Neutron reflectivity: a novel means of studying thin organic films at electrified interfaces
Neutron reflectivity has been employed to study the structure and composition of thin films formed by n-octadecanol (C18OH), 4-pentadecyl-pyridine (C 15-4Py), DMPC (dimyristoylphosphatidylcholine) and sodium dodecyl sulfate (SDS) at gold electrode surfaces. It has been established that it is possible to use neutron reflectivity to determine the thickness and the water content of the films as a function of the applied electrical potential has been established. By measuring the film thickness of a monolayer of C 18OH as a function of potential it has been shown that the tilt-angle of the hydrocarbon tail increases with decreasing (more negative) potential, with a concurrent decrease in the film pressure of the C18OH layer. In the case of C15-4Py, experiments performed with different isotopic contrasts show that at positive potentials a condensed film is formed and the structure of the monolayer film resembles rafts surrounded by cracks filled with water. At very negative potentials, the film is desorbed from the electrode surface. The neutron reflectivity data demonstrated that, at these potentials, the amphiphilic molecules remain in close proximity to the gold surface. A biomimetic film consisting of cholesterol and DMPC was also studied with neutron reflectivity as a function of the charge density on the metal support. When the surface charge density is close to zero, the lipid vesicles fuse directly on the surface to form a bilayer with a small number of defects and hence small water content. When the support's surface is negatively charged the film swells and incorporates water. When the charge density is more negative then -8 [mu]C cm-2 the bilayer starts to detach from the metal surface. However, it remains in close proximity to the metal electrode and is suspended on a thin cushion of electrolyte.