An analogy between biaxial crystal and biaxial nematic lyotropic phase

The biaxial nematic phase (NB), the intermediate phase between the discotic nematic (ND) and calamitic (NC) nematic phases, is investigated from refractive index data in a potassium laurate-decanol-D2O mixture. Temperature dependence of the angle (2V) between two optic axes of this NB phase is obtained with the use of a procedure which is analogous to that used in a biaxial crystal. In this context, biaxial positive (NB+) and biaxial negative (NB−) nematic phases are optically characterized. A transition from a NB+ to a NB− configuration is found at this point where the corresponding angle reaches the maximum value. In this case, the micelle looses its predominant discotic shape to acquire a predominant calamitic shape.