Department of Materials Science and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland.
Langmuir. 2013 Feb 19;29(7):2207-16. doi: 10.1021/la3044736. Epub 2013 Feb 5.
Calcite is among the most abundant minerals on earth and plays a central role in many environmental and geochemical processes. Here we used amplitude modulation atomic force microscopy (AFM) operated in a particular regime to visualize single ions close to the (1014) surface of calcite in solution. The results were acquired at equilibrium, in aqueous solution containing different concentrations of NaCl, RbCl, and CaCl(2). The AFM images provide a direct and atomic-level picture of the different cations adsorbed preferentially at certain locations of the calcite-water interface. Highly ordered water layers at the calcite surface prevent the hydrated ions from directly interacting with calcite due to the energy penalty incurred by the necessary restructuring of the ions' solvation shells. Controlled removal of the adsorbed ions from the interface by the AFM tip provides indications about the stability of the adsorption site. The AFM results show the familiar "row pairing" of the carbonate oxygen atoms, with the adsorbed monovalent cations located adjacent to the most prominent oxygen atoms. The location of adsorbed cations near the surface appears better defined for monovalent ions than for Ca(2+), consistent with the idea that Ca(2+) ions remain further away from the surface of calcite due to their larger hydration shell. The precise distance between the different hydrated ions and the surface of calcite is quantified using MD simulation. The preferential adsorption sites found by MD as well as the ion residence times close to the surface support the AFM findings, with Na(+) ions dwelling substantially longer and closer to the calcite surface than Ca(2+). The results also bring new insights into the problem of the Stern and electrostatic double layer at the surface of calcite, showing that parameters such as the thickness of the Stern layer can be highly ion dependent.
方解石是地球上最丰富的矿物之一，在许多环境和地球化学过程中起着核心作用。在这里，我们使用振幅调制原子力显微镜（AFM）在特定模式下操作，以可视化接近方解石（1014）表面的单个离子在溶液中的情况。结果是在含有不同浓度的 NaCl、RbCl 和 CaCl2 的水溶液中在平衡状态下获得的。AFM 图像提供了不同阳离子在方解石-水界面的某些位置上优先吸附的直接和原子级图像。由于离子溶剂化壳的必要重构会导致能量损失，因此方解石表面的高度有序水层阻止了水合离子与方解石直接相互作用。通过 AFM 针尖从界面上受控地去除吸附离子提供了有关吸附位置稳定性的指示。AFM 结果显示出熟悉的碳酸盐氧原子的“行配对”，吸附的单价阳离子位于最突出的氧原子附近。与 Ca2+相比，吸附阳离子在表面附近的位置似乎对方解石表面附近的单价离子的定义更好，这与 Ca2+离子由于其较大的水合壳而远离方解石表面的想法一致。使用 MD 模拟定量了不同水合离子与方解石表面之间的精确距离。MD 发现的优先吸附位点以及离子在靠近表面的停留时间支持 AFM 的发现，其中 Na+离子在方解石表面附近停留的时间和距离都明显长于 Ca2+。结果还为方解石表面的 Stern 和静电双电层问题提供了新的见解，表明参数（如 Stern 层的厚度）可能高度依赖于离子。
