Static and dynamic properties of mid-size liquid n- alkanes, C12∼C400: a molecular dynamics simulation study

Abstract

ABSTRACTIn this paper, we have extended our previous study of the static and dynamic properties (self-diffusion coefficient Dself and friction coefficient ζ) of liquid n-alkane systems up C400 at several temperatures (∼2300 K) using molecular dynamics (MD) simulations in the canonical ensembles. For the small n-alkanes with n ≤ 120 (n: the chain length), the chains are clearly ⟨R2ee⟩/6⟨R2g⟩ ≥ 1 (1.06 ∼ 1.44), which leads to the conclusion that the liquid n-alkanes are far away from the ideal chain regime. But for the n-alkanes of n ≥ 160, the chains are ⟨R2ee⟩/6⟨R2g⟩ ≈ 1, indicating that they are Gaussian. It is found that the long chains of these n-alkanes at high temperatures show abnormalities in density and friction coefficient. We observed a clear transition in the power law dependence of n-alkane self-diffusion coefficient on the molecular weight (M) of n-alkane, Dself ∼ M−γ, occurs in the range C120∼C160 at temperatures of 318, and 618 K, corresponding to a crossover from the ‘oligomer’ to the ‘Rouse’ regime. The entanglement lengths (Ne) are calculated by the Z1 code and discussed shortly.