Dynamics, Phase Transitions, and Hydrogen Bonding Motifs in Protic Ionic Liquids: Cations Make the Difference

Area of Science:

• Materials Science
• Electrochemistry
• Physical Chemistry

Background:

• Phosphonium ionic liquids (PILs) offer advantages over ammonium analogues, including enhanced thermal stability, conductivity, and lower viscosity.
• These properties make PILs promising candidates for advanced electrolyte applications requiring high discharge capacities and rechargeability.
• Understanding the molecular-level interactions is crucial for optimizing PIL performance.

Purpose of the Study:

• To elucidate the molecular-level interactions and dynamics governing the favorable properties of phosphonium ionic liquids (PILs).
• To compare the behavior of a representative phosphonium ionic liquid ([P444-H][OMs]) with its ammonium analogue ([N444-H][OMs]).
• To investigate the relationship between molecular dynamics, interaction strength, and macroscopic properties like viscosity and phase transitions.

Main Methods:

• Utilized Deuterium Nuclear Magnetic Resonance (2H NMR) spectroscopy in both solid and liquid states.
• Analyzed spectral line shapes and spin relaxation times to probe interaction strength, hydrogen bonding, and phase transitions.
• Performed quantum chemical calculations on ionic liquid clusters to complement experimental findings.

Main Results:

• 2H NMR spectral analysis provided insights into hydrogen bonding arrangements and phase transition behaviors of the ionic liquids.
• Spin relaxation times in the liquid state revealed distinct cation dynamics and allowed for the investigation of microscopic viscosity.
• Computational modeling supported the experimental observations regarding ion pair interactions.

Conclusions:

• Phosphonium ionic liquids demonstrate superior electrolyte characteristics due to favorable molecular interactions and dynamics.
• 2H NMR spectroscopy is a powerful tool for characterizing ionic liquid behavior at the molecular level.
• The study provides a molecular-level understanding of why PILs outperform their ammonium counterparts in electrochemical applications.