锂-, 钠-, 钾水化蒙脱石层间结构的分子动力学模拟

运用分子动力学(molecular dynamics, MD)方法分别研究了含有32, 64和96个水分子的Li-, Na-, K-蒙脱石层间阳离子与水分子的位置和结构. 计算结果表明蒙脱石层间阳离子位置与四面体和八面体电荷位置及离子的大小有关. 一层水合物中可以观察到三种阳离子都能和四面体电荷与八面体电荷位置分别形成内、外配位作用. 二层水合物中, 仍然可以观察到Li^＋和Na^＋与电荷位置的配位作用, 但是已经开始向层中其他方向扩散, 而K^＋仍然在粘土的表面附近. 三层水合物中, Li^＋, Na^＋开始从电荷位置和表面分离, K^＋也开始向层间其他方向扩散. 水分子在所有三种水合物中都分散于层间各个方向. 径向分布函数的分析结果表明层间三种阳离子组织水分子的能力不同, 水化作用随着阳离子半径的增大而减弱; 此外层中水分子的聚合程度随着水分子的增加而加强, 水分子的结构也不同于模拟的液体水分子的结构; 说明蒙脱石层间阳离子的溶剂化作用对水分子的组织起着重要的作用.

Molecular Dynamics Simulation Studies of Interlayered Structure in Lithium-, Sodium- and Potassium-Montmorillonite Hydrate

Molecular structure and position in the interlayers of lithium-, sodium-, and potassium- montmorillonite with 32, 64 and 96 water molecules were investigated by molecular dynamics simulation. Calculated results show that the position of ions in the interlayers of montmorillonite was related with tetra- hedral and octahedral charge sites and ion size. Inner- and outer-sphere surface complexes of all such three cations with tetrahedral and octahedral charge sites were observed in the one-layer hydrates. In the two-layer hydrates, complexes of the Li and Na with charge sites were still observed, but both cations began dif- fuse towards the other direction in the interlayers of montmorillonite, whereas the K still adhered to the charge sites and clay surface. In the three-layer hydrates, the Li and Na became detached from the sur- face and charge sites, while the K were diffused towards the other direction. Water molecules in all three hydrates were spread around all directions in the interlayers of montmorillonite. The analysis results of radial distribution functions indicated that the ability of organizing water molecules were different in three cations, hydration ability was decreased with augmentation of the radius of these cations. Moreover the convergencedegree of water molecules in the interlayers was strengthened with the increment of the number of water molecules, and the structure of water molecules was different from that of simulated water. The solvation of ions was illustrated to be important to the organization of water molecules.