Order-disorder in room-temperature ionic liquids probed via methyl quantum tunneling

Area of Science:

• Materials Science
• Physical Chemistry
• Chemical Engineering

Background:

• Room-temperature ionic liquids (RTILs) are versatile materials with applications in energy storage, pharmaceuticals, and bioprocessing.
• The complex ionic nature of RTILs often leads to the formation of amorphous phases upon cooling.
• Understanding the phase behavior of RTILs is crucial for optimizing their performance in various applications.

Purpose of the Study:

• To investigate the influence of processing conditions on the crystalline and amorphous phase behavior of imidazolium-based RTILs.
• To explore the relationship between processing parameters (thermal history, mixing, pressure) and phase transitions.
• To demonstrate the utility of quantum tunneling as a probe for order-disorder phenomena in RTILs.

Main Methods:

• High-resolution inelastic neutron scattering (INS) was employed to measure quantum tunneling in cation methyl groups.
• Two specific imidazolium-based RTILs, 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and their mixtures were studied.
• Variable processing conditions, including thermal history, liquid mixing, and applied pressure, were systematically varied.

Main Results:

• The study reveals how processing conditions significantly affect the interplay between crystalline and amorphous phases in RTILs.
• Quantum tunneling measurements directly correlate with the degree of order-disorder (crystalline vs. amorphous state) within the RTILs.
• Specific processing pathways were shown to favor either crystalline or amorphous phase formation.

Conclusions:

• Quantum tunneling in cation methyl groups serves as a sensitive probe for characterizing the structural state of RTILs.
• Processing conditions are critical determinants of the phase behavior and resultant properties of RTILs.
• This work provides a novel method for understanding and controlling the solid-state structure of RTILs for targeted applications.