On the relation between reorientation and diffusion in glass-forming ionic liquids with micro-heterogeneous...

Area of Science:

• Physical Chemistry
• Materials Science
• Condensed Matter Physics

Background:

• Ionic liquids (ILs) are promising materials with tunable properties.
• Understanding structure-property relationships is crucial for designing ILs.
• Glass-forming ILs exhibit complex dynamics influenced by their microheterogeneous structures.

Purpose of the Study:

• To investigate the complex relationship between structure and dynamics in glass-forming ionic liquids.
• To elucidate how variations in cation alkyl chain length influence cation and anion dynamics.
• To explore the applicability of the Stokes-Einstein-Debye model to these systems.

Main Methods:

• Utilized nuclear magnetic resonance (NMR) spectroscopy (¹H, ²H, ¹⁹F) to probe cation and anion dynamics.
• Employed spin-lattice relaxation analysis and field-cycling relaxometry for reorientation dynamics.
• Applied stimulated-echo experiments and static field gradients to study translational diffusion.

Main Results:

• Observed Vogel-Fulcher-Tammann temperature dependence for correlation times and diffusion coefficients.
• Found a moderate slowdown of both cation and anion dynamics with increasing alkyl chain length (n).
• Demonstrated that cation dynamics deviate from the Stokes-Einstein-Debye relation for n > 6 due to cation clustering.

Conclusions:

• The study reveals a complex interplay between structural and dynamical properties in ionic liquids.
• Cation clustering significantly influences cation diffusion, leading to a breakdown of the SED relation.
• Anion dynamics consistently follow the SED relation, highlighting distinct behaviors between ionic species.