Mesomorphism thermotropic

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. 

Arsphenamin was the first drug substance with thermotropic mesomorphism [19] to be used therapeutically as Salvarsan during the first half of the 19th century. This drug is effective against microorganisms and offered for the first time a therapy for venereal diseases. Today it has been replaced by antibiotics with less serious side effects. 

Note 2 A polymer composed of molecules that have rigid rod-like groups or chains usually does not show thermotropic mesomorphic behaviour because decomposition occurs below its melting point. 

Liquid crystal As the name implies, this is an ordered yet fluid phase in which water, surfactant, and solubilizate combine to form anisotropic, organized structures. These are called lyotropic mesomorphic phases, as opposed to thermotropic mesomorphs, which form when certain organic crystals are heated. 

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... 

In addition to this thermotropic mesomorphism, a lysotropic mesomorphism is observed [98]. The phase transition temperature, Tt, for the transition from the crystalline to the liquid crystalline state decreases as a function of water content. The decrease in Ttis due to destabilization of the crystal lattice in the head group region by water molecules. This, in turn, decreases the interaction between the fatty acid chains. When the water content reaches a certain level, the phospholipids assume a thermodynamically optimal arrangements whereby the fatty acids are directed to the... 

The recent studies on the structure and properties of polypeptide liquid crystals, which are formed in solution as well as in the solid state, are reviewed in this article. Especially the cholesteric pitch and the cholesteric sense (right-handed or left-handed), which are characteristic factors of cholesteric liquid crystals, are discussed in detail in relation to the effects of temperature, concentration, and solvent. Further cholesteric liquid crystalline structure retained in cast fdms and thermotropic mesomorphic state in some copolypeptides are also discussed. 

For therapeutic purposes, a similar frequently used group of drug compounds is the non-steroidal antiinflammatory drugs (NSAIDs). One of the best known representatives of the aryl acetic acid derivatives in diclofenac and that of aryl propionic acid derivatives ibuprofen. As both have acidic properties, they dissociate while being dissolved and may form salts with amphiphilic properties. Together with appropriate counterions, these amphiphilic organic acids may form lyotropic mesophases with water at even room or body temperature e.g., diclofenac diethylamine or ibuprofen lysinate. Furthermore, some anhy-drates of NSAID, e.g., fenoprofen calcium,exhibit thermotropic mesomorphism after thermal dehydration of the crystalline salt. 

The amphiphilic molecules can occur in lyotropic and thermotropic mesomorph, and the organization of lipid bilaycrs such as liposomes can exist in a temperature range where all the mesophases from gel state to liquid crystal state are favored. In the gel state the phospholipid acyl chains are closely packed and the molecular movements are deteriorized, while in the liquid crystalline slate, the fatty-acid moieties are in a more fluid stale and are able to move more freely. 

Recently it was shown (2) that when a fully saturated phospholipid is heated, a marked change takes place in the appearance of the infrared spectrum, corresponding to a transition from a crystalline to a liquid crystalline phase, many tens of degrees below the reported capillary melting point. Phospholipids therefore exhibit thermotropic mesomorphism. In this paper we discuss the results obtained by other techniques which we have been using in our laboratory for the study of phospholipids. 

In addition to this thermotropic mesomorphism, the complexes formed lyotropic mesophases when dissolved in linear, lipophilic solvents such alkanes, and in chloroform, benzene, octanol, octadecanol, and stearic acid." " In alkanes, the mesophases were stable over wide ranges of temperature and concentration. In general, at high complex concentration, a columnar phase was... 

It has been experimentally found by using a whole complex of physical methods that some polymers can exhibit lyotropic and thermotropic mesomorphism. 

Hence, the analysis of conformational properties of an extensive class of comb-like polymers with mesogenic side groups showed that the appearance of thermotropic mesomorphic structures in the melts of these polymers is determined by the spontaneous formation of liquid crystalline fragments on a molecular level as a result of the interaction between the side groups. 

Thermotropic Liquid Crystals. Also called Thermotropic Mesomorphic Phase. See Mesomorphic Phase. 

Thus, the examined low molecular mass amine salts show no tendency to thermotropic mesomorphism, whereas their concentrated aqueous solutions conform the main regularities valid for the lyotropic mesomorphism of ionogenic surfactants. 

Polymer/surfactant complexes may exhibit thermotropic mesomorphism as well as the lyotropic one discussed above. Among conducting polymers, PANi has been of considerable scientific interest because of its unique electrical and optical properties and environmental stability. Besides the covalent alkyl substitution di.scussed above, a significant approach has been to use functionalized acids, which usually serve as surfactants. Dodedcylbenzenesulfonic acid (DBSA) has been widely used for protonation of PANi and found to improve the solubility and processibility of the conducting polymer. 

Thermotropic mesomorphism in linear polymers had been considered theoretically prior to 1975 (see reference 3 for leading references). However, in 1975, de Gennes explicitly considered linear polymers having alternating rigid mesogenic groups and flexible spacers in the polymeric main chain. He predicted possible nematic behavior for such polymers. 

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