Lyotropic liquid-crystalline solvents

Liquid crystals are classified into lyotropic and thermotropic crystals depending on the way in which the mesomorphic phase is generated. Lyotropic liquid-crystalline solvents are formed by addition of controlled amounts of polar solvents to certain amphiphilic compounds. Thermotropic liquid-crystalline solvents, simply obtained by temperature variations, can be further classified into nematic, smectic, and cholesteric solvents depending on the type of molecular order present. Liquid crystals are usually excellent solvents for other organic compounds. Nonmesomorphic solute molecules may be incorporated into liquid-crystalline solvents without destruction of the order prevailing in the liquid-crystalline matrix (Michl and Thulstrup, 1986). Ordered solvent phases such as liquid crystals have also been used as reaction media, particularly for photochemical reactions (Nakano and Hirata, 1982). 

Cross-polarization in the solution state has been employed for the selective observation of sealar-eoupled quadrupolar nuclei in anisotropic phases. The method has been apphed to a mixture of beryllium fluoride complexes. Despite severe overlap in the Be speetrum, a quadrupolar splitting was observed for BeFs whereas no sueh sphtting was resolved for Bep4 . NMR spectra of C-iodomethane dissolved in thermotropic and lyotropic liquid crystalline solvents have been used to measure and C- H dipolar couplings. From the... 

Some drug substances can form mesophases with or without a solvent [19-26]. In the absence of a solvent, an increase in temperature causes the transition from the solid state to the liquid crystalline state, called thermotropic mesomorphism. Lyotropic mesomorphism occurs in the presence of a solvent, usually water. A further change in temperature may cause additional transitions. Thermotropic and/or lyotropic liquid crystalline mesophases of drug substances may interact with meso-morphous vehicles as well as with liquid crystalline structures in the human organism. Table 1 presents drug substances for which thermotropic or lyotropic mesomorphism has been proved. 

The rigid rod chains in para-aramids tend to form so-called liquid crystals when they are dissolved in polar solvents or heated to certain temperatures. The polymers showing liquid crystalline behavior in melts are called the thermotropic liquid crystalline polymers, and those showing similar properties in solution are called the lyotropic liquid crystalline polymers. These liquid crystals exist in three distinct phases according to their specific structures (Fig. 12.25). 

Lyotropic liquid crystalline properties were observed for poly (arylamide) monodendrons derived from 1,3,5-substituted amino acid chloride hydrochlorides according to Kim [168]. These systems aggregate readily to form high molecular weight species in the absence of complexing ions. Amide solvents containing more than 40 wt.% of the dendrons exhibit nematic phase liquid crystalline texture under a polarizing microscope, whereas, 60 wt.% solutions produce a hard gel under static conditions. 

Table 1. Examples of stiff-chain polymers able to form a lyotropic liquid-crystalline phase when dissolved in the solvent listed in third column... 

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... 

In principle, lyotropic liquid crystalline systems are such systems in which liquid crystalline ordering appears only in the presence of a solvent, the solvent being selective and the dissolved substance having amphiphylic properties. Soaps, lipids, and some dyes in an aqueous medium may serve as examples of such systems. 

Friberg SE, Wohn CS, Lockwood FE. The influence of solvent on nonaqueous lyotropic liquid crystalline phase formed by triethanolamine oleate. / Pharm Sci 1985 74(7) 771-773. 

Solubilities. Rogers et al. (16-18) have prepared and reported on a large number of aromatic polyesters and polyamides containing the 2,2 -disubstituted 4,4 -biphenylene moiety. Some of the polymers were soluble in common solvents such as tetrahydrofuran and acetone, with solubility as high as 50% in one case. The solubilities of the amide monomers synthesized in this work are shown in Table I. These compounds exhibited virtually no solubility in common solvents such as acetone but displayed fairly high solubilities in several amide solvents, with and without added salts. Lyotropic liquid crystallinity was not observed although solubilities were as high as 40% (w/w) in some solvents. 

In contrast to polypeptides that have many possible conformations, poly(hexyl isocynate) is known to have a stiff rodlike helical conformation in the solid state and in a wide range of solvents, which is responsible for the formation of a nematic liquid crystalline phase.45-47 The inherent chain stiffness of this polymer is primarily determined by chemical structure rather than by intramolecular hydrogen bonding. This results in a greater stability in the stiff rodlike characteristics in the solution as compared to polypeptides. The lyotropic liquid crystalline behavior in a number of different solvents was extensively studied by Aharoni et al.48-50 In contrast to homopolymers, interesting new supramolecular structures can be expected if a flexible block is connected to the rigid polyisocyanate block (rod—coil copolymers) because the molecule imparts both microphase separation characteristics of the blocks and a tendency of rod segments to form anisotropic order. 

Siekmeyer, M. Zugenmaier, P. Solvent dependence of lyotropic liquid-crystalline phases of cellulose tricarbanilate. Makromol. Chem. 1990, 191 (5), 1177-1196. 

Zugenmaier, P. Polymer solvent interaction in lyotropic liquid crystalline cellulose derivative... 

As their name implies, liquid crystals are materials whose structures and properties are intermediate between those of isotropic liquids and crystalline solids (2). They can be of two primary types. Thermotropic liquid crystalline phases are formed at temperatures intermediate between those at which the crystalline and isotropic liquid phases of a mesogenic compound exist. Substances which exhibit thermotropic phases are generally rod- or disc-like in shape, and contain flexible substituents attached to a relatively rigid molecular core. Lyotropic liquid crystalline phases are formed by amphiphilic molecules (e.g. surfactants) in the presence of small amounts of water or other polar solvent. In general, the constituent molecules in a liquid crystal possess orientational order reminiscent of that found in the crystalline phase, yet retain some degree of the fluidity associated with the isotropic liquid phase. 

Thermotropic liquid crystals and also lyotropic liquid crystals generate functional molecular assemblies. lyotropic liquid crystalline phases are exhibited by amphiphilic molecules in appropriate solvents. They form nano-segregated structures because the molecular structures consist of hydrophilic and hydrophobic components. In Chapter 6, Gin and co-workers describe how lyotropic liquid crystals may be used to form functional materials. Lyotropic liquid crystals can act as templates for inorganic materials, ion conductors, catalysts, drug delivery systems, and nanofilters. 

Interesting examples of biomolecules that show lyotropic liquid crystallinity by association of identical molecules are guanosines and folic acids [168-171]. These molecules exhibit columnar mesophases due to the formation of hydrogen-bonded complexes in the presence of water. For example, folic acid 80 sodium salt exhibits cholesteric and hexagonal phases with water due to the formation of a tetramer, which stacks as shown in Fig. 23. The introduction of long alkyl chains into the guanosines leads to the assembly of ribbon-like structures in organic solvents [172]. 

Already in 1965, the formation of lyotropic liquid crystalline solutions of poly-(p-aminobenzoic acid) in concentrated sulfuric acid was observed. This work was the basis for the commercialization of high strength, high modulus, and heat-resistant poly-(p-phenylene terephthalate) (PPDT) fibers under the trade name Kevlar (Dupont) and Twaron (Akzo) (Fig. 15) [31]. PPDT is typically prepared by the polycondensation of terephthaloyl chloride and 1,4-pheny-lenediamine in tertiary amidic solvents such as A-methylpyrrolidinone or dimethylacet-... 

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