Ultrasound mobilization of liquid/liquid/solid triple-phase boundary redox systems

Power ultrasound is used to “mobilize” droplets of 1,2-dichloroethane (DCE) on a glassy carbon electrode surface in an aqueous electrolyte environment. Voltammetric methods are employed to investigate the effect of ultrasound on (i) the mass transport in the aqueous phase, (ii) the mass transport in the DCE−aqueous two-phase system, and (iii) the triple-phase boundary anion extraction reaction coupled to oxidation of n-butylferrocene (nBuFc) in the organic phase. Optimized conditions comprise a 13 mm diameter ultrasonic horn (24 kHz) with 15 W/cm2 power output at a distance of 15 mm from a 2.83 cm2 glassy carbon working electrode in 32 cm3 of aqueous solution. Mass transport in the aqueous phase is probed for the reduction of hexaammineruthenium(III) chloride in aqueous 0.1 M KCl supporting electrolyte, and an increase in mass transport induced by the DCE droplets is observed. Triple-phase-boundary ion transfer reactions are studied for the oxidation of nBuFc in DCE in the presence of aqueous 0.1 M NaBPh4, KPF6, NaClO4, and phosphate buffer pH 1. The hydrophobicity of the transferring electrolyte anion is observed to shift the electrochemical response according to the standard transfer potential. For phosphate electrolyte media, rather than phosphate transfer, n-butylferricenium cation transfer into the aqueous phase and iron(III) phosphate formation occur. The beneficial effect of adding tetrabutylammonium hexafluorophosphate electrolyte into the organic DCE phase is demonstrated; however, triple-phase-boundary processes in the absence of intentionally added electrolyte in the organic phase are feasible.