Viscosity and Diffusion Constants Calculation of n-Alkanes by Molecular Dynamics Simulations

Abstract

In this paper we have presented the results for viscosity and self-diffusion constants of model systems for four liquid n-alkanes (C 1 2 , C 2 0 , C 3 2 , and C 4 4 ) in a canonical ensemble at several temperatures using molecular dynamics (MD) simulations. The small chains of these n-alkanes are clearly /6 >1, which leads to the conclusion that the liquid n-alkanes over the whole temperatures considered are far away from the Rouse regime. Calculated viscosity η and self-diffusion constants D are comparable with experimental results and the temperature dependence of both η and D is suitably described by the Arrhenius plot. The behavior of both activation energies, E η and E D , with increasing chain length indicates that the activation energies approach asymptotic values as n increases to the higher value, which is experimentally observed. Two calculated monomeric friction constants ζ and ζ D give a correct qualitative trend: decrease with increasing temperature and increase with increasing chain length n. Comparison of the time auto-correlation functions of the end-to-end vector calculated from the Rouse model for n-dodecane (C 1 2 ) at 273 K and for n-tetratetracontane (C 4 4 ) at 473 K with those extracted directly from our MD simulations confirms that the short chain n-alkanes considered in this study are far away from the Rouse regime.