Characterization of molecular diffusion in electrolyte systems

拟议的研究项目应有助于从根本上理解电解液系统中的扩散传质。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，并用来表征系统选择的体系中的扩散传质。除分子扩散外，电解液中还可能存在团簇扩散。拉曼光谱应该与DLS同时应用，并结合EMD模拟来研究团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分析了处于宏观热力学平衡点的流体中存在的微观浓度波动。这些涨落的时间行为由分子扩散系数和团簇的平移扩散系数决定。通过EMD模拟，可以通过研究纳米尺度上的涨落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，并结合在一起，直接由DLS测量的Fick扩散系数。此外，EMD模拟还可以评估电解液中存在的强大的长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，但这两种方法与拉曼散射的结合提供了对流体结构的近距离了解。应该开发一种灵活的实验装置，包括两种光散射技术，以同时准确地研究不同时间尺度上的扩散系数和潜在的分子相互作用。为了获得有关扩散传质的全面信息，应在广泛的温度和组成范围内研究选定的盐类组合，包括离子液体和有机溶剂。通过系统地改变包括不同分子类型和大小的溶质和溶剂，分析它们的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传质具有单一扩散系数的特点，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模拟获得的可靠的电解液体系扩散系数数据库和从所有采用的方法获得的基本知识应该有助于发展电解液体系中分子扩散的预测工程模型。

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.