钾基蒙脱石的分子力学和分子动力学模拟

SIMILUATION OF MOLECULAR MECHANICS AND MOLECULAR DYNAMICS OF POTASSIUM MONTMORILLONITE

利用分子力学(molecular mechanics,MM)和分子动力学(molecular dynamics,MD)模拟了K-蒙脱石层间阳离子和水的结构及动力学特征,并将模拟的X射线衍射结果与实验值进行比较。结果显示:在K-蒙脱石的最优构型中,层间距随着水含量的增加而增大;X射线衍射的模拟结果和实验结果一致;K+的位置和蒙脱石层间电荷位置有关,K+在1,2层水合物中依附于黏土的表面附近,而在3层水合物中K+则脱离表面开始向层中其它方向扩散。水分子在黏土层间分散于各个方向,而且存在水质子和K+竞争电荷位置的现象。水和K+在钾基蒙脱石中的活动性低于其在水中和溶液中的活动性。径向分布函数分析结果表明K+组织水分子的能力有限,说明K+对蒙脱石的水化膨胀起抑制作用。

Structure and dynamics properties of cation and water in the interlayers of potassium montmorillonite were investigated by molecular mechanics （MM） and molecular dynamics （MD） simulation. The MM simulation results show that the layer spacing increases with water content to the minimum-energy conformation of potassium montmofillonite. The X-ray diffraction results are in agreement with the experimental data. In the MD simulations, the position of K^＋ in the interlayers of montmorillonite are related to the charge sites. In the one-and two-layer hydrates, the K^＋ ions adhere to the clay surface, whereas in the three-layer hydrates, the K^＋ ions become detached from the surface and become diffused towards the opposite direction. Water molecules spread around in all directions in the interlayers of clay; moreover, there is competition between the K^＋ ions and the water protons for negatively charged sites. The results of calculated self-diffusion coefficients for K^＋ and water indicate that their mobilities in the interlayer of potassium montmorillonite remain lower than those observed in bulk water and solution. The analysis results of the radial distribution function illustrate that the ability of K^＋ to the organization of water molecules is limited, it shows that the K^＋ is inhibited by the hydration and swelling of montmorillonite.