X-ray and neutron reflection analysis of the structure and the molecular exchange process in simple and complex fatty acid salt Langmuir-Blodgett multilayers

A Langmuir-Blodgett multilayer film of cadmium arachidate and a complex multilayer system of cadmium stearate bilayers separated by copolymer interlayers have been prepared by sequential transfer of deuterated and nondeuterated bilayers on a solid support. They were investigated at different temperatures using X-ray and neutron specular as well as diffuse reflectivity measurements. The neutron scattering was used to determine the vertical arrangement of deuterated chains within the multilayers and to study the different roughnesses at the chain-chain and chain-metal ion interface separately. The specular reflectivity patterns of both samples could only be interpreted considering the contribution of resonant-diffuse scattering caused by the perfect vertical correlation of individual interface roughnesses within a certain domain. For the two samples the superstructure peak intensity appearing in the respective neutron reflectivity curve decreases dramatically during annealing between 50 and 65 degrees C. This indicates an intermixing of deuterated and nondeuterated molecules within the film. The molecular exchange cannot be suppressed by the incorporation of copolymer interlayers between the deuterated and nondeuterated chains. Because the X-ray reflectivity patterns remain nearly unaffected by the annealing and because the annealing temperature was lower than the melting point of the pure fatty acid phase (about 70 degrees C), we interpret this effect by a vertical movement of fatty acid molecules across the still lamellarly stacked framework of fatty acid salt molecules. The temperature dependence of this process shows Arrhenius-libe behavior. For both samples the estimated activation energy is 1.7 +/- 0.5 eV (164 +/- 48 kJ/mol) and is assigned to the van der Waals bonding energy of single molecules in a 2D hexagonal lattice. From time-dependent measurements we estimated a vertical diffusion coefficient in the order of 10(-22) m(2)/s.