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Anomalous Behavior in the Crossover between the Negative and Positive Biaxial Nematic Mesophases in a Lyotropic Liquid Crystal
Author(s) -
Akpinar Erol,
Reis Dennys,
Figueiredo Neto Antonio M.
Publication year - 2014
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201301003
Subject(s) - liquid crystal , lyotropic , phase diagram , birefringence , anisotropy , materials science , phase (matter) , biaxial nematic , lyotropic liquid crystal , phase transition , crystallography , condensed matter physics , optics , chemistry , organic chemistry , physics , liquid crystalline , optoelectronics
Abstract A novel quaternary lyotropic liquid‐crystalline mixture of dodecyltrimethylammonium bromide (DDTMABr)/sodium bromide/1‐dodecanol/water, presenting the biaxial nematic phase ( N B ) in addition to two uniaxial discotic ( N D ) and calamitic ( N C ) nematic ones, was synthesized. The partial phase diagram of this new mixture was constructed as a function of the DDTMABr molar‐fraction concentration. The phase transitions from uniaxial to biaxial nematic phases were studied by means of the temperature dependence of the optical birefringence. In a particular region of the phase diagram, anomalous behavior was observed in the crossover from ${N_{\rm{B}}^ - }$ to ${N_{\rm{B}}^ + :}$ the contrast of the conoscopic fringes, which allows the birefringence measurements, almost vanishes, and the sample loses its alignment. This behavior, which was not observed before in lyotropics, was interpreted as a decrease in the mean diamagnetic susceptibility anisotropy (Δ χ ) of the sample, which was related to the shape anisotropy of the micelles. Small‐angle X‐ray scattering measurements were performed to evaluate the micellar shape anisotropy; these revealed that this mixture presented a smaller shape anisotropy than those of other lyotropic micellar systems presenting the N B phase.