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Abstract
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 ND + cylindrical NC). This has been detected previously by several methods for a sample with water/SDS molar ratio Mw = 36. Here, this system is investigated changing the ratio Mw to 32, where previously the coexistence region was reported after a discotic ND phase, without the biaxial phase. We report now the existence of a biaxial NB in the temperature range of ~2°C, defining for both values of Mw and the phase sequence as NB – (ND + NC) – NC. 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 ND – NB – (ND + NC). 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 NC matrix. Results are explained in terms of recent theories dealing with stabilisation of mixtures of cylinders and discs.