Molecular Simulation of Quaternary Ammonium Solutions at Low Hydration Levels

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 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.