Time scale of dynamic heterogeneity in model ionic liquids and its relation to static length scale and charge distribution.
Time scale of dynamic heterogeneity in model ionic liquids and its relation to static length scale and charge distribution.
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DOI：
10.1039/c5cp03390j
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发表时间：
2015-10
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影响因子：
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通讯作者：
Sang-Won Park;Soree Kim;YounJoon Jung
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作者：
Sang-Won Park;Soree Kim;YounJoon Jung
通过对离子液体和非带电离子液体两种不同带电模型的分子动力学模拟，研究了结构驰豫的长度尺度和离子电荷分布对离子液体动力学非均质性的影响。在一种离子液体模型中，阳离子中的电荷分布是不对称的，而在另一种模型中，阳离子中的电荷分布是对称的，而它们的中性对应离子与离子不带电荷。结果表明，所有模型都表现出非均匀动力学，表现为次扩散动力学和结构松弛的非指数衰减。我们通过计算三倍关联函数研究了这些系统中动态异质τ(Dh)的寿命，发现τ(Dh)总体上具有关于结构弛豫时间(τ(α)的幂规律行为，即τ(Dh)∝τ(α)(ζ(Dh))。尽管非对称电荷模型的动力学似乎比对称电荷模型的动力学更不均匀，但在本工作所研究的所有模型中，指数ζ(Dh)≈1.2是相似的。无论有没有库仑相互作用，都存在相同的标度关系，即使当结构弛豫的长度尺度足够长时，它也能变成费克扩散。这说明τ(Dh)是有别于τ(α的时间尺度，其动态非均质性主要受短程相互作用和分子结构的影响。
We study how dynamic heterogeneity in ionic liquids is affected by the length scale of structural relaxation and the ionic charge distribution by the molecular dynamics simulations performed on two differently charged models of ionic liquid and their uncharged counterpart. In one model of ionic liquid, the charge distribution in the cation is asymmetric, and in the other it is symmetric, while their neutral counterpart has no charge with the ions. It is found that all the models display heterogeneous dynamics, exhibiting subdiffusive dynamics and a nonexponential decay of structural relaxation. We investigate the lifetime of dynamic heterogeneity, τ(dh), in these systems by calculating the three-time correlation functions to find that τ(dh) has in general a power-law behavior with respect to the structural relaxation time, τ(α), i.e., τ(dh) ∝ τ(α)(ζ(dh)). Although the dynamics of the asymmetric-charge model is seemingly more heterogeneous than that of the symmetric-charge model, the exponent is found to be similar, ζ(dh) ≈ 1.2, for all the models studied in this work. The same scaling relation is found regardless of interactions, i.e., with or without Coulomb interaction, and it holds even when the length scale of structural relaxation is long enough to become the Fickian diffusion. This fact indicates that τ(dh) is a distinctive time scale from τ(α), and the dynamic heterogeneity is mainly affected by the short-range interaction and the molecular structure.