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A material is called lyotropic if it forms liquid crystal phases upon the addition of a solvent. Historically the term was used to describe materials composed of amphiphilic molecules. Such molecules consist of an ionic or non-ionic hydrophilic head-group which is attracted to water, and attached to a hydrophobic group which is repelled by water. Typical hydrophobic groups are saturated or unsaturated hydrocarbon chains. Examples of amphiphilic compounds are the salts of fatty acids; phospholipids. Many simple amphiphiles are used as detergents. Lyotropic liquid crystal phases in these materials are formed by a process of self-assembly that is driven by the hydrophobic effect.
Lyotropic liquid crystal phases are formed on the dissolution of amphiphilic molecules of a material in a solvent. There are several different type of lyotropic liquid crystal structures. Each of these different types has a different extent of molecular ordering within the solvent matrix.
The term lyotropic has also been applied to the liquid crystalline phases that are formed by certain polymeric materials, particularly those consisting of rigid rod-like macromolecules, when they are mixed with appropriate solvents. Examples are suspensions of rod-like viruses as the Tobacco Mosaic Virus as well as man-made colloidal suspensions of non-spherical colloidal particles. Other examples include DNA and Kevlar, which dissolves in sulfuric acid to give a lyotropic phase. It is noted that in these cases the solvent acts to lower the melting point of the materials thereby enabling the liquid crystalline phases to be accessible. These liquid crystalline phases are closer in architecture to thermotropic liquid crystalline phases than to the conventional lyotropic phases. In contrast to the behaviour of amphiphilic molecules, the lyotropic behaviour of the rod-like molecules does not involve self-assembly.