Effect of the surfactant head-group size dependence of the dye-surfactant interactions on the lyotropic uniaxial to biaxial nematic phase transitions

In the present study, the interactions of the anionic azo dye Sunset Yellow with dodecylalkylammonium bromide (DAABr) surfactants in lyotropic mixtures have been investigated for the stabilization of the different lyotropic nematic phases. Sunset Yellow doped mixtures of DAABr surfactants, with different head-group size, were studied by polarizing optical microscopy, laser conoscopy, and small-angle X-ray scattering. It was found that the extent of the interaction of Sunset Yellow with DAABr surfactant head groups on micelle surfaces affects the stabilization of different nematic phases, by causing the change in the micellar structural parameters. The results also indicated that the micelle surface curvature should be different in the three nematic phases. (C) 2021 Elsevier B.V. All rights reserved.