Dynamic Light Scattering Study of Dilute Lamellar Phases

Dynamic light scattering measurements on oriented samples of lyotropic smectics have been performed. The layer compressibility modulus $B$ is extracted from the anisotropic dispersion relation of the so-called slip mode, which arises from the coupling between concentration and layer displacement fluctuations. The samples are diluted with three different solvents (repeat distances 50 to 800 Å): The behavior of $B$ suggests either undulation or electrostatic repulsions between membranes, depending on the swelling solvent.