Much research has focused on the stability of substituted ammonium salts in anion-exchange membranes (AEMs). While cation chemistry dictates AEM stability, chemical degradation has been recently shown to be significantly influenced by the hydration level at which the AEM operates. At low hydration, it is now known that almost every quaternary ammonium may suffer significant decomposition. In this work, we use molecular dynamics simulations to explore the behavior of three common quaternary ammonium cations with stoichiometric hydroxide concentration and at very low hydration. We find that water preferentially solvates hydroxide anions and hence when water is present in sufficient amount (more than four water molecules per ion pair), stability of the cations is expected to significantly improve. However, lower amounts of water result in the formation of isolated molecular clusters and ammonium hydroxide pairing that lead to degradation of the cation. The composition and size of the water–hydroxide–cation clusters that form is shown to be significantly affected by the cation chemistry. Implications of the observed behavior are discussed in view of recent experimental results on the differences in stability of these cations. This study highlights, for the first time, the crucial importance of studying hydroxide–water–cation interactions at low hydration levels.

中文翻译：

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低水合水平下季铵溶液的分子模拟

许多研究都集中在阴离子交换膜（AEM）中取代铵盐的稳定性上。尽管阳离子化学决定了AEM的稳定性，但最近已证明化学降解受AEM运行的水合水平的影响很大。现在已知在低水合作用下，几乎每种季铵盐都可能遭受明显的分解。在这项工作中，我们使用分子动力学模拟来探索三种常见的季铵阳离子在化学计量的氢氧化物浓度和非常低的水合作用下的行为。我们发现水优先使氢氧根阴离子溶剂化，因此，当水以足够的量存在（每个离子对有四个以上的水分子）时，阳离子的稳定性有望大大提高。然而，较低的水量导致形成孤立的分子簇和氢氧化铵对，从而导致阳离子降解。所形成的水-氢氧根-阳离子簇的组成和大小受到阳离子化学性质的显着影响。鉴于最近关于这些阳离子稳定性差异的实验结果，对观察到的行为的含义进行了讨论。这项研究首次强调了在低水合水平下研究氢氧化物-水-阳离子相互作用的至关重要性。鉴于最近关于这些阳离子稳定性差异的实验结果，对观察到的行为的含义进行了讨论。这项研究首次强调了在低水合水平下研究氢氧化物-水-阳离子相互作用的至关重要性。鉴于最近关于这些阳离子稳定性差异的实验结果，对观察到的行为的含义进行了讨论。这项研究首次强调了在低水合水平下研究氢氧化物-水-阳离子相互作用的至关重要性。