Department of Energy and Hydrocarbon Chemistry, Kyoto University , Kyoto 615-8510, Japan.
Environment and Resource System Engineering, Kyoto University , Kyoto 615-8540, Japan.
Langmuir. 2017 Apr 18;33(15):3892-3899. doi: 10.1021/acs.langmuir.7b00436. Epub 2017 Apr 7.
Based on molecular dynamics simulations of eight ions (Na, K, Rb, Cs, Mg, Ca, Sr, and Ba) on muscovite mica surfaces in water, we demonstrate that experimental data on the muscovite mica surface can be rationalized through a unified picture of adsorption structures including the hydration structure, cation heights from the muscovite surface, and state stability. These simulations enable us to categorize the inner-sphere surface complex into two different species: an inner-sphere surface complex in a ditrigonal cavity (IS1) and that on top of Al (IS2). By considering the presence of the two inner-sphere surface complexes, the experimental finding that the heights of adsorbed cations from the muscovite surface are proportional to the ionic radius for K and Cs but inversely proportional to the ionic radius for Ca and Ba was explained. We find that Na, Ca, Sr, and Ba can form both IS1 and IS2; K, Rb, and Cs can form only IS1; and Mg can form only IS2. It is suggested that the formation of IS1 and IS2 is governed by the charge density of the ions. Among the eight ions, we also find that the hydration structure for the outer-sphere surface complexes of divalent cations differs from that of the monovalent cations by one adsorbed water molecule (i.e., a water molecule located in a ditrigonal cavity).
基于对八离子（Na、K、Rb、Cs、Mg、Ca、Sr 和 Ba）在水云母表面的分子动力学模拟，我们证明了实验数据可以通过包括水合结构、阳离子从云母表面的高度以及状态稳定性的吸附结构的统一图像来合理化。这些模拟使我们能够将内球表面络合物分为两种不同的物种：位于三角双锥腔中的内球表面络合物（IS1）和位于 Al 上的内球表面络合物（IS2）。通过考虑两种内球表面络合物的存在，可以解释实验发现，吸附阳离子从云母表面的高度与 K 和 Cs 的离子半径成正比，而与 Ca 和 Ba 的离子半径成反比。我们发现，Na、Ca、Sr 和 Ba 可以形成 IS1 和 IS2；K、Rb 和 Cs 只能形成 IS1；而 Mg 只能形成 IS2。据推测，IS1 和 IS2 的形成受离子电荷密度的控制。在这八个离子中，我们还发现，二价阳离子的外球表面络合物的水合结构与单价阳离子的水合结构不同，相差一个吸附水分子（即位于三角双锥腔中的水分子）。
