Lyotropic liquid crystals aggregate structures

This article will, in addition to a short description of the essential features of surfactant systems in general, concentrate on the energy conditions in premicellar aggregates, the transition premicellar aggregates/inverse micellar structures and the direct transition premicellar aggregates/lyotropic liquid crystals. 

A few non-amphiphilic molecules are able to show liquid crystallinity in solution at a certain range of concentration, such as PBLG, DNA, the tobacco mosaic virus, etc. They are of great molecular mass, very rigid, rod-like and have a very long anisotropic shape. They are typical macromolecules and are lyotropic liquid crystals. This class of liquid crystals does not aggregate to form sphere, column or laminar structures. These lyotropic systems depend on the properties of the solvent. They are one of major interest of this book and will be discussed in detail later. 

The plethora of liquid crystal structures and phases is categorized into two main classes thermotropic and lyotropic liquid crystals. While thermotropic liquid crystals are formed by, e.g., rod- or disc-shaped molecules in a certain temperature range, lyotropic liquid crystals are liquid crystalline solutions, built up by, e.g., aggregates of amphiphilic molecules in a certain concentration range. Many liquid crystal phases are found in thermotropic as well as in lyotropic systems. In some cases, however, the lyotropic analog of a thermotropic phase has never been observed. The probably most interesting of these missing link cases is the thermotropic chiral smectic C (SmC ) phase, which has become famous as the only spontaneously polarized, ferroelectric fluid in nature. 

Structurally, the cubic lyotropic liquid crystal phases are not as well-characterised as the lamellar or hexagonal phases. However, two types of cnbic lyotropic liquid crystal phases have been estabhshed and each can be generated in the normal manner (water continuous) or in the reversed manner (non-polar chain continnous), which makes for a total of fottr different phase types. The most well-known cnbic phase consists of a cubic arrangement of molecular aggregates. The molecttlar aggregates are similar to micelles (Ij phase) or reversed micelles (1 phase). The stractrrre of the normal (1 ) cubic... 

Lyotropic liquid crystals and biological membranes are ordered assemblies of amphiphilic molecules situated in an aqueous environment. By analogy with the other ordered or condensed phases encountered in physics and chemistry, one might suspect that the stability of lyo tropics and membranes derive from favorable attractive interactions between constituent amphiphilic molecules, and that the water only serves to provide the medium in which the ordered aggregates can reside. This notion is, however, incorrect. The participation of the water is far from passive. The role of the water is, in fact, crucial to the formation and stability of lyotropics and membranes. The chief mechanism by which the water acts to promote the various ordered structures is the hydrophobic effect. 

The composition and structure of lyotropic liquid crystals and biological membranes have been examined. An understanding of the varied structures was obtained in terms of the hydrophobic and hydrophilic interactions. That is, aggregates of amphiphilic molecules in aqueous solution always form in such a manner as to minimize the hydrophobic interaction between the hydrocarbon tails and the water, while simultaneously maximizing the hydrophilic interaction of the polar heads with the aqueous solvent. In this way, we saw that the water was not merely the medium in which these phenomena take place. Instead, it became clear that it is the unique properties of the water itself that give rise to the crucial interactions that stabilize lyo-tropics and membranes. 

Other nuclei can be useful in determining structure of lyotropic liquid crystals. In fact, N and NMR have been applied successfully to investigation of the structure of phospholipid aggregates in water. 

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