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    On the coexistence of uniaxial nematic phases in the lyotropic mixture sodium dodecyl sulphate/1-decanol/D2O
    (Taylor & Francis Ltd, 2017) Oliveira-Filho, Arnaldo G.; Akpınar, Erol; Reis, Dennys; Figueiredo Neto, Arnaldo Martins
    Previously examined lyotropicmixtures sodium dodecyl sulphate (SDS) 1-decanol (DeOH) - D2O were prepared in four different molar ratios, M = [D2O]/[SDS], where M = 30, 31, 32 and 33, to study the coexistence of uniaxial discotic (N D) and calamitic (N C) nematic phases as was claimed in recent articles. Those mixtures were investigated in a specific temperature range by laser conoscopy, polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS) to construct partial phase diagram. By combining the results obtained from all these techniques, we could not observe the N-D - N-C phase coexistence in the phase diagram of SDS/DeOH/D2O after the detailed investigation, as it was formerly reported. The results obtained in this study are in a good agreement with symmetry arguments and theoretical predictions but not with literature claims. The present results are also consistent with experimental investigations of other lyotropic mixtures whose phase diagrams were previously identified.
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    A survey on the phase diagram of the system sodium dodecyl sulphate/1-decanol/D2O concerning an eventual coexistence of uniaxial nematic phases
    (Taylor & Francis Ltd, 2018) Oliveira-Filho, Arnaldo G.; Akpınar, Erol; Reis, Dennys; Figueiredo Neto, A. M.
    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 degrees C up to about 52 degrees 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. [Graphics] .