DataSheet1_The hydration of Li+ and Mg2+ in subnano carbon nanotubes using a multiscale theoretical approach.docx

The separation of brines with high Mg/Li mass ratios is a huge challenge. To provide a theoretical basis for the design of separation materials, the hydration of Li+ and Mg2+ in confinement using carbon nanotubes (CNTs) as the 1-D nanopore model was investigated using a multiscale theoretical approach. According to the analysis of the first coordination layer of cations, we determined that the coordination shells of two cations exist inside CNTs, while the second coordination shells of the cations are unstable. Moreover, the results of the structure analysis indicate that the hydration layer of Li+ is not complete in CNTs with diameters of 0.73, 0.87, and 1.00 nm. However, this does not occur in the 0.60 nm CNT, which is explained by the formation of contact ion pairs (CIP) between Li+ and Cl− that go through a unstable solvent-shared ion pair [Li(H2O)4]+, and this research was further extended by 400 ns in the 0.60 nm CNT to address the aforementioned results. However, the hydration layer of Mg2+ is complete and not sensitive to the diameter of CNTs using molecular dynamics simulation and an ab initio molecular dynamics (AIMD) method. Furthermore, the results of the orientation distribution of Li+ and Mg2+ indicate that the water molecules around Mg2+ are more ordered than water molecules around Li+ in the CNTs and are more analogous to the bulk solution. We conclude that it is energetically unfavorable to confine Li+ inside the 0.60-nm diameter CNT, while it is favorable for confining Li+ inside the other four CNTs and Mg2+ in all CNTs, which is driven by the strong electrostatic interaction between cations and Cl−. In addition, the interaction between cations and water molecules in the five CNTs was also analyzed from the non-covalent interaction (NCI) perspective by AIMD.

镁锂质量比（Mg/Li mass ratio）偏高的卤水分离是一项亟待攻克的难题。为给分离材料的设计提供理论依据，本研究以碳纳米管（carbon nanotubes, CNTs）作为一维（1-D）纳米孔模型，采用多尺度理论方法探究了受限环境中Li+与Mg2+的水合行为。通过对阳离子第一配位层的分析，我们发现两种阳离子在CNTs内部均存在配位壳层，而阳离子的第二配位层则处于不稳定状态。此外，结构分析结果表明，在管径为0.73、0.87和1.00 nm的CNTs中，Li+的水合层并不完整；但在0.60 nm的CNTs中并未出现该现象，这可通过Li+与Cl-形成接触离子对（contact ion pairs, CIP）来解释——该过程会经历不稳定的溶剂共享离子对[Li(H2O)4]+。针对上述现象，本研究针对0.60 nm管径的CNTs额外开展了400 ns的模拟。通过分子动力学模拟与从头算分子动力学（ab initio molecular dynamics, AIMD）方法的分析可知，Mg2+的水合层完整且不受CNT管径的影响。进一步的取向分布分析结果显示，在CNTs内部，Mg2+周围的水分子比Li+周围的水分子排布更为规整，更接近本体溶液中的水分子状态。本研究得出结论：将Li+限域于0.60 nm管径的CNTs中在能量上是不利的，而将Li+限域于其余四种CNTs以及将Mg2+限域于所有CNTs中则是有利的，这一过程由阳离子与Cl-之间的强静电相互作用驱动。此外，本研究还通过AIMD从非共价相互作用（non-covalent interaction, NCI）的视角分析了五种CNTs中阳离子与水分子之间的相互作用。