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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 23, Number 11
BKCSDE 23(11)
November 20, 2002 

Diffusion Behavior of n-Alkanes by Molecular Dynamics Simulations
Geun Hoi Goo, Gihong Sung, Song Hi Lee,Taihyun Chang
Molecular dynamics simulation, Diffusion coefficient, n-Alkanes
In this paper we have presented the results of diffusion behavior of model systems for eight liquid n-alkanes (C12-C44) in a canonical (NVT) ensemble at several temperatures using molecular dynamics simulations. For these n-alkanes of small chain length n, the chains are clearly ee2>/6g2>>1 and non-Gaussian. This result implies that the liquid n-alkanes over the whole temperatures considered are far away from the Rouse regime, though the ratio becomes close to the unity as n increases. Calculated self-diffusion constants Dself are comparable with experimental results and the Arrhenius plot of self-diffusion constants versus inverse temperature shows a different temperature dependence of diffusion on the chain length. The global rotational motion of n-alkanes is examined by characterizing the orientation relaxation of the end-to-end vector and it is found that the ratio τ12 is less than 3, the value expected for a isotropically diffusive rotational process. The friction constants ξof the whole molecules of n-alkanes are calculated directly from the force auto-correlation (FAC) functions and compared with the monomeric friction constants ξD extracted from Dself. Both the friction constants give a correct qualitative trends: decrease with increasing temperature and increase with increasing chain length. The friction constant calculated from the FAC’s decreases 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 is found that rotational and translational diffusions of the ends are faster than those of the center.
1595 - 1603
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