Studies of Dynamical Layering in Adsorbed Organic Films

We are using state-of-the-art quasi-elastic neutron scattering (QNS) to study how molecular diffusive motion in prototypical films may correlate with their lubricating properties. Our experiments have demonstrated that the High Flux Backscattering Spectrometer at the National Institute of Standards and Technology Center for Neutron Research has sufficient energy resolution and throughput for probing relatively slow diffusive motions in molecular monolayers of model lubricants such as tetracosane (C24H50 or C24). In particular, these measurements are sensitive to the molecular conformation al dynamics on a nanosecond time scale. It is well known from experiments using a surface force apparatus that organic fluids confined between two surfaces exhibit a layered structure at the molecular level. This static layering has motivated us to consider the possibility that the individual molecular layers in fluid films may also have different dynamical properties. Here we report high-resolution QNS measurements on fluid monolayer, bilayer, and trilayer films of C24 adsorbed on exfoliated graphite (Grafoil). The results indicate slower molecular diffusive motion in the bottom layer of a bilayer C24 film compared to that in a monolayer, while the rate of diffusive motion in the bottom two layers of the trilayer film is comparable to that in a monolayer