2-DIMENSIONAL CRYSTAL-STRUCTURE OF CADMIUM ARACHIDATE STUDIED BY SYNCHROTRON X-RAY-DIFFRACTION AND REFLECTIVITY

Uncompressed arachidic acid films spread over 10(-3) M cadmium chlorid e solution (pH 8.8 adjusted with ammonia) spontaneously form two-dimen sional (2-D) crystalline clusters with coherence lengths of almost-equ al-to 1000 angstrom at 9-degrees-C. Ten distinct low-order in-plane di ffraction peaks and three high-order peaks were observed with grazing incidence X-ray diffraction (GID) using synchrotron radiation. Seven l ow-order peaks were attributed to scattering only from a crystalline c admium layer and the remaining peaks to scattering primarily from the arachidate layer. The molecules in the arachidate layer arrange in a p seudorectangular unit cell with dimensions a = 4.60 angstrom and b = 8 .37 angstrom and gamma = 93.4-degrees with the chains tilted 11-degree s along the b axis. The chains of the two crystallographically indepen dent molecules in the unit cell are related by pseudoglide symmetry al ong the b axis yielding the herringbone motif. The reflections from th e cadmium layer were indexed according to a supercell a(s) = 2a, b(s) = 3(-a + b)/2. Analysis of X-ray specular reflectivity measurements an d the GID data indicated that the counterionic layer consists of a CdO H+ species, bound to the arachidate layer in a stoichometry close to 1 :1. The probable formation of a cadmium-ammonia complex at the high pH = 8.8 was strongly suggested by the X-ray reflectivity measurements e mploying CH3NH2, (CH3)2NH, and (CH3)3N as alternative counterions. The arrangements of the arachidate chains and of the Cd ions were each de termined to near atomic resolution by fitting the GID data, but the re lative offset between the arachidate and Cd ''lattices'' was difficult to ascertain.