Uniaxial and Biaxial Nematic Phases in Sodium Dodecyl Sulphate - Decanol - D2O Mixtures. An Optical Conoscopy Study

The system SDS/decanol/water is investigated by polarized optical microscopy. A reported biaxial island is here confirmed by conoscopy, in the temperature interval 20 degrees C to 26 degrees C. Coexistence regions of the two uniaxial nematic phases (discotic N-D + cylindrical N-C) are also confirmed, in different temperature intervals. While the biaxial phase is stable for at least 10 hours, in a reproducible way, along the same time the coexistence region evolves with time, and drops of the discotic phase grow immersed in a N-C matrix. Results are explained in terms of recent theories dealing with stabilization of mixtures of cylinders and discs.