X-Ray Diffraction and Spectroscopic Techniques for Liquid Surfaces and Interfaces
X-ray and neutron scattering techniques are probably the most effective tools when it comes to determining the structure of liquid interfaces on molecular-length scales. These techniques are, in principle, not different from conventional x-ray diffraction techniques that are commonly applied to three-dimensional crystals, liquids, solid surfaces etc. However, special diffractometers and spectrometers 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 a dedicated liquid surface reflectometer. A basic property of a liquid-vapor interface is the length scale over which the molecular density changes from the bulk value to that of the homogeneous gaseous medium. Inter molecular correlation and capillary waves, which depend on surface tension, gravity, and resolution that determines the length scale over which these correlations averaged, are among the most important factors that define the density profile across the interface and the planar correlations. 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 interface (generally referred to as Langmuir monolayers). The spread compound might “wet” the liquid surface forming a film of homogeneous thickness or cluster to form an inhomogeneous rough surface. Two major diffraction techniques, complimenting one another, are used to determine interfacial structure. One is the x-ray reflectivity (XR) technique which allows one to determine the electron density across such interfaces, from which the molecular density and the total thickness can be extracted. The other is the grazing angle x-ray diffraction (GIXD) technique which is commonly used to obtain lateral arrangements and correlations of the topmost layers at interfaces. GIXD is especially efficient in cases where surface crystallization of the liquid or spread monolayers occurs. GIXD has been expanded to small angles (SA-GIXD, or GI-SAXS) for probing the structure of more complicated macromolecular systems, such as membrane proteins, and polymers. Diffuse x-ray scattering (DXS) near the reflectivity line, which fall into the general GIXD category, are commonly to evaluate interfacial correlations, due to interfacial capillary waves, for instance. Both, XR and GIXD techniques provide structural information that is averaged over macroscopic areas, in contrast to scanning probe microscopies (SPMs), 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 definitively and often requires complementary techniques to support the x-ray analysis. Therefore, whenever the objective of the experiment is not compromised, preparation of well-defined homogeneous interfaces is a key to a more definitive and straightforward interpretation. The X-ray fluorescence spectroscopy technique near total reflection from ion-enriched liquid interfaces were developed at the same time as the diffraction techniques adding insight into ion adsorption and phenomena at liquid interfaces. The facile tunablility of photon energy at synchrotron x-ray facilities enabled the extension of the fluorescence technique to obtain the near-resonance surface x-ray absorption spectroscopy (XANES) of specific interfacial ions. Photon energy tunability also facilitated the application of anomalous diffraction and spectroscopic methods commonly used for bulk materials. Anomalous XR, near edge energy scans at fixed momentum transfers have been implemented and used to determine ion distributions and ionic coordination at charged Langmuir monolayers and liquid-liquid interfaces.