Lyotropic

A liquid crystalline mesophase is called lyotropic if formed by dissolving an amphiphilic mesogen in a suitable solvent, under appropriate conditions of concentration, temperature and pressure.

Historically, the term was used to describe the common behavior of materials composed of amphiphilic molecules upon the addition of a solvent. Such molecules comprise a water-loving hydrophilic head-group (which may be ionic or non-ionic) attached to a water-hating, hydrophobic group.

The micro-phase segregation of two incompatible components on a nanometer scale results in different type of solvent-induced extended anisotropic arrangement, depending on the volume balances between the hydrophilic part and hydrophobic part. In turn, they generates the long-range order of the phases, with the solvent molecules filling the space around the compounds to provide fluidity to the system.

In contrast to thermotropic liquid crystals, lyotropics liquid crystals have therefore an additional degree of freedom, that is the concentration that enables them to induce a variety of different phases. As the concentration of amphiphilic molecules is increased, several different type of lyotropic liquid crystal structures occur in solution. Each of these different types has a different extent of molecular ordering within the solvent matrix, from spherical micelles to larger cylinders, aligned cylinders and even bilayered and multiwalled aggregates.

Examples of amphiphilic compounds are the salts of fatty acids, phospholipids. Many simple amphiphiles are used as detergents. A mixture of soap and water is an everyday example of a lyotropic liquid crystal.

Biological structures such as fibrous proteins showings relatively long and well-defined hydrophobic and hydrophilic ‘‘blocks’’ of aminoacids can also show lyotropic liquid crystalline behaviour.

A typical amphiphilic flexible surfactant can form aggregates through a self-assembly process that results of specific interactions between the molecules of the amphiphilic mesogen and those of the non-mesogenic solvent.

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