Phase Behavior of a Piperidinium-Based Room-Temperature Ionic Liquid Exhibiting Scanning Rate Dependence

Area of Science:

• Physical Chemistry
• Materials Science
• Thermodynamics

Background:

• Room-temperature ionic liquids (RTILs) exhibit diverse physicochemical properties influenced by ion flexibility.
• Nonaromatic RTILs, such as 1-methyl-1-butylpiperidinium bis(fluorosulfonyl)amide ([Pip1,4][FSA]), present unique structural characteristics compared to common imidazolium-based cations.
• Understanding the thermodynamic behavior and phase transitions of these RTILs is crucial for their application.

Purpose of the Study:

• To systematically investigate the thermodynamic properties of nonaromatic RTILs, starting with [Pip1,4][FSA].
• To explore the relationship between ion conformational flexibility (cation ring-flipping and alkyl group rotation) and the observed phase behavior.
• To characterize the dynamics and activation energy associated with structural relaxations in [Pip1,4][FSA].

Main Methods:

• High-sensitivity calorimetry was employed to study phase transitions and thermal behavior.
• Raman spectroscopy was utilized to probe conformational changes in the cation and anion.
• Differential scanning calorimetry was used to analyze heating rate-dependent structural relaxations.

Main Results:

• [Pip1,4][FSA] exhibits two crystalline phases (Cryst-α and Cryst-β) with associated cation and anion conformational changes.
• Observed heating-induced transitions from supercooled liquid to Cryst-α and Cryst-α to Cryst-β are identified as slow structural relaxations, not true phase transitions.
• The activation energy for cation ring-flipping was estimated at 38.8 kJ/mol, demonstrating heating rate dependence.

Conclusions:

• The conformational flexibility of both cation and anion significantly influences the structural dynamics and phase behavior of [Pip1,4][FSA].
• Slow dynamics and structural relaxations, driven by ion conformational changes in a viscous environment, are key features of this nonaromatic RTIL.
• This research provides fundamental insights into the thermodynamic properties and behavior of piperidinium-based RTILs.