Dynamics of n-alkanes: Comparison to Rouse Model

The crossover to Rouse-like behavior for the self-diffusion constant D, the viscosity η, and the equilibrium structural statistics of n-alkanes (6 ≤ n ≤ 66) is studied numerically. For small n the chains are non-Gaussian and the mean squared end-to-end distance 〈R2〉 is greater than 6〈R2 G〉, where 〈R2 G〉 is the mean squared radius of gyration. As n increases, 〈R2〉/〈R2 G〉 → 6(1 + a/n), where a depends on the interaction model. At constant density, the Rouse model is used to extract the monomeric friction coefficient ζ and the viscosity η independently from the diffusion constant D and the longest relaxation time τR. ζD extracted from D is nearly independent of chain length while ζτ obtained from τR is much larger than ζD for small n. The viscosity measured in a non-equilibrium molecular dynamics simulation is closely approximated by the value of η determined from τR while η inferred from D is smaller for small n. For n ∼ 60, the two estimates for both ζ and η agree as predicted from the Rouse model. D calculated from three interaction models is studied for increasing n and compared to experimental data. Typeset using REVTEX