Liquid Crystals

The three primary states of matter are solid, liquid, and gas. A fourth state of matter is the liquid crystalline state or mesophase. Liquid crystals (LCs) represent a unique state of matter existing between the ordered solid and disordered liquid states.

Structure of Liquid Crystals

The two main types of liquid crystals are termed smectic (soaplike or greaselike) and nematic (threadlike). In the smectic state, molecules are mobile in two directions and can rotate about one axis. In the nematic state, the molecules again rotate only about one axis but are mobile in three dimensions. A third type of crystal (cholesteric) exists but can be considered as a special case of the nematic type. In atherosclerosis, it is the incorporation of cholesterol and lipids in human subendothelial macrophages that leads to an insoluble liquid crystalline biologic membrane that ultimately results in plaque formation.

The smectic mesophase is probably of most pharmaceutical significance because it is this phase that usually forms in ternary (or more complex) mixtures containing a surfactant, water, and a weakly amphiphilic or nonpolar additive. In general, molecules that form mesophases are organic compounds characterized by an elongated, linear shape, a rigid structure, and the presence of strong dipoles and highly polarizable groups.

Properties And Significance Of Liquid Crystals

Because of their intermediate nature, liquid crystals have some of the properties of liquids and some of the properties of solids. For example, liquid crystals are mobile and thus can be considered to have the flow properties of liquids. At the same time they possess the property of being birefringent, a property associated with crystals.

Some liquid crystals show consistent color changes with temperature, and this characteristic has resulted in their being used to detect areas of elevated temperature under the skin that may be due to a disease process. Nematic liquid crystals may be sensitive to electric fields, a property used to advantage in developing display systems. The smectic mesophase has application in the solubilization of water-insoluble materials. It also appears that liquid crystalline phases of this type are frequently present in emulsions and may be responsible for enhanced physical stability owing to their highly viscous nature.

The liquid crystalline state is widespread in nature, with lipoidal forms found in nerves, brain tissue, and blood vessels. Atherosclerosis may be related to the laying down of lipid in the liquid crystalline state on the walls of blood vessels. The three components of bile (cholesterol, a bile acid salt, and water), in the correct proportions, can form a smectic mesophase, and this may be involved in the formation of gallstones.

Reference:

  • Sinko, P. (2011). Martin’s Physical Pharmacy and Pharmaceutical Sciences. Baltimore, : Lippincott Williams & Wilkins, a Wolters Kluwer business.

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