Mesomorphic solvents

Liquid erystals or mesomorphie eompounds oeeupy a special position [17-22, 22a, 109, 110]. Compounds capable of forming liquid erystals are long, flat, and fairly rigid along the axis of the moleeule. Most known mesomorphic solvents are systems of the following general structure with polarizable aromatic nuclei held in a planar skeleton ... 

Almost at the same time of this reinvestigation we studied a set of bimolecular reactions(28) all the syntheses described above were unimolecular processes and bimolecular reactions are expected to be more sensitive to the alignment of the reactants imposed by the mesomorphic solvents. 

A Hquid crystal compound in more cases than not takes on more than one type of mesomorphic stmcture as the conditions of temperature or solvent are changed. In thermotropic Hquid crystals, transitions between various phases occur at definite temperatures and are usually accompanied by a latent heat. 

A compound which displays liquid crystal properties is referred to as a mesogen and said to exhibit mesomorphism. Liquid crystals may be considered either as disordered solids or ordered liquids, and their properties are very dependent on temperature and the presence or absence of solvent. In thermotropic liquid crystals the phases of the liquid crystals may be observed to change as the temperature is increased. In lyotropic liquid crystals the ordered crystalline state is disrupted by the addition of a solvent, which is very commonly water. For these systems temperature changes may also be... 

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. 

Note 2 The mesomorphic character of a lyotropic mesophase arises from the extended, ordered arrangement of the solvent-induced micelles. Hence, such mesophases should be regarded as based not on the structural arrangement of individual molecules (as in a non-amphiphilic or a thermotropic mesophase), but on the arrangement within multimolecular domains. 

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). 

The possibility of cis-trans equilibria exists, but has not been explored in depth. One indication for such equilibria comes from the NMR investigation of the parent nickel dithiolene in a nematic solvent only a rapidly equilibrating compound could have produced the observed spectral pattern.35 In the preparation of some mesomorphic dithiolenes related to the one cited above, two products were initially obtained, which during further work-up converted into the trans isomer only.38 In Pt complexes of this type, the existence of two 195Pt- H couplings was taken as an indication36 that the cis and trans isomers coexisted. 

Tihe term lyotropic mesomorphism is used to describe the formation of thermodynamically stable liquid crystalline systems through the penetration of a solvent between the molecules of a crystal lattice. In contrast to the thermotropic mesomorphism shown by many pure substances, lyotropic mesomorphism always requires the participation of a solvent. Lyotropically mesomorphous systems, however, are usually as sensitive to changes in temperature as thermotropic systems. So far, lyotropic mesomorphism has been observed almost exclusively in lipid systems containing water. Lipids that show lyotropic mesomorphism frequently... 

The change from a crystalline into a liquid crystalline state can be brought about by changes in, for example, temperature or pressure. Furthermore, some molecules may be induced to form liquid crystals by the addition of a solvent such as water. This behavior is in reality a liquid crystalline formation in a two component system and is called solvent-induced liquid crystal formation or lyotropic mesomorphism (Small, 1986, p. 49). 

The chief reason for the interest in graft copolymers originates from the incompatibility between polymer chains of different chemical nature. Intramolecular phase separation results, because grafts and backbone repell each other, and these compounds exhibit a marked tendency to form mesomorphic phases like block copolymers and soaps do. When these species are mixed with a solvent that exhibits a preferential affinity for one of the components (grafts or backbone) the incompatibility may be enhanced. This intramolecular phase separation has led to a number of applications. If small amounts of a graft copolymer are included into a homopolymer of the same nature as the grafts (or the backbone), surface modifications can result as described below. 

In this chapter we have described the mesomorphic behavior and ionic conductivities of ionic liquid-based liquid crystalline materials. These ion-active anisotropic materials have great potentials for applications not only as electrolytes that anisotropically transport ions at the nanometer scale but also as ordered solvents for reactions. Ionic liquid crystals have also been studied for uses as diverse as nonliner optoelectronic materials [61, 62], photoluminescent materials [78], structuredirecting reagents for mesoporous materials [79, 80] and ordered solvents for organic reactions [47, 81]. Approaches to self-organization of ionic liquids may open a new avenue in the field of material science and supramolecular chemistry. 

Water and isobutyric acid (IBA) have been chosen as solvents because they form a mesophase with HPC and they have a critical temperature, approximately 26 C for an IBA proportion of approximately 0.4. The phase diagram shows different regions one- or two-phase, mesomorphic, or not. The two-phase region can be segregated. The mesomorphic phase can form a gel or show cholesteric colors (25). The most important Information given by this phase diagram is that the behavior of phase separation of IBA/H2O/HPC system is governed by the extreme preference of HPC for IBA as compared with water. This results in the formation of the ordered phase in dilute HPC solutions as the IBA content in the solvent is reduced. Mesophases can be obtained for low polymer concentrations of a few percent. 

Threshold volume fractions observed for cellulose acetate (CA and CTA), ethyl cellulose (EC) and hydroxypropyl cellulose (HPC), each in various solvents, are presented in Table 3. The results depend to some extent on the solventThe data included are not exhaustive. Other cellulose esters exhibit mesomorphic behavior Chanzy et al. observed mesomorphic behavior in solutions of cellulose itself when dissolved in N-methylmorpholine N-oxide containing water at concentrations of cellulose in the range 20-55 % w/v, depending on the temperature, the water content of the solvent and the degree of polymerization of the cellulose. Solutions of cellulose in mixtures of trifluoroacetic acid with 1,2-dichloroethane or with chloroform are hkewise lyotropic at concentrations of 20% (w/v) and above according to Patel and Gilbert... 

Q = 1.3gcm , lu = 5.2 A and M = 265 (for CA), one obtains = 26 according to Eq. (13), and, from Eq. (11), Vp = 0.284 at incipience of a nematic (or cholesteric) phase. The experimental values in Table 3 are generally somewhat greater than this result, but the differences are not beyond the uncertainty in the characteristic chain dimension, here represented by o/n. Inasmuch as this ratio depends on the solvent, as is well known, dependence of the threshold Vp on the solvent is to be expected. Available data do not permit a definitive correlation, however, between Vp at incipience of the mesomorphic phase and the influence of the solvent on chain flexibility. 

---