Principles and applications of grazing incidence x-ray and neutron scattering from ordered molecular monolayers at the air-water interface

The advent of well collimated, high intensity synchroton X-ray sources and the consequent development of surface-specific X-ray diffraction and fluorescence techniques have recently revolutionized the study of Langmuir monolayers at the air-liquid interface. These methods allowed for the first time the determination of the in-plane and vertical structure of such monolayers with a resolution approaching the atomic level. We briefly describe these methods, including grazing incidence X-ray diffraction, specular reflectivity, Bragg rods, standing waves and surface fluorescence techniques, and review recent results obtained for Langmuir films from their use. The methods have been successfully applied for the elucidation of the structure of crystalline aggregates of amphiphilic molecules at the water surface such as alcohols, carboxylic acids and their salts, α-amino acids and phospholipids. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline self-aggregates as well as structural changes occuring by phase transitions. Furthermore, via the surface X-ray methods, new light is shed on the structure of the underlying attached solvent or solute ionic layer. Examples are given where singly or doubly charged ions bound to the two-dimensional (2D) crystal form either an ordered or diffuse counter-ionic layer. Finally, the surface diffraction methods provide data on transfer of structural information from 2D clusters to 3D single crystals which had been succesfully accomplished by epitaxial-like crystallization both in organic and inorganic crystals.