Crystallinity of the double layer of cadmium arachidate films at the water surface

A crystalline counterionic layer at the interface between an electrolyte solution and a charged layer of insoluble amphiphilic molecules was observed with grazing incidence synchrotron x-ray diffraction. Uncompressed arachidic films spread over 10^-3 molar cadmium chloride solution (pH 8.8) spontaneously form crystalline clusters with coherence lengths of ∼1000 angstroms at 9°C. Ten distinct diffraction peaks were observed, seven of which were attributed to scattering only from a crystalline Cd²⁺ layer and the other three to scattering primarily from the arachidate layer. The reflections from the Cd²⁺ layer were indexed according to a 2 x 3 supercell of the arachidate lattice with three Cd²⁺ ions per cadmium unit cell.