Biaxial phase and coexistence of the two uniaxial nematic phases in the system sodium dodecyl sulphate-decanol-D2O

The system sodium dodecyl sulphate (SDS)/decanol (DeOH)/water presents, with temperature increase, an unusual phase transition between a stable biaxial phase and a coexistence region of the two uniaxial nematic phases (discotic N-D + cylindrical N-C). This has been detected previously by several methods for a sample with water/SDS molar ratio M-w = 36. Here, this system is investigated changing the ratio M-w to 32, where previously the coexistence region was reported after a discotic N-D phase, without the biaxial phase. We report now the existence of a biaxial N-B in the temperature range of ~2 degrees C, defining for both values of M-w and the phase sequence as N-B - (N-D + N-C) - N-C. The change in temperature is followed by conoscopic and orthoscopic optical techniques and also analysed through the curve obtained by the technique of digital image processing of the textures, which reveals a continuous transition N-D - N-B - (N-D + N-C). While the biaxial phase is stable for at least 10 hours, in a reproducible way, 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 stabilisation of mixtures of cylinders and discs.