Effect of the anionic azo dye Sunset Yellow in lyotropic mixtures with uniaxial and biaxial nematic phases Erol Akpinar a, ⁎, Gokhan Topcu a , Dennys Reis b , Antônio Martins Figueiredo Neto b a Bolu Abant Izzet Baysal University, Faculty of Arts and Sciences, Department of Chemistry, 14030 Golkoy, Bolu, Turkey b Universidade de Sao Paulo, Instituto de Fisica, Rua do Matao, no. 1371, 05508-090 Sao Paulo, SP, Brazil abstract article info Article history: Received 3 June 2020 Received in revised form 23 July 2020 Accepted 6 August 2020 Available online 10 August 2020 Keywords: Lyotropic liquid crystal Nematic phase Biaxial nematic phase Anionic azo dye Sunset Yellow Hofmeister series Polarizing optical microscopy Laser conoscopy Small-angle X-ray scattering 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 concentra- tions 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, NaNO 3 , NaClO 3 , NaI, NaSCN, and NaClO 4 . All the samples were investigated by polariz- ing optical microscopy, laser conoscopy, and small-angle X-ray scattering. The results indicated that (a) the an- ionic 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 ne- matic 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. © 2020 Elsevier B.V. All rights reserved. 1. Introduction Surfactant-based lyotropic nematic phases (LNPs) and their phase diagrams as a function of concentration and/or temperature have been investigated in a wide range of studies [1–7]. In general, the mixtures presenting the LNPs are obtained by the dissolution of surfactant mole- cules in water. In most cases, electrolytes and/or cosurfactants are added to those solutions [8–12]. Considering the symmetry of the LNPs, they are classified as uniaxial (discotic, N D , and calamitic, N C ) with D ∞h symmetry and biaxial (N B ) with D 2h symmetry [13–16]. The main characteristic of those uniaxial phases is that the local symmetry axis of their structural units, so-called micelles, exhibits an average alignment along a preferred direction (optical axis or director, n ! ). While only one optical axis exists in the N D and N C phases, the N B phase has two optical axes and three orthogonal two-fold symmetry axes ( l ! , m ! and n ! , where n ! ¼ l ! Â m ! )[15,17,18]. Furthermore, studies in the literature proposed that the three LNPs consist of micelles having the same orthorhombic symmetry [19]. Another important point for the LNPs is that the N B phase is mainly located between the other two uni- axial phases in the phase diagrams of lyotropic mixtures presenting the three nematic phases [20–22]. As theoretically predicted by a Landau- type mean-field theory and experimentally verified, the uniaxial to bi- axial transition is of second-order [20–24]. Another class of lyotropic liquid crystals is the lyotropic chromonic liquid crystals (LCLC) [25–28]. They are obtained from aqueous solu- tions of substances such as drugs (e.g. disodium chromoglycate [29,30]) and dyes (e.g. Sunset Yellow [31,32]). Those kinds of com- pounds, in general, have plank-like molecular structures [32]. While polyaromatic parts form the main body of these structures, ionic groups connected to these parts form the outer part of the structure. When these molecules are dispersed in water, they stack in columns due to the π-π interactions between the polyaromatic parts, and, if the concen- tration of the substance is high enough, they form the LCLC phases [31]. Depending on the aggregation of the molecules, two different LCLC structures were reported in the literature: nematic with orientational ordering, and hexagonal with both translational and orientational or- dering [28]. Not only in surfactant-based LNPs but also in lyotropic chromonic nematic phases (LCNPs), some aspects about the stabilization mecha- nism of those phases have been investigated. For instance, Kumar et al. studied the effect of dye concentration, temperature, and ionic ad- ditives from the inter- and intra-aggregate spacings change point of view via X-ray diffraction and polarizing microscopy [31]. However, the surfactant-based LNPs were more investigated if compared with Journal of Molecular Liquids 318 (2020) 114010 ⁎ Corresponding author. E-mail address: akpinar_e@ibu.edu.tr (E. Akpinar). https://doi.org/10.1016/j.molliq.2020.114010 0167-7322/© 2020 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq