Department of Chemical Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden.
Department of Chemistry, Umeå University, Umeå, Sweden.
J Chem Phys. 2017 Aug 28;147(8):084705. doi: 10.1063/1.4992001.
During the last four decades, numerous studies have been directed to the swelling smectite-rich clays in the context of high-level radioactive waste applications and waste-liners for contaminated sites. The swelling properties of clay mineral particles arise due to hydration of the interlayer cations and the diffuse double layers formed near the negatively charged montmorillonite (MMT) surfaces. To accurately study the cation hydration in the interlayer nanopores of MMT, solvent-solute and solvent-clay surface interactions (i.e., the solvation effects and the shape effects) on the atomic level should be taken into account, in contrast to many recent electric double layer based methodologies using continuum models. Therefore, in this research we employed fully atomistic simulations using classical molecular dynamics (MD) simulations, the software package GROMACS along with the CLAYFF forcefield and the SPC/E water model. We present the ion distributions and the deformation of the hydrated coordination structures, i.e., the hydration shells of Na and Ca in the interlayer, respectively, for MMT in the first-layer, the second-layer, the third-layer, the fourth-layer, and the fifth-layer (1W, 2W, 3W, 4W, and 5W) hydrate states. Our MD simulations show that Na in Na-MMT nanopores have an affinity to the ditrigonal cavities of the clay layers and form transient inner-sphere complexes at about 3.8 Å from clay midplane at water contents less than the 5W hydration state. However, these phenomena are not observed in Ca-MMT regardless of swelling states. For Na-MMT, each Na is coordinated to four water molecules and one oxygen atom of the clay basal-plane in the first hydration shell at the 1W hydration state, and with five to six water molecules in the first hydration shell within a radius of 3.1 Å at all higher water contents. In Ca-MMT, however each Ca is coordinated to approximately seven water molecules in the first hydration shell at the 1W hydration state and about eight water molecules in the first hydration shell within a radius of 3.3 Å at all higher hydration states. Moreover, the MD results show that the complete hydration shells are nearly spherical with an orthogonal coordination sphere. They could only be formed when the basal spacing d ≥ 18.7 Å, i.e., approximately, the interlayer separation h ≥ 10 Å. Comparison between DFT and MD simulations shows that DFT failed to reproduce the outer-sphere complexes in the Stern-layer (within ∼5.0 Å from the clay basal-plane), observed in the MD simulations.
在过去的四十年中，大量研究致力于富蒙脱石的层状黏土在高放射性废物应用和污染场地废物衬垫中的膨胀特性。黏土矿物颗粒的膨胀特性源于层间阳离子的水化作用和带负电荷的蒙脱石（MMT）表面附近形成的扩散双电层。为了准确研究 MMT 层间纳米孔中的阳离子水化作用，需要在原子水平上考虑溶剂-溶质和溶剂-黏土表面相互作用（即溶剂化效应和形状效应），而不是许多最近基于双电层的连续体模型方法。因此，在这项研究中，我们采用了完全原子模拟，使用经典分子动力学（MD）模拟，GROMACS 软件包以及 CLAYFF 力场和 SPC/E 水模型。我们分别展示了 MMT 第一层、第二层、第三层、第四层和第五层（1W、2W、3W、4W 和 5W）水合状态下，层间 Na 和 Ca 的离子分布和水化配位结构的变形，即水化壳。我们的 MD 模拟表明，Na-MMT 纳米孔中的 Na 对黏土层的三配位空腔具有亲和力，并在含水量小于 5W 水合状态下，距黏土中间平面约 3.8 Å 处形成瞬态内球络合物。然而，这些现象在 Ca-MMT 中均未观察到，无论其膨胀状态如何。对于 Na-MMT，每个 Na 在 1W 水合状态下与第一层水合状态中黏土基面的四个水分子和一个氧原子配位，并在所有较高含水量下，第一层水合壳内半径为 3.1 Å 的位置上有五到六个水分子。然而，在 Ca-MMT 中，每个 Ca 在第一层水合壳中约有七个水分子配位，在所有较高水合状态下，第一层水合壳内半径为 3.3 Å 的位置上有八个水分子配位。此外，MD 结果表明，完整的水化壳几乎是球形的，具有正交配位球。只有当基面间距 d≥18.7 Å，即层间间距 h≥10 Å 时，才能形成完整的水化壳。与 MD 模拟的比较表明，DFT 未能重现 MD 模拟中观察到的 Stern 层（距黏土基面约 5.0 Å 内）中的外球络合物。
