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The advent of well-collimated, high-intensity synchrotron 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 from them for Langmuir films. The methods have been successfully applied in the elucidation of the structure of crystalline aggregates of amphiphilic molecules such as alcohols, carboxylic acids and their salts, a-amino acids, and phospholipids at the water surface. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline self-aggregates as well as structural changes occurring by phase transitions. Furthermore, the surface X-ray methods shed new light on the structure of the underlying ionic layer of attached solvent or solute species. Examples are given where singly or doubly charged ions bound to the two-dimensional (2D) crystal form either an ordered or diffuse counterionic layer. Finally, the surface diffraction methods provide data on transfer of structural information from 2D clusters to 3D single crystals, which had been successfully accomplished by epitaxial-like crystallization both in organic and inorganic crystals.

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Uncompressed arachidic acid films spread over 10-3 M cadmium chloride solution (pH 8.8 adjusted with ammonia) spontaneously form two-dimensional (2-D) crystalline clusters with coherence lengths of =loo0 A at 9 "C. Ten distinct low-order in-plane diffraction peaks and three high-order peaks were observed with grazing incidence X-ray diffraction (GID) using synchrotron radiation. Seven low-order peaks were attributed to scattering only from a crystalline cadmium layer and the remaining peaks to scattering primarily from the arachidate layer. The molecules in the arachidate layer arrange in a pseudorectangular unit cell with dimensions a = 4.60 A and b = 8.31 A and y = 93.4O with the chains tilted 1l0 along the b axis. The chains of the two crystallographically independent molecules in the unit cell are related by pseudoglide symmetry along the b axis yielding the herringbone motif. The reflections from the cadmium layer were indexed according to a supercell a, = 2a, b, = 3(-a + b)/2. Analysis of X-ray specular reflectivity measurements and the GID data indicated that the counterionic layer consists of a CdOH+ species, bound to the arachidate layer in a stoichometry close to 1:l. The probable formation of a cadmium-ammonia complex at the high pH = 8.8 was strongly suggested by the X-ray reflectivity measurements employing CH3NH2, (CH&NH, and (CH&N as alternative counterions. The arrangements of the arachidate chains and of the Cd ions were each determined to near atomic resolution by fitting the GID data, but the relative offset between the arachidate and Cd "lattices" was difficult to ascertain.

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