Comparison of single-ion molecular dynamics in common solvents

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Ion mobility in solutions is crucial for electrochemical applications.
• Existing theories often lack molecular specificity for complex ion-solvent interactions.
• Understanding ion dynamics requires detailed analysis of velocity autocorrelation functions (VACFs) and memory functions.

Purpose of the Study:

• To establish a foundation for a molecularly specific theory of ion mobilities.
• To conduct a comprehensive survey of VACFs for Li+ and PF6- ions in diverse solvents.
• To analyze the memory function, γ(t), which governs ion mobility, across different conditions.

Main Methods:

• Simulation and analysis of velocity autocorrelation functions (VACFs).
• Extraction and characterization of the memory function, γ(t).
• Comparative analysis controlling for electrolyte concentration, ion-pairing, van der Waals interactions, and solvent properties.

Main Results:

• Hexafluorophosphate (PF6-) ion exhibited consistent velocity relaxations, including negative tail relaxations in VACF and a distinct second relaxation in γ(t).
• Lithium (Li+) ion showed short-time oscillatory behavior masking longer-time γ(t) relaxation, except for the Li+(H2O)4 cluster.
• The Li+(H2O)4 cluster's dynamics aligned with the general behavior observed for PF6- ions.

Conclusions:

• Ion dynamics, particularly relaxation mechanisms, differ significantly between light and heavy ions in solution.
• Solvent structure and ion-solvent interactions play a critical role in determining ion mobility.
• The findings provide essential data for refining molecular theories of ion transport in practical electrolyte systems.