Charge transport and dipolar relaxations in phosphonium-based ionic liquids

Area of Science:

• Materials Science
• Physical Chemistry
• Condensed Matter Physics

Background:

• Ionic liquids (ILs) are salts that are liquid at room temperature, with applications in various fields.
• Understanding charge transport and molecular dynamics in ILs is crucial for optimizing their performance.
• Tributyloctylphosphonium ILs present a unique system for studying anion effects due to their structure.

Purpose of the Study:

• To investigate the influence of anions on charge transport and localized dipolar relaxations in tributyloctylphosphonium ionic liquids.
• To elucidate the microscopic mechanisms governing ion conduction and molecular dynamics in these systems.
• To compare relaxation dynamics with other classes of ionic liquids.

Main Methods:

• Broadband dielectric spectroscopy was employed to analyze dielectric spectra and identify relaxation processes.
• Rheology measurements provided complementary data on the viscoelastic properties of the ionic liquids.
• The random barrier model and Havriliak-Negami functions were used for quantitative analysis of ion transport and dipolar relaxations.

Main Results:

• Two secondary relaxations were observed below the glass transition temperature, attributed to cation libration and a Johari-Goldstein-like process.
• The faster relaxation showed an anion-independent activation energy, while the slower one had a higher activation energy in phosphonium systems.
• Anion variation caused significant differences (∼2.5 decades) in dc ionic conductivity and charge transport rates, which converged upon scaling with the glass transition temperature.

Conclusions:

• The study reveals a strong dependence of charge transport on anion choice in phosphonium ionic liquids.
• A glass transition-assisted hopping mechanism is proposed as the unifying microscopic basis for ion conduction.
• The findings contribute to a deeper understanding of structure-property relationships in ionic liquids.