Electron paramagnetic resonance of sonicated powder suspensions in organic solvents

Héla Laajimi; Michela Mattia; Robin S Stein; Claudia L Bianchi; Daria C Boffito

doi:10.1016/j.ultsonch.2021.105544

Electron paramagnetic resonance of sonicated powder suspensions in organic solvents

Ultrason Sonochem. 2021 May:73:105544. doi: 10.1016/j.ultsonch.2021.105544. Epub 2021 Mar 26.

Authors

Héla Laajimi¹, Michela Mattia², Robin S Stein³, Claudia L Bianchi², Daria C Boffito⁴

Affiliations

¹ Polytechnique Montréal - Department of Chemical Engineering, C.P. 6079, Centre Ville, H3C 3A7 Montréal, QC, Canada.
² Università degli Studi di Milano - Chemistry Department, via Golgi 19, 20133 Milan, Italy.
³ McGill University - Chemistry Department, 801 Rue Sherbrooke Ouest, Montréal QC H3A 0B8, QC, Canada.
⁴ Polytechnique Montréal - Department of Chemical Engineering, C.P. 6079, Centre Ville, H3C 3A7 Montréal, QC, Canada. Electronic address: daria-camilla.boffito@polymtl.ca.

Abstract

The chemical effects of the acoustic cavitation generated by ultrasound translates into the production of highly reactive radicals. Acoustic cavitation is widely explored in aqueous solutions but it remains poorly studied in organic liquids and in particular in liquid/solid media. However, several heterogeneous catalysis reactions take place in organic solvents. Thus, we sonicated trimethylene glycol and propylene glycol in the presence of silica particles (SiO₂) of different sizes (5-15 nm, 0.2-0.3 µm, 12-26 µm) and amounts (0.5 wt% and 3 wt%) at an ultrasound frequency of 20 kHz to quantify the radicals generated. The spin trap 5,5-dimethyl-1-pyrrolin-N-oxide (DMPO) was used to trap the generated radicals for study by electron paramagnetic resonance (EPR) spectroscopy. We identified the trapped radical as the hydroxyalkyl radical adduct of DMPO, and we quantified it using stable radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as a quantitation standard. The concentration of DMPO spin adducts in solutions containing silica size 12-26 µm was higher than the solution without particles. The presence of these particles increased the concentration of the acoustically generated radicals by a factor of 1.5 (29 µM for 0.5 wt% of SiO₂ size 12-26 µm vs 19 µM for 0 wt%, after 60 min of sonication). Ultrasound produced fewest radicals in solutions with the smallest particles; the concentration of radical adducts was highest for SiO₂ particle size 12-26 µm at 0.5 wt% loading, reaching 29 µM after 60 min sonication. Ultrasound power of 50.6 W produced more radicals than 24.7 W (23 µM and 18 µM, respectively, at 30 min sonication). Increased temperature during sonication generated more radical adducts in the medium (26 µM at 75 °C and 18 µM at 61 °C after 30 min sonication). Acoustic cavitation, in the presence of silica, increased the production of radical species in the studied organic medium.

Keywords: EPR; Free radicals; Organic solvent; Solid particle; Sonochemical activity quantification; Spin trapping; Ultrasound.