Phase behaviour of ultrathin crystalline n-heptane films on graphite: an atomistic simulation study

The thermal behaviour of n-heptane films adsorbed on the basal plane of graphite has been investigated using atomistic molecular dynamics simulations performed under constant temperature conditions. An uniaxially commensurate monolayer (UCM), an uniaxially commensurate bilayer (UCB) and a fully commensurate bilayer (FCB) of n-heptane have been studied in order to distinguish the contributions from coverage and in-plane density to the melting process. These ordered adlayers containing molecules whose zig-zag planes were parallel to the substrate, were stabilized at low temperatures. The calculated intermolecular energy per molecule shows that the FCB structure is more stable than the UCB structure, consistent with X-ray and neutron scattering experiments. In all of these systems, disordering processes in the solid state involves a fraction of molecules rotating about their backbone axes, in agreement with scanning tunneling microscopy studies. The molecules present in uniaxially commensurate structures (UCM and UCB) are facile to rotate about their long axes which causes these adlayers to melt at lower temperatures compared to the fully commensurate structure. The spatial correlation functions between such rotated molecules show interesting temperature dependence that indicates a loss of long-range and short-range correlations in these overlayers with increasing temperature. The importance of conformational defects for the melting of these quasi-two-dimensional systems has also been explored. Axial rotations of molecules present in the first adsorbed layer of FCB are facile between bulk and monolayer melting temperatures. This process leads to the observation of an average angle of rotation of around 40 degrees in excellent agreement with X-ray and neutron scattering experiments.