Dynamics of n-alkanes: Comparison to Rouse Model

Abstract

The crossover to Rouse-like behavior for the self-diffusion constant D, the viscosity $\eta$, 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 $\zeta$ and the viscosity $\eta$ independently from the diffusion constant D and the longest relaxation time ${\tau }_R$. ${\zeta }_D$ extracted from D is nearly independent of chain length while ${\zeta }_{\tau }$ obtained from ${\tau }_R$ is much larger than ${\zeta }_D$ for small n. The viscosity measured in a non-equilibrium molecular dynamics simulation is closely approximated by the value of $\eta$ determined from ${\tau }_R$ while $\eta$ inferred from D is smaller for small n. For $n\agt 60$, the two estimates for both $\zeta$ and $\eta$ agree as predicted from the Rouse model. D calculated from three interaction models is studied for increasing $n$ and compared to experimental data.

21 pages, 8 postscript figures, uses revtex

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