PASTEUR, Département de chimie , École normale supérieure, PSL University, Sorbonne Université, CNRS , 75005 Paris , France.
CNRS Laboratoire de Biochimie Théorique , Institut de Biologie Physico-Chimique, PSL University, Sorbonne Paris Cité , 13 rue Pierre et Marie Curie , 75005 Paris , France.
J Phys Chem B. 2019 Apr 18;123(15):3312-3324. doi: 10.1021/acs.jpcb.9b01053. Epub 2019 Apr 9.
Aqueous ionic solutions are ubiquitous in chemistry and in biology. Experiments show that ions affect water dynamics, but a full understanding of several questions remains needed: why some salts accelerate water dynamics while others slow it down, why the effect of a given salt can be concentration-dependent, whether the effect of ions is rather local or more global. Numerical simulations are particularly suited to disentangle these different effects, but current force fields suffer from limitations and often lead to a poor description of dynamics in several aqueous salt solutions. Here, we develop an improved classical force field for the description of alkali halides that yields dynamics in excellent agreement with experimental measurements for water reorientational and translational dynamics. These simulations are analyzed with an extended jump model, which allows to compare the effects of ions on local hydrogen-bond exchange dynamics and on more global properties like viscosity. Our results unambiguously show that the ion-induced changes in water dynamics are usually mostly due to a local effect on the hydrogen-bond exchange dynamics; in contrast, the change in viscosity leads to a smaller effect, which governs the retardation only for a minority of salts and at high concentrations. We finally show how the respective importance of these two effects can be directly determined from experimental measurements alone, thus providing guidelines for the selection of an electrolyte with specific dynamical properties.
水相离子溶液在化学和生物学中无处不在。实验表明离子会影响水的动力学性质，但仍有几个问题需要深入理解：为什么有些盐会加速水的动力学过程，而有些盐则会减缓它；为什么给定盐的影响可能取决于浓度；离子的影响是局部的还是全局性的。数值模拟特别适合于揭示这些不同的影响，但目前的力场存在局限性，并且经常导致对几种水盐溶液动力学的描述不佳。在这里，我们开发了一种改进的经典力场，用于描述碱金属卤化物，该力场能够极好地描述水的重新取向和平移动力学的实验测量结果。这些模拟结果用扩展跳跃模型进行了分析，该模型允许比较离子对局部氢键交换动力学和更全局性质（如粘度）的影响。我们的结果明确表明，离子对水动力学的影响通常主要是由于氢键交换动力学的局部效应；相比之下，粘度的变化导致较小的影响，仅在少数盐和高浓度下会导致动力学的延迟。我们最后还展示了如何仅从实验测量就可以直接确定这两种效应的相对重要性，从而为选择具有特定动力学性质的电解质提供了指导。
