The local structure in the BmimPF₆/acetonitrile mixture: the charge distribution effect

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Physics

Background:

• Understanding the local structure of ionic liquid mixtures is crucial for predicting their macroscopic properties.
• The influence of charge distribution models on intermolecular interactions in ionic liquid-solvent systems requires detailed investigation.

Purpose of the Study:

• To investigate the changes in local structure of 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6) and acetonitrile mixtures across the entire composition range.
• To analyze the effect of different ionic charge distribution models on hydrogen bonding and cation-cation stacking interactions.
• To correlate structural changes with experimentally observed physical chemical properties.

Main Methods:

• Utilized two distinct charge distribution models for ions: constant fractional charges and mole fraction-dependent scaled charges.
• Introduced characteristic distances to quantify hydrogen bonding between ions and ions-solvent.
• Defined a coordinate system to analyze cation-anion orientation and nearest neighbor arrangements.

Main Results:

• The variable charge model preserves cation-anion arrangements down to a lower ionic liquid mole fraction (xIL = 0.10) compared to the constant charge model (xIL = 0.20).
• At xIL = 0.10-0.20, two preferred anion arrangements around cations emerge: hydrogen-bonded and non-hydrogen-bonded.
• The relative population of these arrangements is sensitive to ionic liquid concentration, correlating with experimental property changes.

Conclusions:

• A significant transition in the system's behavior occurs in the xIL = 0.10-0.20 range.
• This transition marks a shift from cation-anion hydrogen bonding dominance to molecular solvent solvation determining macroscopic properties.
• The choice of charge distribution model significantly impacts the predicted local structure and interaction dynamics in ionic liquid mixtures.