Characterization of molecular diffusion in electrolyte systems

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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.