Effect of cadmium arachidate on molecular packing of zinc arachidate LB multilayers

Mixed zinc arachidate–cadmium arachidate (ZnA–CdA) LB multilayers deposited from varying subphase compositions have been characterized by FTIR and X-ray reflection (XR). The COO− symmetric stretching band of CdA and changes in CH2 scissoring band were used to monitor the CdA content and nature of molecular packing in the mixed multilayers. The XR pattern of pure ZnA multilayer with bilayer period of ∼47 Å corresponds to a rotator phase like ‘loose packing’ in a hexagonal layer cell. However, the presence of low concentrations of cadmium ion in subphase (2–5 mol%) results in biphasic multilayers with bilayer periods of ∼47 and ∼51 Å. In the cadmium concentration range of 10–25 mol%, the multilayers exhibit sharp (0 0 l) peaks in XR pattern, corresponding to a single layered structure with alkyl chains tilted by ∼23° from layer-normal. Increase in the cadmium concentration (30–50 mol%) results in biphasic multilayers with bilayer periods of ∼52 and ∼55 Å, the later corresponding to a herringbone type packing of alkyl chains, perpendicular to the layer plane. At higher concentrations of CdA, the layered structure and molecular packing resemble that of pure CdA multilayer.