Size matters! On the way to ionic liquid systems without ion pairing

Area of Science:

• Materials Science
• Physical Chemistry
• Electrochemistry

Background:

• Ionic liquids (ILs) are tunable solvents with applications in various fields.
• Understanding the relationship between IL structure and properties is crucial for their design.
• The influence of anion size on IL properties, such as ion mobility and conductivity, remains an active area of research.

Purpose of the Study:

• To synthesize and characterize novel ionic liquids with imidazolium and ammonium cations and two distinct anions: [NTf2](-) and [Al(hfip)4](-).
• To investigate the impact of anion size on the physicochemical properties of ILs, including viscosity, conductivity, and self-diffusion.
• To correlate experimental findings with theoretical models to elucidate the mechanisms governing ion transport.

Main Methods:

• Synthesis and characterization of new ionic liquids.
• Temperature-dependent measurements of viscosity and conductivity.
• Pulsed-gradient stimulated-echo (PGSTE) NMR for ion-specific self-diffusion measurements.
• Hirshfeld analysis and modified Marcus theory for theoretical insights.

Main Results:

• Ionic liquids with the larger [Al(hfip)4](-) anion showed significantly higher self-diffusion constants and conductivities compared to those with the smaller [NTf2](-) anion.
• [Al(hfip)4](-)-based ILs exhibited higher ionicities, reaching near 100% in some cases.
• Theoretical analysis indicated that shallow Coulomb potential wells, influenced by anion size, are responsible for the enhanced ion mobility.

Conclusions:

• The size of the anion plays a critical role in determining the ion mobility and overall properties of ionic liquids.
• [Al(hfip)4](-)-based ILs demonstrate near-ideal ionic behavior due to their large size and associated shallow Coulomb potentials.
• Modified Marcus theory successfully predicts the observed differences in ion transport, highlighting its utility in designing high-performance ILs.