Characterization of molecular diffusion in electrolyte systems

提出的研究项目将有助于对电解质系统中弥漫性质量输运的基本理解。为此，应进一步发展动态光散射（DLS）实验和平衡分子动力学（EMD）模拟，并利用它们来表征系统选择系统中的弥漫性质量输运。电解质中除了分子扩散外，还存在团簇扩散。拉曼光谱应与DLS同时应用，并结合EMD模拟来研究簇的形成及其对分子扩散的影响。为了确定质量扩散系数，DLS分析了宏观热力学平衡下流体中存在的微观浓度波动。这些波动的时间行为是由分子扩散系数和团簇的平动扩散系数决定的。通过EMD模拟，可以通过研究纳米尺度上的波动来获得分子扩散系数。在这里，计算麦克斯韦-斯特凡扩散系数和热力学因子并结合起来，得到DLS直接测量的菲克扩散系数。此外，EMD模拟允许评估存在于电解质中的强远程分子间静电相互作用对扩散的影响。虽然DLS结果用于验证EMD模拟，但两种方法与拉曼散射的结合可以更深入地了解流体结构。为了同时准确地研究不同时间尺度上的扩散率和潜在的分子相互作用，需要开发一种灵活的实验装置，包括光散射技术。为了获得关于弥漫性质量输运的全面信息，应该在广泛的温度和组成范围内研究盐（包括离子液体）和有机溶剂的选择组合。通过溶质和溶剂的系统变化，覆盖不同的分子类型和大小，分析其物理特性对扩散过程的影响。虽然三组分二元混合物的扩散输运以单一扩散系数为特征，但本研究项目的研究也应扩展到与二阶扩散系数矩阵相关的四组分体系。此外，从DLS实验和EMD模拟中获得的可靠的电解质系统扩散率数据库以及从所有采用的方法中获得的基础知识将有助于电解质系统中分子扩散的预测工程模型的发展。

The proposed research project should contribute to a fundamental understanding of the diffusive mass transport in electrolyte systems. For this purpose, dynamic light scattering (DLS) experiments and equilibrium molecular dynamics (EMD) simulations should be further developed and used to characterize the diffusive mass transport in systematically selected systems. Besides molecular diffusion, also cluster diffusion can be present in electrolytes. Raman spectroscopy should be applied simultaneously with DLS and combined with EMD simulations to study the formation of clusters and their influence on molecular diffusion. For the determination of mass diffusivities, DLS analyzes microscopic concentration fluctuations present in fluids at macroscopic thermodynamic equilibrium. The temporal behavior of these fluctuations is governed by the molecular diffusion coefficient and the translational diffusion coefficient of clusters. By EMD simulations, the molecular diffusion coefficients can be accessed by investigating fluctuations on a nanometer scale. Here, the Maxwell-Stefan diffusion coefficient and the thermodynamic factor are calculated and combined to access the Fick diffusion coefficients directly measured by DLS. Furthermore, EMD simulations allow to evaluate the influence of the strong long-range intermolecular electrostatic interactions present in electrolytes on diffusion. While the DLS results are used to verify the EMD simulations, the combination of both methods with Raman scattering provides a close insight into the fluid structure. A flexible experimental setup including both light scattering techniques should be developed for the simultaneous and accurate investigation of diffusivities on different time scales and underlying molecular interactions. For gaining comprehensive information on the diffusive mass transport, selected combinations of salts, including ionic liquids, and organic solvents should be studied over broad ranges of temperature and composition. By a systematic variation of the solute and solvent, covering different molecule types and sizes, the influence of their physical characteristics on the diffusive process should be analyzed. While the diffusive mass transport for binary mixtures with three species is characterized by a single diffusion coefficient, the investigations in this research project should also be extended to four-species systems associated with a second-order diffusion coefficient matrix. Furthermore, the reliable database of diffusivities in electrolyte systems obtained from DLS experiments and EMD simulations and the fundamental knowledge gained from all employed methods should contribute to the development of a predictive engineering model for the molecular diffusion in electrolyte systems.