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 synchrotron X-ray source s 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 allowe d for the first time the determination of the in-plane and vertical st ructure of such monolayers with a resolution approaching the atomic le vel. 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 f or Langmuir films from their use. The methods have been successfully a pplied for the elucidation of the structure of crystalline aggregates of amphiphilic molecules at the water surface such as alcohols, carbox ylic acids and their salts, alpha-amino acids and phospholipids. In ad dition, it became possible to monitor by diffraction the growth and di ssolution of the crystalline self-aggregates as well as structural cha nges occurring by phase transitions. Furthermore, via the surface X-ra y methods, new light is shed on the structure of the underlying attach ed solvent or solute ionic layer. Examples are given where singly or d oubly charged ions bound to the two-dimensional (2D) crystal form eith er an ordered or diffuse counter-ionic layer. Finally, the surface dif fraction methods provide data on transfer of structural information fr om 2D clusters to 3D single crystals which had been successfully accom plished by epitaxial-like crystallization both in organic and inorgani c crystals.