Molecular Dynamics Simulation of Salt Diffusion in Polyelectrolyte Assemblies
Molecular Dynamics Simulation of Salt Diffusion in Polyelectrolyte Assemblies
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聚电解质组件中盐扩散的分子动力学模拟
DOI：
10.1021/acs.jpcb.8b02644
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发表时间：
2018
影响因子：
3.3
通讯作者：
Shi Tongfei
中科院分区：
文献类型：
--
作者：
Zhang Ran;Duan Xiaozheng;Ding Mingming;Shi Tongfei
盐离子和带电探针分子在聚乙烯（PE）组装体中的扩散通常被假设为遵循理论跳跃模型，其中扩散离子基于电中性在链的带电位点之间跳跃。然而，在这样的微观尺度上的扩散途径的实验验证是困难的，相应的分子机制仍然是难以捉摸的。在这项研究中，我们进行全原子分子动力学模拟盐在聚（4-苯乙烯磺酸钠）（PSS）和聚（二烯丙基二甲基氯化铵）（PDAC）的PE组装。除了离子跳跃模式，扩散轨迹被发现目前的跳跃过程中，即，受到PE松弛，在结构中的水口袋打开和关闭的共同特点，因此，离子可以从一个口袋移动到另一个。由于这些水囊中的捕获场景，观察到离子和水的异常次扩散。与离子跳跃相比，跳跃事件要罕见得多，但随着温度的升高，盐扩散显著增加。我们的研究结果强烈表明，盐在水合PDAC/PSS中的扩散是一个离子跳跃和跳跃运动的组合过程。这为盐运动与链运动的耦合以及玻璃化转变温度下盐扩散的非线性增加提供了一个新的分子解释。
The diffusion of salt ions and charged probe molecules in polyelectrolyte (PE) assemblies is often assumed to follow a theoretical hopping model, in which the diffusing ion hops between charged sites of chains based on electroneutrality. However, experimental verification of diffusing pathway at such microscales is difficult, and the corresponding molecular mechanisms remain elusive. In this study, we perform all-atom molecular dynamics simulations of salt diffusion in the PE assembly of poly(sodium-4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDAC). Besides the ion hopping mode, the diffusing trajectories are found to present common features of a jump process, that is, subjecting to PE relaxation, water pockets in the structure open and close; thus, the ion can move from one pocket to another. Anomalous subdiffusion of ions and water is observed because of the trapping scenarios in these water pockets. The jump events are much rarer compared with ion hopping but significantly increases salt diffusion with increasing temperature. Our result strongly indicates that salt diffusion in hydrated PDAC/PSS is a combined process of ion hopping and jump motion. This provides a new molecular explanation for the coupling of salt motion with chain motion and the nonlinear increase of salt diffusion at glass-transition temperature.