Sans Diffuse Pattern in Very Swollen Lamellar Phases

The x-ray or neutron diffraction profiles of very swollen surfactant lamellar phases generally exhibit a small-angle diffuse scattering pattern not expected for media with quasi–long-range smectic order. We show that it arises from the concentration fluctuations associated with the thermal displacement of the layers. The analysis of experimental diffraction profiles of very swollen samples further proves that the steric interaction scaling like k c -1 d -2 (where k c is the layer rigidity and d the interlayer distance) is dominant at large d .