蒙脱石层间阳离子交换的分子模拟  

Molecular simulation of interlayer cation exchange of montmorillonite

作　　者：李勤 陆现彩[1] 张立虎[1] 程永贤 刘鑫[1] Li Qin;Lu Xiancai;Zhang Lihu;Cheng Yongxian;Liu Xin(State Key Laboratory for Mineral Deposits Research,School of Earth Sciences and Engineering,Nanjing University,Nanjing,210023,China)

机构地区：[1]内生金属矿床成矿机制研究国家重点实验室南京大学地球科学与工程学院

出　　处：《南京大学学报（自然科学版）》2019年第6期879-887,共9页Journal of Nanjing University（Natural Science）

基　　金：国家自然科学基金(41425009)

摘　　要：蒙脱石是地表最为常见的一种粘土矿物,层间阳离子交换是蒙脱石矿物的基本属性之一,并因此而成为被广泛应用的矿物材料.通过研究钠基蒙脱石(两层水状态)的阳离子交换特征,在分子层次上探究了K^+,Mg^2+,Ca^2+,Ba^2+从环境溶液进入蒙脱石层间并与Na+离子的交换行为.模拟结果显示不同离子交换量顺序为:Ba^2+>Ca^2+>K^+>Mg^2+,即水化能力弱的阳离子更容易和层间Na^+离子发生交换;离子的水化能力还与其在蒙脱石层间的空间分布密切相关,水化能力最强的Mg^2+离子远离蒙脱石表面而倾向于出现在层间区域的中间,Ca2+与Ba^2+离子则部分出现在结构层表面四面体取代位置;K+离子的分布具有特殊性,被紧缚在硅氧烷六元环中央空穴处.阳离子进入蒙脱石层间是自由能下降的过程,进入层间的阳离子活动性远低于孔隙流体中的离子,其中Ba^2+离子的自扩散系数最低.本文在原子层次上揭示的蒙脱石阳离子交换动力学过程有助于加深对粘土矿物-流体相互作用机制的理解.Montmorillonite is the most commonly distributed clay mineral in earth surface system.Exchangeability of interlayer cation is one of its characteristic properties,which makes it an important natural materials for various applications.Different cations(K^+,Mg^2+,Ca2+,Ba^2+)exchange from montmorillonite interparticle pore fluid into clay interlayer space has been studied at atomistic level by using classical molecular dynamics simulation.The final cation exchange capacities follow the order of Ba^2+>Ca2+>K+>Mg^2+,as opposed to the hydration ability of cations.In the montmorillonite interlayer,Mg^2+ is absolutely hydrated by waters and far away from the montmorillonite surface.Parts of Ca2+ and Ba^2+ ions may closely hover above the tetrahedral substitution position of the clay,however,most K+ions are bound by the six-membered ring of the clay surface.The migration of cation from fluid to clay interlayer space corresponds a process with decrease in free energy.The mobility of interlayer cations is much lower than that in fluid,especially the self-diffusion of Ba^2+ is the lowest.The disclosed dynamic process of cation exchange of montmorillonite at atomistic level will enhance the understanding of clay-fluid interactions.

关 键 词：蒙脱石 阳离子交换 水化能力 活动性 

分 类 号：P574[天文地球—矿物学]