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Magic-number water molecules speed up the hydrated ions at interfaces
江颖、王恩哥等在Nature发文揭示水合离子的微观结构和幻数效应

It is well known that salts will dissolve in the water and the water molecules are bounded with the dissolved ions, forming hydrated ions (Figure 1). The existence of the hydrated ions has already been realized at the end of 19 century. To date, the first experimental trial to confirm this was done by German chemist Walther Nernst (Nobel Laureate, 1920) and his students using so-called transference experiments. For the following more than one hundred year, massive experiments and theoretical calculations have been carried out to understand the microscopic structure and dynamics of hydrated ions. However, many key issues are still under debate so far, such as the water number and configuration in the hydration shells, the effect of hydrated ions on the water structure and dynamics, the microscopic factors that govern the transport of the hydrated ions, and so on. The situation becomes even more complicated when the interfaces are introduced, due to the competing interaction among ions, water and surfaces. The main reason lies in the lack of experimental tools, which can really 'see' and 'manipulate' the hydrated ions with atomic precision.

 

Figure 1 NaCl is dissolved in water. The water molecules are bounded with the dissolved ions, forming hydrated ions.

 

Now, the teams led by Prof. Ying Jiang, Prof. Limei Xu and Prof. Enge Wang of International Center for Quantum Materials (ICQM) of Peking University, in collaboration with Prof. Yiqin Gao of College of Chemistry and Molecular Engineering of Peking University and Dr. Pavel Jelínek from Institute of Physics, the Czech Academy of Sciences, unravel, for the first time, the microscopic structures of Na+ ion hydrates on the NaCl surface and discover a magic-number effect on the transport of ion hydrate, through a combined study using scanning probe microscope (SPM), density functional theory (DFT) calculations and molecular dynamics (MD) simulations. This work is published in Natureon May 14, 2018.

The first challenge of this work is to prepare single ion hydrates on a surface, with tunable number of water molecules. To this end, the researchers figured out a novel method to manipulate individual ions and water molecules by scanning tunneling microscopy (STM). They were able to construct individual Na+ hydrates containing one-to-five water molecules on a NaCl(001) surface, which paves the way for high-resolution imaging of the ion hydrates. Another challenge is to avoid the disturbance of the scanning probes on the ion hydrates, which are highly fragile and flexible. To overcome this difficulty, the researchers developed a weakly-perturbative imaging technique, which relies on the weak high-order electrostatic force by noncontact atomic force microscopy (AFM). Such a technique yields the first-ever atomically resolved images of the Na+ hydrates, in direct comparison with DFT calculations and simulations (Figure 2).

 

Figure 2 The atomically resolved images of different Na+ ion hydrates. From the left to right column: atomic model (Geometry), scanning tunneling microscopy (STM), atomic force microscopy (AFM) and AFM simulation. Image size: 1.5 nm ×1.5 nm.

 

Furthermore, the researchers compared the mobility of different ion hydrates in a well-controlled manner via the inelastic electron tunneling technique. They found an interesting magic-number effect: the Na+ hydrated with three water molecules diffuses one to two orders of magnitude faster than other Na+ hydrates and even much faster than the Na+ in dilute bulk solution (Figure 3). Ab initio calculations and MD simulations revealed that such high ion mobility arises from the degree of the symmetry match between the hydrate and substrate. The magic-number effect applies within a wide range of temperatures (up to room temperature) according to the classical MD simulations. Besides, they found that the magic-number effect applies for many salt ions, suggesting its generality.

 

Figure 3 Magic number effect in the transport of ion hydrates. a, Schematic showing that the Na+ hydrated with three water molecules diffuses much faster than other Na+ hydrates. b, Mean Square Displacement (MSD) in 1 ns of Na+•nH2O (n = 1–5) between 225 K and 300 K.

 

This work established, for the first time, direct correlation between the atomic structure and transport mechanism of hydrated ions, which may completely renovate the traditional understanding of ion transport in nanofluidic systems. In addition, those results point out a new way to control the ion transport in nanofluidic systems by interfacial symmetry engineering, which is of great importance for an extremely wide range of technologically and biologically relevant processes, including corrosion, water desalination, electrochemistry, and biological ion channel, etc.. The techniques developed in this work can be easily extended to different ions and other hydration systems, opening up the possibility of studying various hydration processes down to atomic scale.

Dr. Jinbo Peng (now a Humboldt fellow), Duanyun Cao of International Center for Quantum Materials (ICQM) of Peking University and Zhili He of College of Chemistry and Molecular Engineering of Peking University are the co-first authors of this work. Prof. Ying Jiang (SPM), Prof. Enge Wang (DFT), Prof. Yiqin Gao (MD) and Prof. Limei Xu (DFT) are the co-corresponding authors.This work received supports from National Natural Science Foundation of China, Ministry of Science and Technology of China, Chinese Academy of Sciences, and Collaborative Innovation Center of Quantum Matter.

 

Paper link:J. Peng, D. Cao, Z. He, J. Guo, P. Hapala, R. Ma, B. Cheng, J. Chen, W.-J. Xie, X.-Z. Li, P. Jelínek, L.-M. Xu*, Y.-Q. Gao*, E.-G Wang*, Y. Jiang*, 'The effect of hydration number on the interfacial transport of sodium ions', Nature, DOI: 10.1038/s41586-018-0122-2 (2018) (https://doi.org/10.1038/s41586-018-0122-2)

近日,北京大学量子材料科学中心江颖课题组、徐莉梅课题组、北京大学化学与分子工程学院高毅勤课题组与北京大学/中国科学院王恩哥课题组合作,继2014年获得世界首张亚分子级分辨的水分子图像后,再次取得突破,首次得到了水合钠离子的原子级分辨图像,并发现了一种水合离子输运的幻数效应。该工作以“The effect of hydration number on the interfacial transport of sodium ions”为题于5月14日发表在国际顶级学术期刊《自然》上。

图1 水分子使氯化钠(NaCl)溶解形成离子水合物

 

众所周知,盐放入水中会发生溶解,溶解的离子与水分子结合在一起形成的团簇称为水合离子或离子水合物(图1)。水合离子的微观结构和动力学一直是学术界争论的焦点。早在19世纪末,人们就意识到离子水合作用的存在并开始了系统的研究,最早的实验研究可以追溯到1900年德国著名物理化学家Walther Nernst的迁移实验(Transference experiments)。虽然经过了一百多年的努力,离子的水合壳层数、各个水合层中水分子的数目和构型、水合离子对水氢键结构的影响、决定水合离子输运性质的微观因素等诸多问题,至今仍没有定论。究其原因,关键在于缺乏原子尺度的实验表征手段,以及精准可靠的计算模拟方法。传统的谱学和衍射技术空间分辨能力较差,只能得到平均效应,无法探测局域环境的影响,实验数据的解释异常困难,甚至得出完全矛盾的结论,因此受到很大的限制。另一方面,由于水分子具有全量子化效应,且水分子与离子相互作用也非常微弱,这对理论计算也是巨大的挑战。

图2 钠离子水合物的原子级分辨成像。从左至右,依次为五种离子水合物的原子结构图、扫描隧道显微镜图、原子力显微镜图和原子力成像模拟图。图像尺寸:1.5 nm ×1.5 nm。

 

为了突破实验上的瓶颈,研究人员基于扫描隧道显微镜发展了一套独特的离子操控技术,在氯化钠表面上可控的制备出了单个水合钠离子,水分子的数目精确可调,为高分辨成像创造了条件。在此基础上,他们利用之前发展起来的非侵扰式原子力显微镜成像技术,依靠及其微弱的高阶静电力,克服了针尖对弱键合水合离子的扰动并首次实现了原子级分辨表征,精确确定了其微观吸附构型(图2)。这也是水合离子的概念提出一百多年来,首次在实验中直接“看到”水合离子的原子级图像。

进一步,研究人员利用带电的针尖作为电极,控制单个水合离子在氯化钠表面上的定向输运,发现了一种有趣的幻数效应:包含有特定数目水分子的钠离子水合物具有异常高的扩散能力,迁移率比其他水合物要高1-2个量级,甚至远高于体相离子的迁移率(图3)。结合第一性原理计算和经典分子动力学模拟,他们发现这种幻数效应来源于离子水合物与表面晶格的对称性匹配程度,而且可以在很大一个温度范围内存在(包括室温)。此外,研究人员还发现这种幻数效应具有一定的普适性,适用于相当一部分盐离子体系。

图3 钠离子水合物在NaCl表面输运的幻数效应。a,效果图:包含3个水分子的水合物具有异常强的扩散能力。b,分子动力学模拟得到的不同离子水合物在225K-300K下1ns时间内扩散的均方位移。

 

水溶液中的离子输运研究长期以来都是基于连续介质模型,而忽略了离子与水相互作用以及离子水合物和界面相互作用的微观细节。该工作首次建立了离子水合物的微观结构和输运性质之间的直接关联,刷新了人们对于受限体系中离子输运的传统认识。该项研究的结果表明,可以通过改变表面晶格的对称性和周期性来控制受限环境或纳米流体中离子的输运,从而达到选择性增强或减弱某种离子输运能力的目的,这对很多相关的应用领域都具有重要的潜在意义,比如:离子电池、防腐蚀、电化学反应、海水淡化、生物离子通道等等。此外,该工作发展的实验技术也首次将水合相互作用的研究精度推向了原子层次,未来有望应用到更多更广泛的水合物体系,开辟全新的研究领域。

该工作得到了Nature三个不同领域审稿人的一致好评和欣赏(Overall, I enjoyed reading this manuscript),认为该工作“会马上引起理论和应用表面科学领域的广泛兴趣”(The results presented in this manuscript are of immediate interest to the communities dealing with theoretical and applied surface science),“为在纳米尺度控制表面上的水合离子输运提供了新的途径并可以拓展到其他水合体系”(This result may open a venue for controlling diffusion transport on nano-engineered crystal surfaces and it may be also extended to other hydration systems)。

北京大学量子材料科学中心江颖课题组2012级博士生彭金波(扫描探针实验,现德国洪堡学者)、北京大学/中国科学院王恩哥课题组2015级博士生曹端云(第一性原理计算和模拟)和北京大学化学与分子工程学院高毅勤课题组2013级博士生何智力(经典分子动力学)是文章的共同第一作者,江颖教授、王恩哥院士、高毅勤教授和徐莉梅教授为文章的共同通讯作者。这项工作得到了国家自然科学基金委、科技部、中科院和量子物质科学协同创新中心的经费支持。

 

论文链接:J. Peng, D. Cao, Z. He, J. Guo, P. Hapala, R. Ma, B. Cheng, J. Chen, W.-J. Xie, X.-Z. Li, P. Jelínek, L.-M. Xu*, Y.-Q. Gao*, E.-G Wang*, Y. Jiang*, 'The effect of hydration number on the interfacial transport of sodium ions', Nature, DOI: 10.1038/s41586-018-0122-2 (2018) (https://doi.org/10.1038/s41586-018-0122-2)