Perturbations to water, both by ions and confining media, have been the focus of numerous experimental and theoretical studies. Yet, several open questions remain, including the extent to which such perturbations modify the structural and dielectric properties of the liquid. Here, we present a first-principles molecular dynamics study of alkali cations in water (Li⁺, Na⁺, and K⁺) as well as of water and LiCl and KCl solutions under confinement within carbon nanotubes (CNTs) of small diameter (1.1–1.5 nm). Our simulations support the view that the water structure is only modified locally in the presence of cations. We found that molecular polarizabilities are fingerprints of hydrogen bonding modifications, which occur at most up to the second solvation shell for all cations in bulk water. Under confinement, we found that the overall value of the molecular polarizability of water molecules near the surface is determined by the balance of two effects, which are quantitatively different in CNTs of different radii: the presence of broken hydrogen bonds at the surface leads to a decrease in the polarizabilities of water molecules, while the interaction with the CNT enhances polarizabilities. Interestingly, the reduction in dipole moments of interfacial water molecules under confinement is instead driven only by changes in the water structure and not by interfacial interactions. As expected, confinement effects on water molecular polarizabilities and dipole moments are more pronounced in the case of the 1.1 nm CNT.

中文翻译：

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分子极化率作为离子和约束作用对水的扰动的指纹

离子和约束介质对水的扰动一直是众多实验和理论研究的重点。然而，仍然存在一些悬而未决的问题，包括这种扰动在多大程度上改变了液体的结构和介电性能。在这里，我们介绍了水中碱性阳离子（Li ⁺，Na ⁺和K ⁺）以及水，LiCl和KCl溶液在较小直径（1.1-1.5 nm）的碳纳米管（CNT）中的限制。我们的模拟支持以下观点：仅在存在阳离子的情况下才对水结构进行局部修饰。我们发现分子极化率是氢键修饰的指纹，氢键修饰最多出现在散装水中所有阳离子的第二溶剂化壳层上。在限制条件下，我们发现水分子在表面附近的分子极化能力的总价值取决于两种作用的平衡，这两种作用在不同半径的CNT中在数量上是不同的：在表面存在断裂的氢键会导致水分子的极化率降低，而与CNT的相互作用增强极化率。有趣的是 相反，限制条件下界面水分子偶极矩的减少仅由水结构的变化而不是界面相互作用驱动。不出所料，在1.1 nm CNT的情况下，对水分子极化率和偶极矩的限制作用更加明显。