There are two main classes of liquid crystals, the detail of which is given below:
THERMOTROPIC LIQUID CRYSTALS
These crystals are formed by heating a solid or by cooling a liquid. When the solid which forms a liquid crystal is heated it undergoes transformation into a turbid system that is both fluid and bireftingent. The consistency of the fluid varies with different compounds from paste to a free flowing liquid. When the turbid quephenyl etc. Thermotropic liquid crystals are either nematic or smectic. Nematics are further subdivided into ordinary and cholesteries.
LYOTROPIC LIQUID CRYSTALS
These are not pure substances, but are solutions of a substance in a highly polar liquid such as water. Lyotropic liquid crystal often has an amphiphilic component. a compound with a polar head attached to a long hydrophobic tail. Sodium stearate and lecithin arc typical examples of amphiphiles. Starting with a solid amphiphile and adding water, the lamellar structure (molecular packing in layers) is formed. By stepwise addition of water, the molecular packing may take on a cubic structure, and then hexagonal, then micellar, followed by true solution. The process is reversed by withdrawing water. Thousand of compounds will form liquid crystal on heating, and still more will do so if two or more components are mixed. Sodium stearate, a-Lecithin, cell membranes, lipids, or fat molecules in membranes belong to this class.