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-12, C-20, C-32, and C-44) in a canonical ensemble at several temperatures using molecular dynamics (MD) simulations. The small chains of these n-alkanes are clearly <Ree(2)>/6 <R-g(2)>>(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 eta and self-diffusion constants D are comparable with experimental results and the temperature dependence of both 17 and D is suitably described by the Arrhenius plot. The behavior of both activation energies, E-eta 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 zeta and zeta(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-12) at 273 K and for n-tetratetracontane (C-44) 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.