Effect of the anionic azo dye Sunset Yellow in lyotropic mixtures with uniaxial and biaxial nematic phases

Abstract

The effects of the anionic azo dye Sunset Yellow on the stabilization of lyotropic uniaxial and biaxial nematic phases and on the uniaxial to biaxial phase transitions were studied. The dye was added in different concentrations to the host ternary mixture of cationic surfactant dodecyltrimethylammonium bromide, 1-dodecanol, and water. Furthermore, an investigation examined the role of the dye in the lyotropic host mixture in comparison to some inorganic salts containing anions of the Hofmeister series. The following inorganic salts were added to the same host mixture: NaBr, NaNO3, NaClO3, NaI, NaSCN, and NaClO4. 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 last 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 the 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 micelles' surfaces, just like a conventional ion, and that on increasing concentration the dye molecules induces higher aggregation number of the micelles.