can behave as both thermotropic and lyotropic liquid crystals. 

Amphiphilic Molecules. In just about all cases of lyotropic liquid crystals, the important component of the system is a molecule w ith two very different parts, one that is hydrophobic and one that is hydrophilic. These molecules arc called amphiphilic because when possible they migrate to the interface between a polar and nonpolar liquid. 

A characteristic feature of molecules that form lyotropic liquid crystals is their surface activity. Because of the amphiphilic nature of the molecules, they orient upon contact with solvent molecules, giving rise to polar and nonpolar regions that are separated by the polar end groups. All structures known fit one of those made possible by the various curvatures of the interface between two liquid regions, with molecular size taken into consideration. It is therefore not surprising that the earlier treatment of the structure of lyotropic liquid crysals was unsuccessful since the molecules were regarded as stiff rods. 

Order and Mobility are two basic principles of mother nature. The two extremes are realized in the perfect order of crystals with their lack of mobility and in the high mobility of liquids and their lack of order. Both properties are combined in liquid crystalline phases based on the selforganization of formanisotropic molecules. Their importance became more and more visible during the last years in Material science they are a basis of new materials, in Life science they are important for many structure associated functions of biological systems. The main contribution of Polymer science to thermotropic and lyotropic liquid crystals as well as to biomembrane models consists in the fact that macromolecules can stabilize organized systems and at the same time retain mobility. The synthesis, structure, properties and phototunctionalization of polymeric amphiphiles in monolayers and multilayers will be discussed. 

The unusual optical properties of liquid crystals had been remarked upon and described for several centuries before their uniqueness as a state of matter was recognised. Their early reports described the strange melting behaviour and appearance of some naturally occurring materials, either as pure compounds or as gels in water, which have now been shown to be thermotropic or lyotropic liquid crystals. Thermotropic liquid crystalline phases are formed under the action of heat, see Figures 2.1 and 2.2, and the lyotropic liquid crystalline phases are formed by the action of a solvent, such as water, usually with an amphiphilic compound. However, the nature of these materials, or indeed their exact... 

The phase regions for micellar solutions and lyotropic liquid crystals form a complicated pattern in water/amphlphile/hydrocarbon systems. The present treatment emphasizes the fact that they may be considered as parts of a continuous solubility region similar to the one for water/short chain amphiphilic systems such as water/ethanol/ethyl acetate. 

Extension of the molecular sieves to the mesoporosity range is possible nsing lyotropic liquid crystal mesophases (Figine 25.20) as removable templates. These mesophases result from the self-assembly of surfactants or amphiphilic molecules and can be thermally or chemically eliminated after the formation of the inorganic network. This approach enables the preparation of materials exhibiting an ordered... 

If then we abandon the standard three-dimensional Euclidean perspective and adopt this non-Euclidean two-dimensional view, it can be seen that stable polymorphs are characterised by a global geometric constraint surface density 2"1, and a local constraint Gaussian curvature, . We shall see in Chapter 4 that this description is identical to one that accounts for the mesophase behaviour of lyotropic liquid crystals in amphiphile-water mixtures. 

Liquid crystal phases, or mesophases, are characterized by a partial order, intermediate between the full orientational and translational disorder of the isotropic liquid phase and the full orientational and translational order of the crystalline phase. Thermotropic liquid-crystal phases are obtained for a given compound (or possibly a mixture) as a function of temperature, while the so-called lyotropic liquid-crystal phases are obtained as a function of the concentration of a given solute in a solvent Typical examples of the latter systems are the various types of aggregates formed by amphiphilic molecules either in water or in organic solvents. In this chapter we will be interested only in thermotropic systems. An interesting review on lyotropic ionic liquid crystals can be found in Ref. [2],... 

While there are several examples of metal complexes which are amphiphilic in nature, it is in very few cases that lyotropic liquid crystals mesophases have been characterized. Although numerous and strictly classifiable as metallomesogens, in this article we exclude discussion of the amphiphiles with a simple metal ion as the cation (e. g. sodium salts of carboxylic acids), rather concentrating on amphiphiles in which the metal cation is an integral part of the amphiphile. 

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.)... 

---