Coherent X-ray scattering and speckle pattern of solid-supported multilayers of surfactant bilayers

Charged surfactant bilayers deposited in thick multilayer films and swollen in vapor or in solution have been studied by coherent x-ray reflectivity. We report a pronounced static speckle pattern which persists under conditions of full hydration, despite the fact that the bilayers are fluid, relatively soft, and consisting of soluble amphiphiles (DDABr). Characteristic of the macroscopic domains size, the speckle pattern is observed in the plane of incidence (rocking curves) but not in out-of-plane scans, where smaller distances are probed and diffuse scattering from thermal fluctuations is seen. Many phospholipids and synthetic surfactant bilayers spontaneously form lamellar phases in water. These stacks with a microstructure characteristic of smectic-A lyotropic liquid crystals are model systems for studying structure, interactions and elasticity of membranes, which are of intense interest in soft matter and biophysical research. One central issue here is the role of thermal fluctuations, and more generally of collective motions in fluid membrane assemblies. In the past, elastic x-ray scattering has been used extensively to study the collective motions such as bilayer undulations, assuming that the diffuse scattering around the lamellar Bragg peaks (on the diffuse sheets) reflects thermal diffuse scattering. Here we use coherent reflectivity [1] as a sensitive test to whether the diffuse scattering measured in a given range of wave vectors can be fully understood in terms of fluctuating membranes. For the scattering experiments on membrane multilayers, we have recently presented a new approach [2] illustrated in Fig.1, which combines highly oriented surfactant lamellar phases on a flat solid support, interface sensitive scattering techniques and the control of the osmotic pressure. High pressures (dehydrated membranes) are obtained by vapor of controlled humidity, while low pressures (fully hydrated bilayers) are imposed by direct contact with a polyelectrolyte solution of the same sign as the membranes, so it does not penetrate into the multilayer. We used the synthetic cationic surfactant DDABr with bilayer thickness of 23Å at imposed osmotic pressures corresponding to swellings in the range of a few layers of water molecules up to of a few hundred Å. Under these conditions, specular, non-specular and grazing incidence diffuse scattering x-ray reflectivity measurements can be carried out along the different symmetry axes of the oriented membranes. All measurements were carried out at the European Synchrotron Radiation Facility (ESRF). The Preprint submitted to Elsevier Science 18 September 2004 0 10 10