Biological membranes are vital components of all living organisms. They form the boundaries between the various compartments of cells and constitute platforms for essential biochemical processes like enzymatic reactions, molecular transport, or the formation of functional lipid domains. Importantly, structural insight is often a prerequisite to understand the details of these processes. X-ray and neutron reflectometry enable the structural characterization of model biological membranes at sub-nanometer resolution and the investigation of interactions of membranes with a variety of biomolecules . This has only become possible with the simultaneous development of methods for membrane immobilization in planar geometries, such as solid-supported membranes or membranes floating on polymers, lipids, or soft tethers [1,2]. However, when using these approaches, studies on molecules crossing the membrane or deeply penetrating into the bilayer chain region (see toxins, amyloid peptides, membrane proteins in general) turned out to be difficult, because membrane mobility often suffered from the presence of the solid surface. In fact, the water gap beneath solid-supported floating membrane is rarely thicker than 2nm.
Here we present an alternative route, the immobilization of lipid membranes near functionalized liquid/liquid (L/L) interfaces. The latter are are intrinsically soft, self-healing, defect-free and the replacement of the solid support with a liquid phase enables manipulation of the interface through both liquids. Negatively charged lipid bilayers were immobilized via vesicle fusion onto oil/water interfaces functionalized with positively charged lipids and structurally investigated using specular neutron reflectometry. The interaction between the bilayer and the L/L interface was tuned by variation of the ionic strength, as evidenced by a bilayer displacement relative to the interface.
 H. P. Wacklin, Curr. Opin. in Coll. & Int. Sci. 2010, 15, 445-454.
 G. Fragneto, T. Charitat, J. Daillant, E. Biophys. J., 2012, 41, 863-874.