The domain structure of LB multilayers prepared from fatty acid salts

Langmuir-Blodgett (LB) multilayers from fatty acid salts can be characterized by finite sized scattering aggregates. High-resolution atomic force microscopy maps reveal a substructure of molecular ordering. These blocks are to be considered as 3D domains. Furthermore, macroscopic holes and scratches appear at the surface. These defect structures have to be taken into account when interpreting observed X-ray and neutron reflectivity curves. The density loss due to the holes can be detected from the angular shift of the critical angle of total external reflection. Off-diagonal scans demonstrate a slight inclination of domains relative to each other. Neutron reflectivity patterns of sequentially deuterated and hydrogenated carbon chains provide information about the disorder of the organic chains. In the second part of the paper a structure model of the molecular order within the domains is developed using 16 in-plane Bragg peaks and their truncation rods measured by X-ray grazing incidence diffraction. The refined 3D unit cell contains at least four monolayers in a herringbone arrangement of the hydrocarbon chains. Its stability is given by the inter-chain coupling between two neighbouring monolayers.