水化镁基蒙脱石的分子动力学模拟

Molecular Dynamics Simulation of Magnesium-Montmorillonite Hydrates

利用分子动力学 (MD)模拟了 3 0 0K时镁基蒙脱石 (粘土 )层间水和镁离子的结构和动力学性质 .模拟结果显示水在粘土层间分为二层 ,只有一小部分水被粘土表面吸附 ,与粘土结构中的羟基形成氢键 ,不同分布位置的水处于动态平衡 .层间水分子氢键配位数比普通水少 2 4%左右 ,水在粘土中自扩散系数D =5 3 5 5× 10 -10 m2 ·s-1,约为主体相水的 1/4 .镁离子在粘土层间形成一层 ,其与水分子配位数约为 6.进一步讨论了温度对粘土层中水的结构和动力学性质的影响 .随着温度升高 ,水层的局部密度 ρ(z)降低 ,水在XY方向的扩散系数不断增大 .当温度达到 60 0K后 ,层间水分子间的氢键断裂 ,与超临界状态下水的结构相似 ,层间水的扩散系数达最大值 ,温度进一步升至 70 0K时 ,其值基本无变化 .

Molecular dynamics simulation of Mg-montmorillonite (clay) hydrates was performed at T = 300 K in order to elucidate the structure and dynamics properties of interlayer water molecules and magnesium ions. The water molecules are divided into two layers in the clay and only a few water molecules are absorbed on the surface of the clay with hydrogen bonds, and the water molecules at different distribution sites are in dynamic equilibrium. The calculations reveal that the confined water molecules are of 24% fewer H-bond coordination numbers per molecule than the molecules in the bulk liquid water. The simulated self-diffusion coefficient of the interlayer water, D, is 5.355 x 10(-10) m(2) . s(-1), which is about 1/4 of that for the bulk water molecules. It is found that the confined magnesium cations form one layer and the coordination number with water molecules is about 6. The effect of temperature on the structure and dynamic properties of the interlayer water molecules were investigated. The local densities of the water layers decrease, and the self diffusion coefficient of water in the X-Y direction increases with the increase of temperature. When the temperature reaches above 600 K, similar to the supercritical water, the hydrogen bonds between the interlayer water molecules are broken up, and diffusion coefficient reaches a maximum.