LIQUID CRYSTALS

From Big Medical Encyclopedia

LIQUID CRYSTALS (synonym: mesomorphic bodies, anisotropic liquids, paracrystals) — substances which in a certain interval of temperatures form the liquid phase having at the same time properties of liquid and crystalline solid. As liquids Zh. to. have flowability (see. Liquids ), as crystalline solids — anisotropy of optical, electric, magnetic, mechanical and other properties (see. Anisotropy , Crystals ).

Many organic matters (polynucleotides, nucleinic to - you, phospholipids, cerebrosides, etc.) form. to. Many components of living tissue have liquid crystal structure (myelin covers of nerve fibrils, various biol, membranes, etc.) that defines an originality fiziol, functions of these educations. At diagnosis of a number of the diseases which are followed by local changes of body temperature liquid crystal finds the increasing application termografiya (see). The separate systems containing. to., in particular the water and lipidic, forming lyotropic. to., use as models at a research of structure and function of cellular membranes (see. Membranes biological ). Assume that. to. played a crucial role at early stages of emergence of life on Earth (J. Bernal).

Fig. 1. A type of anisotropic liquid (liquid crystals) in sight of the polarizing microscope: liquid crystals in the form of brightly lit educations whereas drops of usual liquid do not pass light and the field of microscope remains dark.

. to. were open in 1888 avstr. scientist F. Reinitzer. He found out that crystals of the connection synthesized by it — a holesterilbenzoat melted at t ° 145 °, forming the muddy liquid passing into the transparent fusion at t ° 179 ° which is not changing at further heating; during the cooling to t ° 145 ° below liquid turned into crystalline state again. Investigated this substance in 1889 it. the physicist Lehmann (O. of Lehmann) also established that the liquid phase in the range of temperatures 145 — 179 represents the anisotropic liquid possessing double refraction (see). Anisotropy. to. most clearly it is shown in double refraction. At the same time drop. to. under polyarizatsion a ny microscope takes a form of the color brightly lit education whereas the drop of usual liquid in these conditions does not pass light and the field of vision of the polarizing microscope remains to dark (fig. 1). Lehmann showed also that similar properties also other organic compounds have (e.g., paraoxyanisole, ammonium oleate, etc.). All these substances, and them there are several thousands, have that feature that each of them in a certain interval of temperatures forms the liquid phase having at the same time properties of liquid and crystalline solid. Lehmann called such liquids liquid crystals

. to. can be received by melting of firm crystals of many organic compounds which molecules have usually extended form. Such crystals call termotropny. to. also can be received by dissolution of firm crystals of some substances in certain solvents, napr, salts of higher fatty acids in water. In this case. to. call lyotropic.

Viscosity of many. to. only slightly exceeds viscosity of water therefore they have the good flowability and ability to form the drops merging with each other at contact. The form of drops can be not only spherical, but also rhabdoid, cone-shaped, etc.

Pellicles. to., possessing an ordered structure, differently reflect light waves of different length therefore in a reflected light they get bright coloring. This coloring changes at action on a film of electric and magnetic fields, during the heating and their deformation.

Anisotropy. to. it is caused not by a structure of molecules of the substance having properties Zh. to., and the arranged spatial relationship of these molecules. Distinguish two main structural types Zh. to. — nematic (grech, nema thread) and smectic (grech, smegma washed).

Fig. 2. Diagrammatic representation of a nematic liquid crystal: molecules are oriented along the axes, and their centers of gravity are located chaotically.
Fig. 3. Diagrammatic representation of a smectic liquid crystal: molecules are oriented along the axes and located layers.

Nematic. to. are formed by molecules which long axes are oriented in one direction, but the centers of gravity are located chaotically (fig. 2). In sight of the polarizing microscope in a continuous layer. to. characteristic filamentary structures are visible. Separate drops have spherical shape. Example of nematic. to. paraazoxyanisole can serve 117 — 135 in a temperature band °. Nematic. to. subdivide into two versions: actually nematic and cholesteric. to. The last are formed by cholesterol and some of its derivatives which molecules are in addition twirled in the direction, perpendicular to long axes of molecules. Owing to such structure pellicles of cholesteric. to. look brightly painted in a reflected light.

Smectic. to. are formed by molecules, the located layers (fig. 3). From each other explain with sliding of layers the flowability of smectic. to. In alternation of layers strict frequency is established, the edge is not observed in an arrangement of molecules in each layer. In the polarizing microscope of a drop of smectic. to. have the rod, cone-shaped or step form. Example of smectic. to. can serve fusion paraazoxybenzoic to - you (in the range of temperatures 114 — 120 °), water solutions of potassium oleate, etc.

Sometimes the same substance, napr, ethyl ether paraetoksibenzalyaminokorichny to - you, can exist in two mesomorphic modifications. At the same time the temperature band, in Krom exists more arranged smectic modification, lies below a temperature band of less arranged nematic modification.

Smectic. to. in some solvents often form the peculiar textures called by myelin forms which represent thin coats of smectic. to., curtailed into tubes to dia. 0,01 — 0,02 mm. Myelin forms easily arise, e.g., in system cholesterol — glycerin, during the heating to a certain temperature. As glycerin plays a role of solvent Zh. to., myelin forms should be considered as lyotropic. to. Structure of lyotropic. to. more difficult, than at termotropny. Their base units are not molecules, but micelles, is arranged distributed in solvent.



Bibliography: Vistin L. K. and Chistyakov I. G. Liquid crystals, M., 1975, bibliogr.; Lehman O. Liquid crystals and theories of life, the lane with it., Odessa, 1908, bibliogr.; Demus D., Demus H. u. Z a s with h k e H. Fliis-sige Kristalle in Tabellen, Lpz., 1974, Bibliogr.

B. P. Mishin.

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