Computer simulation of the role of torsional flexibility on mass and momentum transport for a series of linear alkanes.

We present the results obtained from a systematic equilibrium molecular dynamics study of the effect of torsional flexibility on the diffusion and viscosity of a series of linear alkanes. To make unambiguous comparisons between molecules with torsional flexibility and those without, we use the frozen distribution sampling (FDS) method introduced by Travis et al. [J. Chem. Phys. 98, 1524 (1993); J. Chem. Phys. 102, 2174 (1995)] but modified and updated for increased efficiency. We first demonstrate comprehensively that FDS guarantees corresponding thermodynamic states. We then show that removal of torsional flexibility results in a significant lowering of the diffusion coefficient (and corresponding increase in shear viscosity) and furthermore that this effect increases with increasing chain length. The results are discussed in terms of the possible mechanism giving rise to this dynamic coupling phenomenon.