Sonoelectrochemistry: Transient cavitation in acetonitrile in the neighbourhood of a polarized electrode

The voltammograms recorded with ultrasound at a stationary electrode present a sigmoidal shape and their limiting current is the sum of steady-state and transient components. A semi-quantitative description of the integrated transient component is proposed in the case of the one-electron reduction of methyl viologen in acetonitrile at a platinum disc electrode of small radius (250 mu m) under the action of ultrasound (acoustic intensity = 1.4 W cm(-2), i.e. acoustic pressure = 1.7 atm). With the help of the basic theoretical and experimental data provided by Neppiras, Plesset and Chapman, it can be estimated that transient cavitation involves gas- or vapour-filled bubbles of similar radius R(M) (160-200 mu m) collapsing at the surface of the electrode. Jets of liquid of almost cylindrical shape develop, which repeatedly strike the electrode surface with a constant velocity (228 m s(-1)) and submicrosecond duration. Hence 4.2 x 10(-9) C are expected to be consumed in the first reduction step of methyl viologen (0.23 mM) when the largest collapsing bubbles (R(M) = 200 mu m) are involved. The experimental result (4.2 x 10(-9) C) is consistent with the above description. The voltammetric results show that the increase of the electrolytical current due to sonication is mainly given by the transient component. The consequences of the formation of violent jets striking the electrode surface are discussed.