水的离子水化及氢键网络缺陷：MgCl $_2$水溶液中第一水化壳层外慢取向水分子的观察
Ion Hydration and Associated Defects in Hydrogen Bond Network of Water: Observation of Reorientationally Slow Water Molecules Beyond First Hydration Shell in Aqueous Solutions of MgCl$_2$
摘要：
通过经典的分子动力学模拟，使用两个众所周知的非极化水模型的离子的存在下，在中等到高浓度，对水分子的动力学性质的影响进行了研究。模拟结果表明，氯化镁（MgCl $_2$）的存在下诱导扰动的氢键网络的水，导致形成散装状域与\textquoteleft缺陷网站\textquoteright-在这样的域的边界上：在这样的缺陷网站的水分子具有比那些在散装水的氢键数量较少。计算了不同离子浓度下缺陷水分子偶极矢量的重取向自相关函数，并与纯水体系进行了比较。早期的实验和模拟研究表明，在许多溶解离子的第一水化壳层中，水分子的重取向动力学存在显着差异。这项研究的结果表明，缺陷水分子，这是超出了第一水合壳的离子，也经历了显着放缓的重定向时间作为浓度的函数的情况下，氯化镁$_2$。然而，向水中加入氯化铯（CsCl）即使在较高浓度下也不会显著干扰水的氢键网络。MgCl $_2$和CsCl之间的这种行为差异与众所周知的Hofmeister系列一致。
英文摘要：
Effects of presence of ions, at moderate to high concentrations, on dynamical properties of water molecules are investigated through classical molecular dynamics simulations using two well known non-polarizable water models. Simulations reveal that the presence of magnesium chloride (MgCl$_2$) induces perturbations in the hydrogen bond network of water leading to the formation of bulk-like domains with \textquoteleft defect sites\textquoteright~on boundaries of such domains: water molecules at such defect sites have less number of hydrogen bonds than those in bulk water. Reorientational autocorrelation functions for dipole vectors of such defect water molecules are computed at different concentrations of ions and compared with system of pure water. Earlier experimental and simulation studies indicate significant differences in reorientational dynamics for water molecules in the first hydration shell of many dissolved ions. Results of this study suggest that defect water molecules, which are beyond the first hydration shells of ions, also experience significant slowing down of reorientation times as a function of concentration in the case of MgCl$_2$. However, addition of cesium chloride(CsCl) to water does not perturb the hydrogen bond network of water significantly even at higher concentrations. This difference in behavior between MgCl$_2$ and CsCl is consistent with the well-known Hofmeister series.
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