Lyotropic liquid crystals birefringence

Liquid crystalsare an intermediate state in which the molecules in a crystal can undergo a secondary phase transition to a mesophase, which gives them mobility in 1-2 directions. They are birefringent, but possess low properties like a liquid phase. Lyotropic liquid crystals form on uptake of water into a system that increases its mobility, and thermotropic liquid crystals can be disrupted by heating above a transition temperature. Cromolyn sodium (Cox et al., 1971), the HMG-CoA reductase inhibitor SQ33600 (Brittain et al., 1995), and the leukotriefienffagonist L-660,711 (Vadas et al., 1991) are examples of pharmaceuticals that can form liquid crystals. 

A more detailed description of the structure and properties of the mesophases in Table 3.1 is provided in the following subchapters. In principal, the properties and textures of analog phases are also similar due to the equivalent strucmre of the mesophases and thus are discussed simultaneously. However, the texmres of lyotropic liquid crystals often appear less colorful. This is due to the lack of aromatic units in most of the typically used surfactant molecules, as the aromatic cores of thermotropic liquid crystal largely contribute to their birefringence. Exemplary texture images of the discussed thermotropic mesophase are shown in Refs. [11, 12], while texture images of lyotropic mesophases are found in Refs. [13, 14, 15]. 

The assembly of the chiral nematic structure happens in two steps. As the solvent evaporates from the suspension, the critical concentration of the lyotropic liquid crystal phase is reached. The CNCs form discrete localised clusters of chiral nematic phase, called tactoids, as predicted by the phase separation of liquid crystals. These tactoids can be observed between cross-polarisers under a microscope. The first repotted image of tactoids in a cellulose suspension is reproduced in Figure 17.10. The tactoids are visible from the birefringent stripes, which represent axes of crystallite arrangement parallel to the stripes. ... 

All of the lyotropic liquid crystals except the cubic phases exhibit birefringence. Due to their high content of water and the low electronic polarizability of the surfactant molecules the difference. An = between... 

The birefringence of lyotropic liquid crystals is almost an order of magnitude smaller due to the water content and because of the hydrocarbon chain, which takes relatively large part of the amphiphilic molecules, contributes to birefringence only weakly. 

J. P. McClymer, C. A. Oxborrow, P. H. Keyes, Light scattering and magnetic birefringence measurements at the isotropic-nematic and nematic-reentrant isotropic transitions in a lyotropic nematic liquid crystal, Phys. Rev. A 1990,42, 3449-3452. 

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