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 tetrahedral 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 diffuse 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 surface 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 convergence degree 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.

Key words: molecular dynamics, montmorillonite, radial distribution function