Abstract The semipermeability of clay aggregates in mudstones, shales, and marine sediments gives rise to geological ultrafiltration of ions and water. This behavior has been numerically analyzed based on the theory of the electric double layer (EDL). The model of EDL is suited for describing ionic partitions in clay pore with a width of a few nanometers. However, the interlayer space of smectite might be hardly described by the EDL model. An explicit atomic-scale description is more appropriate for the interlayer space. Besides, clay swelling or collapse corresponding to a variation of environmental conditions, and its impact on partitions of water and ions between interlayers and environment, have not been taken into account numerically. With atomic-scale molecular dynamics simulations and thermodynamic integrations based on them, the coupling between montmorillonite swelling/collapse and partitions of water and cations under different aqueous activity conditions is systematically disclosed here. With the combination of the Horinek’s force field to describe cations and ClayFF force field for atoms in solid layers, consistent swelling/collapse of Na- and K-montmorillonite with experimental observations as a function of water activity is shown. The swelling/collapse is connected to the transition between monolayer and bilayer hydration states. Decomposition of the swelling/collapse free energy shows both energy and entropy contributions are important in determining the thermodynamically stable hydration state. Taking environmental water activity and Na+/K+ ionic activity ratio as variables, a picture of cationic partition in response to those variables is established. It shows, under all circumstances, K+ ions are enriched in montmorillonite, which might explain K+ depletion in pore fluids of sediments. A low water activity improves K+ enrichment in montmorillonite. In addition, cationic partition accompanies variations of hydration states and alters the ionic concentration of the aqueous environment. These results can be applied to explain water and ionic partitions during subsurface water flow and the diagenesis processes of sediments.

中文翻译：

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粘土溶胀/塌陷与阳离子分配之间的耦合

摘要 泥岩、页岩和海相沉积物中粘土聚集体的半渗透性导致离子和水的地质超滤。这种行为已经基于双电层 (EDL) 理论进行了数值分析。EDL 模型适用于描述宽度为几纳米的粘土孔隙中的离子分配。然而，EDL 模型可能很难描述蒙脱石的层间空间。明确的原子尺度描述更适合于层间空间。此外，粘土膨胀或坍塌对应于环境条件的变化，及其对夹层和环境之间水和离子分配的影响，尚未被数值考虑。通过原子尺度的分子动力学模拟和基于它们的热力学积分，在此系统地公开了在不同水活性条件下蒙脱石溶胀/塌陷与水和阳离子的分配之间的耦合。结合描述阳离子的 Horinek 力场和固体层中原子的 ClayFF 力场，显示了 Na- 和 K-蒙脱石的一致膨胀/塌陷以及作为水活性函数的实验观察结果。膨胀/塌陷与单层和双层水合状态之间的转变有关。溶胀/塌陷自由能的分解表明，能量和熵的贡献对于确定热力学稳定的水合状态很重要。以环境水分活度和Na+/K+离子活度比为变量，建立了响应这些变量的阳离子分配图。它表明，在所有情况下，K+ 离子都富含蒙脱石，这可能解释了沉积物孔隙流体中 K+ 的枯竭。低水分活度可提高蒙脱石中 K+ 的含量。此外，阳离子分配伴随水合状态的变化并改变水性环境的离子浓度。这些结果可用于解释地下水流和沉积物成岩过程中的水和离子分配。