Abstract The physicochemical properties of clay minerals strongly depend on their hydration characteristics which therefore have drawn great concerns from different research communities. In the present work, the effects of charge density of montmorillonite (Mt) on the hydration characteristics of its interlayer spaces, particularly the siloxane surface, were studied using classical molecular dynamics (MD) simulations. Four Mt. models with various octahedral charges are established, and these charges are compensated with tetramethylammonium cation (TMA). The simulation results showed that water molecules within the hydration layer of siloxane surface will donate hydrogen atoms to form H-bond with the surface oxygen atoms, while those surrounding TMA only slightly have their oxygen atoms point towards TMA. In addition, water molecules prefer to first hydrate the siloxane surface and then the TMA as the water content increases. These findings indicate that water molecules have stronger hydration interaction with siloxane surface than with TMA, and TMA can be ideal counterbalance cation in terms of studying the hydration characteristics of siloxane surface. Charge density can significantly influence the hydration of TMA-Mt. Although increasing charge density will not lead to the formation of stronger H-bond (i.e., no obvious reduction of H-bond length) between water molecules and siloxane surface, water molecules are more likely to be drawn to the siloxane surface and form more H-bonds between them. Subsequently, the hydration energy increases and the mobility of water molecules decreases as the charge density rises. These findings show that charge density can evidently influence the hydrophobicity of siloxane surface, which may further influence its interaction with organic species, e.g., the adsorption of organic contaminants.

中文翻译：

---

电荷密度对蒙脱石硅氧烷表面水化的影响：分子动力学模拟研究

摘要 粘土矿物的理化性质很大程度上取决于它们的水化特性，因此引起了不同研究界的极大关注。在目前的工作中，使用经典分子动力学 (MD) 模拟研究了蒙脱石 (Mt) 的电荷密度对其层间空间，特别是硅氧烷表面的水合特性的影响。四山 建立了具有各种八面体电荷的模型，并用四甲基铵阳离子 (TMA) 补偿这些电荷。模拟结果表明，硅氧烷表面水化层内的水分子会捐出氢原子与表面氧原子形成H键，而TMA周围的水分子只有轻微的氧原子指向TMA。此外，随着水含量的增加，水分子倾向于先水合硅氧烷表面，然后再水合 TMA。这些发现表明，水分子与硅氧烷表面的水合相互作用比与 TMA 强，在研究硅氧烷表面的水合特性方面，TMA 是理想的平衡阳离子。电荷密度可以显着影响 TMA-Mt 的水合作用。虽然增加电荷密度不会导致水分子与硅氧烷表面形成更强的氢键（即，氢键长度没有明显减少），但水分子更有可能被吸引到硅氧烷表面并形成更多的氢——他们之间的羁绊。随后，随着电荷密度的增加，水合能增加，水分子的迁移率降低。