Pseudo-lamellar ordering in uniaxial and biaxial lyotropic nematics : a synchrotron X-ray diffraction experiment

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Abstract
Synchrotron X-ray measurements are performed in both uniaxial (discotic and calamitic) and biaxial nematic lyomesophases of the mixture K-laurate, 1-decanol and D2O. A first-order band and a second-order band with spacing ratio 1 : 2 characteristic of a pseudo-lamellar structure are observed in the three nematic phases. The X-ray diffraction results are interpreted in terms of the orientational fluctuations of intrinsically biaxial correlation volumes. It is argued that the calamitic and discotic phases are not built up of cylindric-like and disk-like micelles, but of aggregates of statistically biaxial shape, similar in the three nematic phases. The only change at the uniaxial-biaxial nematic transitions is the long-range order