Ion Pairing Controls Physical Properties of Ionic Liquid-Solvent Mixtures

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Room temperature ionic liquids (RTILs) dissolved in organic solvents can improve ion dynamics.
• Previous molecular dynamics (MD) simulations studied the ionic liquid [BMIM+][Tf2N-] in 22 solvents, revealing correlations between ion diffusivity, solvent properties, and conductivity.
• Key findings included monotonic increases in ion diffusivity with RTIL composition and a peak in Nernst-Einstein conductivity at intermediate compositions.

Purpose of the Study:

• To investigate the structure of [BMIM+][Tf2N-] in organic solvents using the same parameter space as previous dynamic studies.
• To explore ion correlations and their impact on transport properties through various structural and thermodynamic analyses.
• To identify solvent characteristics that effectively screen ion-ion interactions and enhance ion dynamics.

Main Methods:

• Utilized molecular dynamics (MD) simulations across a wide range of concentrations for [BMIM+][Tf2N-] in 22 organic solvents.
• Calculated structural properties: liquid densities and pair correlation functions.
• Assessed thermodynamic properties: ion pairing and ion caging lifetimes, and free energy calculations.

Main Results:

• Identified significant differences in the effectiveness of solvents at screening ion-ion interactions.
• Observed that strong ionic liquid pairing generally hinders transport properties.
• Demonstrated that certain solvents can effectively screen these interactions, leading to substantially enhanced ion dynamics.

Conclusions:

• Solvent choice critically influences the structure and dynamics of ionic liquid mixtures.
• Effective screening of ion-ion interactions by solvents is key to improving ion transport.
• These structure-transport relationships provide insights for designing advanced electrolytes for energy storage applications like batteries and capacitors.