X-ray Diffraction Techniques to Characterize Liquid Surfaces and Monomolecular Layers at Gas-Liquid Interfaces

X-ray and neutron scattering techniques are probably the most effective tools to be employed in order to determine the structure of liquid interfaces on molecular length scales. These are not different in principle from conventional X-ray diffraction techniques that are usually applied to three dimensional crystals, liquids, solid surfaces etc. However, special diffractometers that enable scattering from fixed horizontal surfaces are required to carry out the experiments. Indeed, systematic studies of liquid surfaces had not begun until the introduction of the first liquid surface reflectometer. [1]. A basic property of a liquid-gas interface is the length scale over which the molecular density changes from the bulk value to that of the homogeneous gaseous medium. Molecular size and capillary waves, that depend on surface tension and gravity, are among the most important factors that shape the density profile across the interface and the planar correlations [14,9,49]. In some instances the topmost layers of liquids are packed differently than in the bulk, giving rise to layering phenomena at the interface. Monolayers of compounds different than the liquid can be spread at the gas-liquid at interface, and are termed Langmuir monolayers [17,50]. The spread compound might wet the liquid surface to form a film of homogeneous thickness or cluster to form an inhomogeneous rough surface. The X-ray reflectivity (XR) technique allows one to determine the electron density across such interfaces from which the molecular density and the total thickness can be extracted. The grazing angle diffraction (GID) technique is commonly used to determine lateral arrangements and correlations of the topmost layers at interfaces. GID is especially efficient in cases where surface crystallization of the liquid or spread monolayers occurs. Both techniques (XR and GID) provide structural information that is averaged over macroscopic areas in contrast to scanning probe microscopies (SPM’s) where local arrangements are probed. For an inhomogeneous interface the reflectivity is an incoherent sum of reflectivities, accompanied by strong diffuse scattering which in general is difficult to interpret with definitive answers and often requires complementary techniques to support the X-ray analysis. Therefore, preparation of well-defined homogeneous interfaces is a key to a more definitive and straightforward interpretation.