Molecular Mechanics And Molecular Dynamics Simulation Studies Of Interlayer Structure In Montmorillonites

Montmorillonite has been widely used in the industrial and agricultural production and scientific research application because of its excellent performances. The swelling properties and the hydration mechanism of interlayer cations are the researchful focus at home and abroad. Computer molecular simulation is a powerful tool combining the function of theoretical research with experimental characterization, which bases on the basal principle of statistical mechanics and characterizes the microcosmic structures and calculate the macrograph properties of molecules. Molecular simulations of interlayer structure in montmorillonite helps to fully understand the interlayer structure and particle behavior from threedimensional structure and molecular level, and makes the research of montmorillonite to reach a new high degree.On basis of previous research, a series of 15 superlattice models of L i-, N a-,and K -montmorillonites i n which w ater molecules w as increased systematically from 0 to 96 were established from Cerius in this thesis. Potential energy, bond energy and non-bond energy of the minimum-energy conformation were obtained by molecular force field method (UFF). At the same time, the minimum-energy conformation obtained from the force-field calculations was used to simulate the X-ray diffraction and IR spectrum of the three kinds of montmorillonites in which water content were 32, 64 and 96 molecules, the results demonstrated that these models were feasible.The minimum-energy conformations calculated by molecular mechanics indicated that the interactions of Coulumb's force and angle torsion were important to the formation and stabilization of montmorillonite structure; and layer spacing increased with water content while density decreased in the minimum-energy conformations o f L i-, N a-, and K -montmorillonite, which r epeated the swelling property of montmorillonite in nature.The minimum-energy conformation obtained from the force-field calculations was used as initial configurations for molecular dynamics simulation, the interlayer structure and position of Li-, Na-, and K-montmorillonite were simulated, in which water content are 32, 64, 96 molecules. The results showed that the mobility and position of interlayer cations and water molecules are different in the montmorillonites. The position of cations related to the charge sites of tetrahedron and octahedron; self-diffusion coefficient calculated by mean square displacement showed that the diffusion speed of cations and water molecules in montmorillonites was slower than that in the solution and water, which indicated the particles in the interlayers of montmorillonites were affected by charge sites and restricted by interlayer structure; the analysis of radial distribution function indicated that the hydration ability of cations decreased with their radius enlargement, and the structure of interlayer water molecules was different from that in the bulk water, which illustrated that the solvation of interlayer cations played an important role on the organization of water molecules, and the effect was different for different cations.