Roy Susmita, Yashonath Subramanian, Bagchi Biman
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India.
J Chem Phys. 2015 Mar 28;142(12):124502. doi: 10.1063/1.4915274.
A self-consistent mode coupling theory (MCT) with microscopic inputs of equilibrium pair correlation functions is developed to analyze electrolyte dynamics. We apply the theory to calculate concentration dependence of (i) time dependent ion diffusion, (ii) intermediate scattering function of the constituent ions, and (iii) ion solvation dynamics in electrolyte solution. Brownian dynamics with implicit water molecules and molecular dynamics method with explicit water are used to check the theoretical predictions. The time dependence of ionic self-diffusion coefficient and the corresponding intermediate scattering function evaluated from our MCT approach show quantitative agreement with early experimental and present Brownian dynamic simulation results. With increasing concentration, the dispersion of electrolyte friction is found to occur at increasingly higher frequency, due to the faster relaxation of the ion atmosphere. The wave number dependence of intermediate scattering function, F(k, t), exhibits markedly different relaxation dynamics at different length scales. At small wave numbers, we find the emergence of a step-like relaxation, indicating the presence of both fast and slow time scales in the system. Such behavior allows an intriguing analogy with temperature dependent relaxation dynamics of supercooled liquids. We find that solvation dynamics of a tagged ion exhibits a power law decay at long times-the decay can also be fitted to a stretched exponential form. The emergence of the power law in solvation dynamics has been tested by carrying out long Brownian dynamics simulations with varying ionic concentrations. The solvation time correlation and ion-ion intermediate scattering function indeed exhibit highly interesting, non-trivial dynamical behavior at intermediate to longer times that require further experimental and theoretical studies.
我们发展了一种具有平衡对关联函数微观输入的自洽模式耦合理论（MCT）来分析电解质动力学。我们应用该理论来计算以下各项的浓度依赖性：（i）随时间变化的离子扩散，（ii）组成离子的中间散射函数，以及（iii）电解质溶液中的离子溶剂化动力学。使用含隐式水分子的布朗动力学和含显式水的分子动力学方法来检验理论预测。从我们的MCT方法评估得到的离子自扩散系数的时间依赖性以及相应的中间散射函数与早期实验结果和当前的布朗动力学模拟结果在定量上吻合。随着浓度增加，由于离子氛的更快弛豫，发现电解质摩擦的色散出现在越来越高的频率处。中间散射函数F(k, t)的波数依赖性在不同长度尺度上表现出明显不同的弛豫动力学。在小波数下，我们发现出现了阶梯状弛豫，这表明系统中存在快速和慢速时间尺度。这种行为与过冷液体的温度依赖性弛豫动力学存在有趣的类比。我们发现，标记离子的溶剂化动力学在长时间呈现幂律衰减——这种衰减也可以拟合为拉伸指数形式。通过进行不同离子浓度的长时间布朗动力学模拟，检验了溶剂化动力学中幂律的出现。溶剂化时间关联和离子 - 离子中间散射函数在中间到更长时间确实表现出高度有趣、非平凡的动力学行为，这需要进一步的实验和理论研究。
