Molecular Dynamics Simulations of Neat Alkanes:  The Viscosity Dependence of Rotational Relaxation

Molecular dynamics simulations of neat octane, dodecane, hexadecane, and eicosane at 323 K are compared with experiment and analyzed with respect to terms that contribute to reorientational relaxation of CH vectors. Agreement with experiment for translational diffusion constants and rotational relaxational times is excellent. It is found that gauche−trans isomerization rates, while weakly dependent on chain position, are essentially independent of chain length and, hence, shear viscosity. In contrast, rotational correlation times of the symmetry (or “long”) axis of the instantaneous moment of inertia tensor increase monotonically with chain length, and, taking the shape dependence into account with a hydrodynamic cylinder model, are proportional to viscosity. Rotation about the long axis is strongly coupled with isomerization. Using an approximate separation of internal and overall motion, reorientation of octane is shown to be largely “rigid body”. For the central carbons in eicosane flexibility contributes at most 40% to the total relaxation time that would be observed in, for example, NMR T 1 measurements.