X-Ray Studies of Lyotropic Liquid Crystals

Abstract

There are two broad classes of liquid crystalline systems, the thermotropic and the lyotropic. The historical difference between these two is that the lyotropic systems are always mixtures (or solutions) of unlike molecules in which one is a nonmesogenic liquid. Solutions of soap and water are typical examples of lyotropics. Their mesomorphic phases appear as a function of either concentration or temperature. In contrast, the thermotropic systems are usually formed from a single chemical component, and the mesomorphic phases appear primarily as a result of temperature changes. The molecular distinction between the two is that one of the molecules in the lyotropic solution always has a polar hydrophilic part, often called the “head group”, and one or more hydrophobic alkyl chains called “tails”. Although these molecules can form mesogenic phases as single component systems, they more easily do so in solution with either water or oil. This is due to a competition between the hydrophilic and hydrophobic interactions, as well as other things such as packing and steric constraints^1,2. The molecules that form thermotropic liquid crystalline phases also have hydrophilic and hydrophobic parts, the disparity in the affinity of these parts for either water or oil is much less and most of these molecules are relatively insoluble in water. Nevertheless, it is interesting that different parts of typical thermotropic molecules do have some of the same features as the lyotropic molecules. For example, although the rod-like thermotropic molecules always have an acyl chain at one or both ends of a more rigid section, the chain lengths are rarely as long as those of the lyotropic molecules. Also, the solubility of different parts of the thermotropic molecules, when separated, are not as disparate as those of the lyotropic molecules, are definitely different.