Guidelines for abstract preparation for OLC 2015
Crossover between optically positive and negative biaxial nematic phase in lyotropic liquid crystals
E. Akpinar1, D. Reis2 and A.M. Figueiredo Neto2
1 Department of Chemistry, Arts and Sciences Faculty, Abant Izzet Baysal University, 14280, Bolu,Turkey
2 Instituto de Física, Universidade de S?o Paulo, caixa postal 66318, 05314-970, S?o Paulo, SP, Brazil
Lyotropic mixtures with a surfactant and co-surfactant may present three nematic phases, two uniaxial and one biaxial (NB) [1]. The biaxial phase has three diamagnetic susceptibilities, each one along their respective two-fold symmetry axes or directors. The alignment behavior of the NB in the presence of may be used to classify the biaxial phase as being of the NB+ or NB- type, however belonging to the same and unique biaxial phase. Mixtures of dodecyltrimethylammonium bromide (DTB)/sodium salt from the Hofmeister anions series/1-dodecanol/water, presenting the NB, were prepared to investigate the effect of these anions on the existence of the NB phase, and the uniaxial to biaxial phase transitions. Laser conoscopy, optical microscopy and X-ray scattering were used to characterize the phases and phase transitions. The results indicated that the Hofmeister anions were bounded to the head groups of DTB molecules at the micelles' surfaces, which significantly affect the different orientational fluctuations responsible for the formation of different nematic phases, the biaxial phase domains and the uniaxial to biaxial phase transition temperatures. The partial phase diagrams of the mixtures were established. In particular regions of the phase diagrams, an anomalous behavior was observed [2] in the crossover NB- to NB+: the contrast of the conoscopic fringes almost vanishes, and the sample looses its alignment. This behavior was interpreted as a decrease in the mean diamagnetic susceptibility anisotropy of the sample, which is related to the shape anisotropy of the micelles. We observed that these mixture present smaller micellar shape anisotropy when compared to other lyotropic micellar systems presenting the NB phase. Financial support: INCT-FCx, NAP-FCx, CNPq, FAPESP, CAPES, TüB?TAK.
[1] Figueiredo Neto, A.M. and Salinas, S.R. The Physics of Lyotropic Liquid Crystals, Oxford University Press, Oxford, New York (2005).
[2] Akpinar, E., Reis, D. and Figueiredo Neto, A.M., Anomalous Behavior in the Crossover between the Negative and Positive Biaxial Nematic Mesophases in a Lyotropic Liquid Crystal, Chemphyschem., 15, 1463-9, 2014.