Ionic liquids to study the plasma formed during acoustic cavitation

Ionic liquids are organic salts that exist in liquid form around ambient temperature. An interesting attribute of these liquids is their negligible vapor pressure. Sonochemistry is the chemical activity observed under ultrasonic irradiation of a liquid. Its origin has been credited to acoustic cavitation, although its formation mechanism is still a subject of debate. Presently, it is widely accepted that a plasma forms in acoustic cavitation bubbles at collapse. Ionic liquids, owing to the aforementioned property, have been deemed ideal solvents for sonochemistry since they a priori do not interfere in chemical reactions inside bubbles. However, despite the growing interest in the combined use of ionic liquids and ultrasound, acoustic cavitation in these media has been poorly characterized. Even more so, the formation of degradation products opened the way for an experimental determination of the reached temperatures in sonicated ionic liquid. Till now, the only reported characterization temperature is shrouded in discrepancies ascribable to biases of the method used, i.e. the methyl radical recombination method. Therefore, other experimental determinations of the reached temperatures in ionic liquids are needed.

Recently, the Institute for Separation Chemistry of Marcoule researchers Dr. Rachel Pflieger and Dr. Sergey Nikitenko in collaboration with Dr. Manuel Lejeune, and Pr. Micheline Draye at University of Savoy Mont Blanc – Chambery and Dr. Cédric Noel, Dr. Thierry Belmonte at University of Lorraine reported on a sonoluminescence spectrum obtained during the sonication of N-butyl-N-ethyl-piperidinium bis(trifluoromethylsulfonyl)imide ([BEPip][NTf₂]) at 20 kHz under argon gas and the rovibronic temperatures obtained from spectroscopic fitting of the observed molecular emissions. Their work is currently published in the research journal, Physical Chemistry Chemical Physics.

In brief, the research team started by measuring the sonoluminescence spectra of a very dry (< 10 ppm H₂O) [BEPip][NTf₂] ionic liquid in the first minutes of sonication under Ar. Using the intense sonoluminescence, the researchers were able to monitor the time-evolution of the sonoluminescence spectra, where several molecular emissions were observed. Lastly, the rovibronic temperatures of C₂ and CN were determined.

Several molecular emissions were observed, the origin of which is the direct degradation of the ionic liquid and that of its degradation products. Interestingly, the authors noted that the rovibronic temperatures of C₂ and CN remained constant during the first minutes of sonolysis, as long as absorption of SL light by the sonolysis degradation products did not prevent from simulating the emissions. As such, due to this discrepancy, it was concluded that the rovibronic temperatures could not be used as estimates of the plasma electron properties. Additionally, two electronic systems of CH were observed in the sonoluminescence spectra of [BEPip][NTf₂]. Their relative intensities strongly decreased in the first two minutes of sonication, reflecting the uptake of volatile degradation products inside the bubbles.

In summary, the French study presented the sonication of a very dry [BEPip][NTf₂] ionic liquid under Ar, which produced an intense sonoluminescence, that allowed the researchers to monitor the time-evolution of the sonoluminescence spectra. Generally, the experimental data presented will help to the reassessment of the model of formation of the sonochemical plasma, and its comparison with ‘‘more conventional’’ plasmas. Also, future studies should evaluate and compare more molecular emissions in sonoluminescence, to shed more light on the mechanisms of formation of the excited species and to determine which useful information can be derived from them.