Dynamics of n-alkanes: Comparison to Rouse Model

Abstract

The crossover to Rouse-like behavior for the self-diffusion constant D, the viscosity $η$, and the equilibrium structural statistics of n-alkanes $(6 \le n \le 66)$ is studied numerically. For small n the chains are non-Gaussian and the mean squared end-to-end distance $R^2$ is greater than $R_G^2$, where $R_g^2$ is the mean squared radius of gyration. As n increases, $R^2/R_G^2 \to 6(1+b/n)$, where b 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 $τ_R$. $ζ_D$ extracted from D is nearly independent of chain length while $ζ_τ$ obtained from $τ_R$ is much larger than $ζ_D$ for small n. The viscosity measured in a non-equilibrium molecular dynamics simulation is closely approximated by the value of $η$ determined from $τ_R$ while $η$ inferred from D is smaller for small n. For $n\agt 60$, 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.