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

Synchrtron X-ray measurements are performed in both uniaxial (discotic and calamitic) and biaxial nematic lyomesophases of the mixture K-laurate, 1-decanol and D 2 O. 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 aggrgates of statistically biaxial shape, similar in the three nematic phases. The only change at the uniaxial-biaxial nematic transitions is the long-range order Mesures realisees sur un melange de laurate de potassium, decanol-1 et D 2 O. Dans les trois phases discotiques, calomitiques et biaxes, on observe le premier et le second ordre d'une bande de diffraction caracteristique d'une structure pseudo-lamellaire. Les phases calomitiques et discotiques sont composees d'agregats de forme statistiquement biaxe, seul l'ordre a longue distance change aux transitions uniaxes-biaxes