Investigation of molecular diffusion across organic multilayers using neutron specular reflectivity

Langmuir-Blodgett (LB) multilayers of fatty acid salt molecules have been investigated by neutron specular reflectivity (NSR) at the reflectometer ADAM at the Institute Laue-Langevin in Grenoble, France. Due to the high flux at the sample site, the NSR curve could be recorded over more than six orders of magnitude, corresponding with q(z) less than or equal to 0.3 Angstrom(-1). Because of the incoherent hydrogen scattering, the detectable coherent NSR curve is limited to 10(-5). The incoherent background of the multilayer systems could be determined for the first time by means of a spin-polarized NSR experiment. The observed intermixing at freshly prepared LB multilayers increases during thermal annealing below the phase transition temperature, whereas the vertical structure remains. This is explained by thermally induced vertical reorganisation of molecules across the multilayer system. The molecular exchange can be described as an analogue to an order-disorder transition. The evaluated activation energy of about 1.7 eV is related to the breaking of van der Waals bonds between one particular molecule and its hexagonal neighbourhood. Time-resolved measurements at particular temperatures allow us to estimate a vertical "diffusion constant" in the range of 10(-23) m(2)/s. (C) 1999 Elsevier Science S.A. All rights reserved.