Observation of Two Biaxial Nematic Mesophases in the Tetradecyltrimethylammonium Bromide/Decanol/Water System

The phase diagram of the nematic mesophases present in the tetradecyltrimethylammonuim bromide/ n -decanol/water system was determined. Two distinct biaxial nematic mesophases with opposite diamagnetic and optical anisotropies were identified using 2 H NMR spectroscopy and optical microscopy with polarized light. Two nematic biaxial phases were observed in a very narrow region between the two uniaxial nematic mesophases. The sequence of phase changes, hexagonal−calamitic N C −biaxial N BX + −biaxial N BX - −discotic N D −lamellar mesophases, was observed with increasing decanol content. The phase transition between the two biaxial mesophases is first order, while the phase transitions between the N C and N BX + and the N BX - −N D mesophases are second order. The order parameters of the ionic headgroup of the surfactant, determined from the 2 H quadrupolar splittings, indicate that the average micelle size decreases with the decanol content in the N C and N BX + mesophases and increases with the decanol content in the N BX - and N D mesophases.