S. Gupta et al., SEGMENTAL DYNAMICS AND RELAXATION OF N-OCTANE AT SOLID-LIQUID INTERFACES, The Journal of chemical physics, 100(11), 1994, pp. 8444-8453
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.