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 o f n-octane liquids confined between planar 100 fcc solid surfaces. The systems studied were wide enough to develop a bulk-like region throug hout the middle portion of the film between two well-separated interfa ces. Our work Focused on the effects of increasing solid-segment adhes ion. For adhesive energies per segment much lower than the thermal ene rgy (weak physisorption) the structure inside the interface and the mo bility of octane molecules were found to be liquid-like and quantitati vely not very different from the bulk. In strong physisorption cases ( adhesive energy of 1-2 kT) we observed the development of qualitativel y new structural patterns inside the first interfacial layer. Octane c hains lay flat on the surface, adopted very extended almost rod-like c onformations and formed two-dimensional liquid crystalline domains wit h smectic order. The directors of these domains were set by the topogr aphy of the underlying solid matrix. For adhesive energies of about 1. 5 kT per segment we witnessed a sharp transition from ''horizontal'' t o ''tilted'' smectic domains. These structural patterns affected profo undly the dynamics of octane molecules. Chain desorption froze for adh esive energies higher than 1.5 kT and rotational relaxation times were at least three orders of magnitude higher than the bulk. However, tra nslational diffusion parallel to the surface remained significant insi de the first layer. Surface migration of octane molecules acquired gra dually the characteristics of a one-dimensional random walk along the director of the chain's liquid crystalline domain.