J Phys Chem B. 2019 Aug 8;123(31):6882-6891. doi: 10.1021/acs.jpcb.9b04534. Epub 2019 Jul 26.
We present the results of the atomistic molecular dynamics modeling of different protonation states of Nafion at varying hydration levels. Previous experiments have shown that the degree of deprotonation (DDP) of the sulfonic acid groups in a Nafion membrane varies significantly upon hydration. Our goal is to provide insights into the effects of variable protonation states and water content on the internal structure and vehicular transport inside the Nafion membrane. The Nafion side chain lengths showed a weak increasing trend with increasing DDP at all hydration levels, exposing more of the sulfonic acid groups to the hydrophilic/water phase. The water-phase characteristic size/diameter decreased with increasing DDP, but, interestingly, the average number of water molecules per cluster increased. The probability of water-hydronium hydrogen bond formation decreased with increasing DDP, despite an increase in the total number of such hydrogen bonds. The water diffusion was largely unaffected by the state of deprotonation. In contrast to that, the hydronium ion diffusion slowed down with increasing DDP in the overall membrane. The hydronium ion residence times around the sulfonic acid group increased with increasing DDP. Our simulations show a strong connection between the morphology of the water domains and protonation states of Nafion. Such a connection can also be expected in polyelectrolyte membranes similar to Nafion.
我们呈现了不同质子化状态的 Nafion 在不同水合水平下的原子分子动力学模拟结果。先前的实验表明，Nafion 膜中磺酸基团的离解度（DDP）在水合作用下会发生显著变化。我们的目标是深入了解可变质子化状态和含水量对 Nafion 膜内部结构和车辆传输的影响。在所有水合水平下，随着 DDP 的增加，Nafion 侧链长度呈现出微弱的增加趋势，使更多的磺酸基团暴露于亲水性/水相。水相特征尺寸/直径随 DDP 的增加而减小，但有趣的是，每个簇的平均水分子数增加。尽管氢键总数增加，但水合氢离子形成氢键的概率随 DDP 的增加而降低。尽管如此，水的扩散受离解度的影响不大。相比之下，在整个膜中，随着 DDP 的增加，氢离子的扩散速度会减慢。磺酸基团周围的氢离子停留时间随 DDP 的增加而增加。我们的模拟表明，水域的形态与 Nafion 的质子化状态之间存在很强的联系。在类似于 Nafion 的聚电解质膜中也可以预期到这种联系。
