Results on the diffusion behavior of model systems was studied, for eight liquid n-alkanes (C12-C44) in a canonical ensemble at several temperatures, by using molecular dynamics simulations. For these n-alkanes of small chain length n, the chains were clearly <Ree2>/6<Rg2>>1 and non-Gaussian. This result implied that the liquid n-alkanes over the whole temperatures considered were far away from the Rouse regime, though the ratio came close to unity as n increased. Calculated self-diffusion constants Dself were comparable with experimental results and the Arrhenius plot of self-diffusion constants versus inverse temperature showed a different temperature dependence of diffusion on the chain length. The global rotational motion of n-alkanes was examined by characterizing the orientation relaxation of the end-to-end vector and it was found that the ratio τ1/τ2 was less than 3, the value expected for a isotropically diffusive rotational process. The friction constants Ξ of the whole molecules of n-alkanes were calculated directly from the force auto-correlation functions and compared with the monomeric friction constants ΞD extracted from Dself. Both the friction constants gave correct qualitative trends: decreased with increasing temperature and increased with increasing chain length. The friction constant calculated from the force auto-correlations decreased very slowly with increasing temperature, while the monomeric friction constant varies rapidly with temperature. By considering the orientation relaxation of local vectors and diffusion of each site, it was found that rotational and translational diffusions of the ends were faster than those of the center.

Diffusion Behavior of n-Alkanes by Molecular Dynamics Simulations. Goo, G.H., Sung, G., Lee, S.H., Chang, T.: Bulletin of the Korean Chemical Society, 2002, 23[11], 1595-603