Local structures in ionic liquids probed and characterized by microscopic thermal diffusion monitored with picosecond...

Area of Science:

• Physical Chemistry
• Materials Science
• Spectroscopy

Background:

• Room temperature ionic liquids (RTILs) are unique solvents with potential applications in various fields.
• Understanding energy transfer dynamics in RTILs is crucial for optimizing their performance.
• Previous studies in molecular liquids showed a correlation between vibrational cooling and thermal diffusivity.

Purpose of the Study:

• To investigate the relationship between vibrational cooling rates and bulk thermal diffusivity in RTILs.
• To explore the role of local structures in RTILs' energy transfer properties.
• To estimate the size of these characteristic local structures.

Main Methods:

• Picosecond time-resolved Raman spectroscopy was used to measure the vibrational cooling rate of trans-stilbene's S(1) state.
• The transient grating method was employed to determine the bulk thermal diffusivity of seven RTILs.
• Numerical simulations were utilized to model vibrational cooling kinetics and estimate local structure size.

Main Results:

• Vibrational cooling rates in the studied RTILs showed no correlation with their bulk thermal diffusivity.
• Observed vibrational cooling rates were similar across RTILs despite significant differences in thermal diffusivity.
• Numerical simulations suggest the presence of local structures with diameters larger than 10 nm.

Conclusions:

• RTILs possess characteristic local structures that govern energy transfer dynamics.
• Vibrational cooling is influenced by energy transfer within local structures, while macroscopic heat conduction depends on inter-structure transfer.
• The estimated size of these local structures in RTILs is between 10 nm and 100 nm.