Heuberger M P, Zachariah Z, Spencer N D, Espinosa-Marzal R M
Laboratory for Advanced Fibers, Empa Materials Science & Technology, St. Gallen, Switzerland.
Phys Chem Chem Phys. 2017 May 31;19(21):13462-13468. doi: 10.1039/c7cp01439b.
Hydrated ions can enter nanometre pores that are smaller than the hydrated ion diameter - the associated dehydration mechanism is still poorly understood. Using an adjustable model slit pore between negatively charged mica surfaces, we have followed the dehydration of highly confined Na counter-ions as a function of salt concentration. We applied external load to the slit pore and resolved the induced sub-nanometre film-thickness transitions, in order to gain information about any structural elements present. At a given concentration, the pull-off force required to reopen the collapsed pore is a sensitive measure for the final hydration state of the confined ions at the interface. Remarkably, we observe a two-step evolution of pull-off force, suggesting two-stage collective ion dehydration. There is a notable coincidence between this process and the occurrence of hydrated-ion layering, as previously observed for K ions, suggesting that a similar mechanism is at work. The gained insights into equilibrium collective ion dehydration in nano pores add to our fundamental understanding of confined electrical double layers. This may be ultimately translated into design criteria for future nano-porous electrode materials and nanofiltration membranes used for water treatment, or electrical-double-layer capacitors.
水合离子能够进入比其水合直径更小的纳米级孔隙——相关的脱水机制仍未得到充分理解。利用带负电荷的云母表面之间可调节的模型狭缝孔隙，我们研究了高度受限的钠离子作为盐浓度函数的脱水过程。我们对狭缝孔隙施加外部负载，并解析了由此引发的亚纳米级膜厚转变，以便获取有关任何存在的结构元素的信息。在给定浓度下，重新打开塌陷孔隙所需的拉脱力是界面处受限离子最终水合状态的灵敏度量。值得注意的是，我们观察到拉脱力呈现两步演变，这表明存在两阶段的集体离子脱水。这一过程与之前观察到的钾离子的水合离子分层现象存在显著巧合，表明类似的机制在起作用。对纳米孔隙中平衡集体离子脱水的深入了解增进了我们对受限电双层的基本认识。这最终可能转化为用于水处理的未来纳米多孔电极材料和纳滤膜或双电层电容器的设计标准。
