SEGMENTAL DYNAMICS AND RELAXATION OF N-OCTANE AT SOLID-LIQUID INTERFACES

In this paper we present our results from a molecular dynamics study o f n-octane liquids confined between planar bcc solid surfaces. The sys tems studied were wide enough to develop a bulklike region throughout the middle portion of the film and two well-separated interfaces. Our work focused on segmental dynamics and relaxation of ''adsorbed'' octa ne molecules. In particular, we investigated the role of architectural and dynamical features peculiar to short chain molecules (almost fixe d bend angles and restricted torsional rotations) on the dynamics of ' 'adsorbed'' chains. We found that the relaxation of octane molecules e xhibits the same qualitative trends as those observed in molecular sim ulations of generic ''bead-spring'' oligomer films. The most important effect is the dramatic slow down of rotational motions (up to a facto r of 1000) for chains adsorbed on strongly physisorbing surfaces (adhe sion energy per segment of 1-2 kT). Despite the qualitative similariti es with bead-spring chains, the dynamics of realistic short hydrocarbo n chains are affected much more strongly by the interfacial, environme nt than their bead-spring counterparts. These stronger effects origina te largely from the suppression of torsional angle transitions inside the extremely dense first layer (in cases of strong physisorption). Th e frequency of torsional transitions was found to be correlated direct ly with the amount of ''free volume'' available inside the crowded fir st layer.