Two-dimensional crystal structure of cadmium arachidate studied by synchrotron x-ray diffraction and reflectivity

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 ≈1000 Å 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 Å and b = 8.37 Å and γ = 93.4° with the chains tilted 11° 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 a8 = 2a, b₈ = 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 1:1. 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, (CH₃)₂NH, and (CH₃)₃N 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.