Interaction of short-chain alkanes with the surface and interfaces of multilayer films built from amphiphilic molecules: An in-situ X-ray and neutronscattering probe

Partially deuterated Cd-arachidate Langmuir-Blodgett multilayers were subje cted to a gas atmosphere of n-pentane and n-hexane at room temperature. The gas-film interaction was studied by in-situ X-ray and neutron specular sca ttering through a gas-flow box. Both methods show an increase of the small- angle Bragg peak intensities during the gas treatment while the intensities of Kiessig oscillations decrease at the same time. The change of intensiti es is partially reversible after purging with air. Neutron scattering revea ls that the increase of structural Bragg peaks is also accompanied by a dec rease of superstructure peaks originated by the scattering length density d ifference between the sequentially stacked deuterated and nondeuterated cha ins. This indicates a gas-assisted intermixing of amphiphilic molecules in the vertical direction. The observed effect is much larger than in the case that the short-chain molecules occupy defect sites of the virgin Langmuir- Blodgett film. It can be explained by assuming that the gas molecules tempo rarily enter the space between the amphiphilic molecules. This locally redu ces the van der Waals interaction and increases the mobility of amphiphilic molecules. The gas-assisted mobility improves the quality of internal inte rfaces by increasing the density at the Cd-ion sites, but it increases the surface roughness by the vertical pile up of molecules.