Coherent X-ray scattering and speckle pattern of solid-supported multilayers of surfactant bilayers
Résumé
Charged surfactant bilayers deposited in thick multilayer films and swollen in vapor or in solution have been studied by coherent X-ray scattering. 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). Contrarily, out-of-plane scans, which probe smaller lateral distances along the multilayer, exhibit a smooth diffuse scattering curve characteristic of thermal fluctuations of membranes. r 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, 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 see whether the diffuse scattering measured in a given range of wave vectors can fully be 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 ARTICLE IN PRESS www.elsevier.com/locate/physb 0921-4526/$-see front matter r