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.

About the author

Cédric NOËL earned his Ph.D. degree from the Institut National Polytechnique de Lorraine in 2009. He joined the Institut Jean Lamour as a CNRS Research Engineer in 2008. His research focuses on plasma diagnostics techniques (OES, laser spectroscopy, mass spectrometry, …) and plasmas created in liquids.


About the author

Dr. Sergey Nikitenko now is a Director of Research at CNRS, Head of the Laboratory of Sonochemistry in Complex Fluids (LSFC) at the Marcoule Institute for Separation Chemistry (ICSM), France. Sergey Nikitenko obtained Ph.D. in chemistry at Moscow State University, Russia. Currently his research interests focus on the mechanisms of sonochemical reactions and on the development of technologies for carbon-free energy, more specifically, on the synthesis of functional nanomaterials, nuclear chemistry, sono- and photo-catalysis.

Dr. Nikitenko published more than 120 peer-reviewed papers, delivered more than 100 conference contributions, 3 book chapters and hold 3 patents. He is an Editorial Board Member of the Ultrasonics Sonochemistry and Heliyon (Elsevier), Board Member of the European Society of Sonochemistry (ESS) and Director of GDR Cavitation.

About the author

Thierry Belmonte is research director at CNRS, France. He graduated as engineer in Grenoble where he also obtained his PhD. He works in the field of plasma nanoscience. He studies the synthesis of nanostructures by atmospheric pressure plasmas, additive manufacturing by chemical vapour deposition and nanoparticles generation by discharges in dielectric liquids for applications in the field of energy. A particular attention is paid to plasma-surface interactions and modelling. Thierry Belmonte published 177 papers in international journals with peer review, 7 chapters in books and 6 patents. He supervised 32 PhD students and 11 post-docs.

He was awarded the bronze Medal of the CNRS, the Jean RIST medal of the Société Française de Métallurgie et des Matériaux and the 1st prize of the Région Lorraine. He is the current director of the Institut Jean Lamour, a laboratory of 500 persons in the field of material science located in Nancy, France.

About the author

Micheline Draye is a professor of chemistry and head of the “Green Chemistry” group at the Laboratory of Molecular Chemistry and Environment, University of Savoie Mont Blanc. Her research interests include use ultrasound for organic synthesis, green organic synthesis and catalysis, ionic liquids, biomass valorisation, green/sustainable separation science and technology, study of organic molecules degradation and radiochemistry. She served on the National Research Council’s Committee on ‘Research Needs for High-Level Waste Stored in Tanks and Bins at the US Department of Energy Sites’ (

Her awards include the 1991 prize of the French Society of Nuclear Energy, a fellowship with CEA and a fellowship with COGEMA, the 2017 title of distinguished Fellow of the Chemical Society of France. She was for seven years an associate Professor at the Ecole Nationale Supérieure de Chimie de Paris where she conducted her research in the ‘separation processes and radiochemistry group’ (Pr. G. Cote, head of the group). She worked for two years as a visiting scientist in the Nuclear Engineering Department of the Massachusetts Institute of Technology (Pr. Kenneth Czerwinski, head of the group) after having received her PhD in Chemistry from the University of Lyon (Pr. M. Lemaire, supervisor).

About the author

Manuel Lejeune was born in Epinal, in northeastern France. He studied organic chemistry with Prof. Jean-Pierre Sauvage and Prof. Dominique Matt during his master degree at the University of Strasbourg. Then, he obtained his PhD degree in coordination chemistry under the guidance of Prof. Dominique Matt and Prof. Pierre Lutz in the same university (2006). His thesis work was devoted to the preparation of new fast phosphorus calix[4]arene based nickel catalysts and their catalytic behavior in dimerization and polymerization of olefins. After, he joined successively the University of Geneva (Prof. Michel Geoffroy, Switzerland) and the University of Brussels (Prof. Ivan Jabin, Belgium) as a postdoctoral researcher, where he has interested to the synthesis of bioinspired catalysts.

In 2013, he moved to the group of Prof. Draye at the University Savoie Mont-Blanc (France) as a part-time teacher in chemistry. The next year, he worked under the supervision of Prof. Micheline Draye as a postdoctoral researcher in the SILEXE project, a new sustainable industrial process for the recycling of strategic metals from electronic waste. Manuel Lejeune is currently a project manager in the field of sustainable development and circular economy at CEA Marcoule, France.

About the author

Dr. Rachel Pflieger is a senior research scientist from CEA (French Alternative Energies and Atomic Energy Commission) working at Marcoule Institute for Separation Chemistry (ICSM), France. She studied chemistry at ECPM in Strasbourg (France) then did her PhD in high-temperature thermodynamics applied to nuclear materials at the Institute for Transuranium Elements (JRC Karlsruhe, Germany).

Her fields of research focus on fundamental sonochemistry, sonoluminescence and plasma emission spectrometry, and ultrasound-assisted decontamination of solids. She is member of the Editorial Board of Ultrasonics Sonochemistry.


Rachel Pflieger, Manuel Lejeune, Cédric Noel, Thierry Belmonte, Sergey I. Nikitenko, Micheline Draye. Diagnosing the plasma formed during acoustic cavitation in [BEPip][NTf2] ionic liquid. Phys.Chem.Chem.Phys., 2019, volume 21, page 1183

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