正构烷烃动力学：与Rouse模型的比较
Dynamics of n-alkanes: Comparison to Rouse Model
摘要：
数值研究了正构烷烃$(6 \le n \le 66)$的自扩散常数D、粘度$η$和平衡结构统计向类Rouse行为的跨越。对于小的n，链是非高斯的，并且均方端到端距离$R^2$大于$R_G^2$，其中$R_g^2$是均方回转半径。随着n的增加，$R^2/R_G^2 \to 6(1+b/n)$，其中b取决于相互作用模型。在恒定密度下，Rouse模型用于独立于扩散常数D和最长弛豫时间$τ_R$提取单体摩擦系数$ζ$和粘度$η$。从D中提取的$ζ_D$几乎与链长无关，而从$τ_R$中获得的$ζ_τ$在小n时远大于$ζ_D$。在非平衡分子动力学模拟中测量的粘度与由$τ_R$确定的$η$的值非常接近，而对于小的n，由D推断的$η$较小。对于$n\agt 60$，$ζ$和$η$的两个估计值与Rouse模型预测的一致。D计算从三个相互作用模型进行了研究，增加$n$和实验数据进行比较。
英文摘要：
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.
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