Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, USA.
J Phys Chem B. 2012 Oct 4;116(39):12007-13. doi: 10.1021/jp306847t. Epub 2012 Sep 21.
The effect of salt on the dynamics of water molecules follows the Hofmeister series. For some "structure-making" salts, the self-diffusion coefficient of the water molecules, D, decreases with increasing salt concentration. For other "structure-breaking" salts, D increases with increasing salt concentration. In this work, the concentration and temperature dependence of the self-diffusion of water in electrolyte solutions is studied using molecular dynamics simulations and pulsed-field-gradient NMR experiments; temperature-dependent viscosities are also independently measured. Simulations of rigid, nonpolarizable models at room temperature show that none of the many models tested can reproduce the experimentally observed trend for the concentration dependence of D; that is, the models predict that D decreases with increasing salt concentration for both structure-breaking and structure-making salts. Predictions of polarizable models are not in agreement with experiment either. These results suggest that many popular water models do not accurately describe the dynamic nature of the hydrogen bond network of water at room temperature. The simulations are in qualitative agreement, however, with experimental results for the temperature dependence of water dynamics; simulations and experiment show an Arrhenius dependence of D with temperature, T, with added salt, that is, ln D ∼ 1/T, over a range of temperatures above the freezing point of water.
盐对水分子动力学的影响遵循 Hofmeister 序列。对于一些“形成结构”的盐，水分子的自扩散系数 D 随着盐浓度的增加而降低。对于其他“破坏结构”的盐，D 随着盐浓度的增加而增加。在这项工作中，使用分子动力学模拟和脉冲梯度 NMR 实验研究了电解质溶液中水分子的自扩散浓度和温度依赖性；还独立测量了温度依赖性粘度。在室温下对刚性、不可极化模型的模拟表明，在所测试的许多模型中，没有一个模型可以重现 D 随浓度变化的实验观察趋势；也就是说，这些模型预测对于破坏结构和形成结构的盐，D 都随着盐浓度的增加而降低。可极化模型的预测也与实验不符。这些结果表明，许多流行的水模型不能准确描述室温下水的氢键网络的动态性质。然而，模拟结果与水动力学的温度依赖性实验结果定性一致；模拟和实验都表明，在水的冰点以上的一系列温度下，添加盐后，D 随温度 T 的变化呈阿伦尼乌斯依赖性，即 ln D∼1/T。
