Structural patterns and molecular mobility inside the interface between an fcc solid and liquid n-octane

In this paper we present our results from a molecular dynamics study of n-octane liquids confined between planar 100 fcc solid surfaces. The systems studied were wide enough to develop a bulk-like region throughout the middle portion of the film between two well-separated interfaces. Our work focused on the effects of increasing solid-segment adhesion. For adhesive energies per segment much lower than the thermal energy (weak physisorption) the structure inside the interface and the mobility of octane molecules were found to be liquid-like and quantitatively not very different from the bulk. In strong physisorption cases (adhesive energy of 1–2 kT) we observed the development of qualitatively new structural patterns inside the first interfacial layer. Octane chains lay flat on the surface, adopted very extended almost rod-like conformations and formed two-dimensional liquid crystalline domains with smectic order. The directions of these domains were set by the topography of the underlying solid matrix. For adhesive energies of about 1.5 kT per segment we witnessed a sharp transition from “horizontal” to “tilted” smectic domains. These structural patterns affected profoundly the dynamics of octane molecules. Chain desorption froze for adhesive energies higher than 1.5 kT and rotational relaxation times were at least three orders of magnitude higher than the bulk. However, translational diffusion parallel to the surface remained significant inside the first layer. Surface migration of octane molecules acquired gradually the characteristics of a one-dimensional random walk along the director of the chain's liquid crystalline domain.