A study on lyotropic biaxial chromonic nematic phases via polarising light microscopy, laser conoscopy and small-angle x-ray scattering

Abstract

Lyotropic nematic phases have been widely studied in the literature because of their structural units 'micelles' exhibit a tendency of orientations in a certain direction with their local directors and, as a consequence, they present susceptibility against magnetic field. In recent years, the sub-class of lyotropic liquid crystals, so-called lyotropic chromonic liquid crystals, have also been attracted the attention of scientists intensely, however, the studies on them remained limited. Because of some properties of this type of phases, it highly appears in the future that they have potential use in the fields of biotechnology and optical technological applications (e.g. LCD displays). However, the studies in the literature have not exactly revealed which parameters (such as the addition of salt as a guest molecule) affect the formation of lyotropic chromonic liquid crystals. Since the orientational fluctuations of local directors of the micelles give rise to the formation of different lyotropic nematic phases, in lyotropic chromonic liquid crystals, the phases are formed by the self-assembly of plank-like planar dye molecules into columnar aggregates. The orientational fluctuations that micelles have in a certain direction cause the formation of three different nematic phases. They are both uniaxial discotic (ND) and calamitic (NC) nematic phases and biaxial (NB) nematic phase. Among these phases, since the optical director of ND phase aligns perpendicular to the magnetic field direction, that of NC phase is parallel to the magnetic field. In addition, it is also important to determine which factors affect the exhibiting wider and stable biaxial phase domain on the phase diagrams. In the frame of this project, the surfactant molecules having ionic head groups will be used in the preparation of lyotropic mixtures presenting uniaxial and biaxial nematic phases. By this way, it is provided that micelles will have ionic surfaces. Oppositely charged dye molecules (sunset yellow) will be bound to the ionic surface of micelles and, by this way, lyotropic uniaxial and biaxial chromonic nematic phases will be reported in the literature for the first time. With these novel phases: (a) a new scientific and technological research area which could be potentials in optical applications will be created for researchers, (b) the production of new materials will be provided for technological applications, (c) how the interactions with dye molecules at the micelle surfaces affect the formation of both biaxial and uniaxial nematic phases will be determined and finally (d) a role of dye-surfactant interactions in lyotropic chromonic systems will be investigated. For these purposes, dodecyltrimethylammonium bromide (DTMABr) is being planned to be used as the main surfactant molecules. The head group of this molecule exhibits chaotropic property and the chaotrope counter-ions are favorable to be bound to the micelle surfaces formed by DTMABr. Hence, it will be provided that molecules, such as sunset yellow, including chaotrope groups and presenting lyotropic chromonic phases, are bound to the micelle surfaces. Then, in these novel lyotropic biaxial chromonic nematic phases, the uniaxial-to-biaxial phase transitions will be determined from the temperature dependences of the birefringences of each phase via laser conoscopy. The textures of the novel sub-class of lyotropic liquid crystals phases will be characterized by polarising optical microscopy. Small-angle x-ray scattering will be used to determine the micelle sizes. We believe that the results obtained in the frame of this joint project will make contributions to the literature (i) by introducing novel lyotropic uniaxial and biaxial chromonic nematic phases, which may have high potential use in the technological applications in the future, and (ii) by clarifying the discussions, especially, on the lyotropic biaxial nematic phases which are still continuing in the literature. (AU)