Investigation of uniaxial and biaxial nematic phase properties of dye-doped lyotropic liquid crystals

In this study, the effects of the anionic azo dye Sunset Yellow on the stabilization of lyotropic uniaxial and biaxial nematic phases and the uniaxial to biaxial phase transitions were investigated. The dye was added in different concentrations to the host ternary mixture of cationic surfactant dodecyltrimethylammonium bromide, 1-dodecanol, and water. Furthermore, a comparative investigation was performed to examine the role of the dye in lyotropic mixtures by adding some inorganic salts to the same host mixture. Salts used for this purpose were NaBr, NaNO3, NaClO3, NaI, NaSCN, and NaClO4. The lyotropic nematic liquid crystal properties of all the samples were investigated by polarizing optical microscopy, laser conoscopy, and small-angle X-ray scattering. The results indicated that (a) the anionic dye Sunset Yellow has a chaotropic character between that of the I‒ and SCN‒ ions, being similar to the second one, (b) it gives a larger biaxial nematic phase temperature range in the partial phase diagram in comparison with the SCN‒ ion and other inorganic salts, and (c) it shifts the temperatures of both uniaxial to biaxial nematic phase transitions. The small-angle X-ray scattering results also indicated that, at least in the investigated concentration range, the dye is mainly located on the micelle surfaces, just like a conventional ion, and that on increasing concentration of the dye induces higher surfactant aggregation number in the micelles.