Dc. Koopman et al., STRUCTURAL PATTERNS AND MOLECULAR MOBILITY INSIDE THE INTERFACE BETWEEN AN FCC SOLID AND LIQUID N-OCTANE, Chemical Engineering Science, 49(17), 1994, pp. 2907-2920
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.