Dynamics of n-alkanes: Comparison to Rouse model

The crossover to Rouse-type 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〈RG2〉, where 〈RG2〉 is the mean squared radius of gyration. As n increases, 〈R2〉/〈RG2〉→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 nonequilibrium 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.