Liquid Crystals, 2018, vol 45, 4, pp. 475-484
We examine an eventual coexistence of discotic and calamitic nematic uniaxial phases in the sodium dodecyl sulphate (SDS)/1-decanol (DeOH)/D2O mixture phase diagram. This coexistence is claimed in some previous works. Four different molar ratios [D2O]/[SDS] were investigated, where the square brackets represents the molar concentration of the compound, and 33, with temperatures ranging from 20°C up to about 52°C. The experimental techniques employed for phase identification were laser conoscopy, polarised optical microscopy and small-angle X-ray scattering. In the investigated surface of the phase diagram, no such nematic phases coexistence region was found. The discotic and biaxial nematic phases were identified in some regions of the phase diagram. Coexistence domains of the nematic ND/lamellar, and lamellar/micellar isotropic were identified. Our results agree with the theoretical predictions and symmetry arguments that consider the nematic/nematic phase transitions of second order, well described by a mean-field Landau-type model, preventing the nematic/nematic coexistence domains. Moreover, these results are consistent with all the experimental results we encountered in more than 30 different lyotropic mixtures investigated by using complementary experimental techniques to univocally identify the mesophases and phase-coexistence regions.