Surfactant-water liquid crystal phases

Lyotropic liquid crystals are formed on the dissolution of most surfactants in water, and are more common than micellar solutions. Four major varieties of phase structures are known, these being hexagonal, lamellar, reversed hexagonal and cubic. The molecular arrangements within the first three are well established. For cubic phases further work is required to determine the detailed structures, but it is likely that three or four different arrangements occur. Numbers of other structures have been proposed, but their existence has not been established beyond reasonable doubt. In addition, several phases occur with compositions between hexagonal and lamellar phases, where the structure has not been definitely elucidated. Anhydrous surfactants form a variety of thermotropic liquid crystals and here, while the lamellar and reversed hexagonal structures are well-established, other proposed structures require verification. The dependence of phase structure on surfactant chemical structure remains almost uninvestigated. Major factors that determine lyotropic phase structures are electrostatic interactions, surfactant head group hydration and steric repulsions, and alkyl chain conformations. A general outline of the relationship between phase structure and overall composition can be obtained from a consideration intra- and inter-micellar interactions. These also give some insight into the relationship between rheological properties and phase structure. Over the past ten years much has been learned about molecular order and mobility through the application of modern spectroscopic techniques, particularly magnetic resonance. While the general picture is known details of molecular interactions remain to be explained.