Anomalous Behavior in the Crossover between the Negative and Positive Biaxial Nematic Mesophases in a Lyotropic Liquid Crystal

Abstract

A novel quaternary lyotropic liquid-crystalline mixture of dodecyltrimethylammonium bromide (DDTMABr)/sodium bromide/1-dodecanol/water, presenting the biaxial nematic phase (NB ) in addition to two uniaxial discotic (ND) and calamitic (NC) 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-B to N+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 (Δχ) 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 NB phase.

Effect of Hofmeister anions on the existence of the biaxial nematic phase in lyotropic mixtures of dodecyltrimethylammonium bromide/sodium salt/1-dodecanol/water

“…[14] There, we attributed this phenomenon, named anomalous behaviour, to the presence of micelles with small shape anisotropy, fact that leads to smaller values of the anisotropy of diamagnetic susceptibility of the phase. Present results clearly show that the anomalous behaviour depends not only on the main amphiphilic molecule concentration [14] but also on the type of the salt present in the lyotropic mixture. We may argue that kosmotropic ions weakly bounded to the surfactant head group cause the decrease in the micellar shape anisotropy.…”

“…In the case of the N C phase, as the director orients parallel to the applied magnetic field, well-aligned samples are easily From [14]. c Since this salt consists of two Na + ions and its anion has the charge 2 − , its mole number was multiplied by the factor 2 in the calculations.…”

“…In the case of samples presenting the anomalous behaviour (see, e.g. [14]), the birefringence was evaluated using the Berek compensator and monochromatic light (λ ¼ 546 nm).…”