Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, United States.
J Phys Chem B. 2021 Jan 14;125(1):350-359. doi: 10.1021/acs.jpcb.0c09965. Epub 2020 Dec 31.
The effect of ions on the properties of aqueous solutions is often categorized in terms of the Hofmeister series that ranks them from chaotropes ("structure-breakers"), which weaken the surrounding hydrogen-bond network to kosmotropes ("structure-makers"), which enhance it. Here, we investigate the Hofmeister series in ∼1 M sodium-halide solutions using molecular dynamics simulations to calculate the effect of the identity and proximity of the halide anion on both the water diffusion coefficient and its activation energy. A recently developed method for calculating the activation energy from a single-temperature simulation is used, which also permits a rigorous decomposition into contributions from different interactions and motions. The mechanisms of the salt effects on the water dynamics are explored by separately considering water molecules based on their location relative to the ions. The results show that water diffusion is accelerated moving down the halide group from F to I. The behavior of the diffusion activation energy, , is more complex, indicating a significant role for entropic effects. However, water molecules in the first or second solvation shell of an ion exhibit a decrease in moving down the halide series and Na exhibits a larger effect than any of the anions. The for water molecules within the second solvation shell of an ion are modest, indicating a short-ranged nature of the ion influence.
离子对水溶液性质的影响通常可以根据豪夫迈斯特序列进行分类，该序列将它们分为离质体（“破坏氢键网络的物质”）和同质体（“增强氢键网络的物质”）。在这里，我们使用分子动力学模拟研究了约 1 M 氯化钠溶液中的豪夫迈斯特序列，以计算卤化物阴离子的身份和位置对水扩散系数及其活化能的影响。我们使用了一种最近开发的从单温模拟计算活化能的方法，该方法还允许对来自不同相互作用和运动的贡献进行严格分解。通过分别考虑根据其与离子的相对位置定位的水分子，我们探索了盐对水动力学的影响机制。结果表明，从 F 到 I，水的扩散沿着卤化物基团加速。扩散活化能的行为更为复杂，表明熵效应的重要性。然而，处于离子第一或第二溶剂化壳层内的水分子的 随着卤化物系列的下降而降低，并且 Na 比任何阴离子的影响都大。处于离子第二溶剂化壳层内的水分子的 适中，表明离子影响的短程性质。
