Nanostructuration of ionic liquids: impact on the cation mobility. A multi-scale study

Area of Science:

• Condensed Matter Physics
• Materials Science
• Physical Chemistry

Background:

• Ionic liquids (ILs) present a paradox: behaving as strong electrolytes at macroscopic scales but weak ionic solutions at molecular levels.
• Understanding the multi-scale dynamics of ILs is crucial for reconciling these seemingly contradictory behaviors.
• The nanostructuration of ILs, such as OMIM-BF4, plays a key role in their complex dynamics.

Purpose of the Study:

• To reconcile the macroscopic and microscopic behaviors of ionic liquids.
• To investigate the nanometer/nanosecond and micrometer/millisecond dynamics of OMIM-BF4.
• To develop a unified model explaining the observed dynamics across unprecedented spatial and temporal ranges.

Main Methods:

• Quasi-elastic neutron scattering (QENS) and neutron spin-echo (NSE) were employed to probe nanometer/nanosecond dynamics.
• Pulsed field gradient nuclear magnetic resonance (PFG-NMR) was used to investigate microscopic (micrometer/millisecond) dynamics.
• A novel physical model was introduced to interpret neutron scattering data, accounting for side-chain and whole-molecule dynamics.

Main Results:

• A unified description of local cation dynamics was achieved, combining localized motion within nanodomains and long-range translational motion.
• QENS, NSE, and NMR experiments consistently identified the same long-range translational process, despite probing different scales.
• Diffusion coefficients obtained from different methods varied by over an order of magnitude, highlighting the impact of nanostructuration.

Conclusions:

• The apparent discrepancy in diffusion coefficients is a direct consequence of the IL's inherent nanostructuration.
• A coherent structural and dynamical picture emerges, explaining the multi-scale behavior of ionic liquids.
• The findings provide a comprehensive understanding of ionic liquid dynamics, bridging molecular and macroscopic observations.