An electrochemically driven air bubble on the O|W interface under the three-phase boundary reactions of ferrocene

The three-phase boundary (TPB) reaction may be generated when a sensor electrode is inserted in living tissues, piercing oil|water interfaces. Most of TPB reactions have varied with the measurement time such as potential scan rates and the number of potential cycles because supporting electrolytes transfer between the two phases to mix and extend the area of the reacting surface, explored by Scholz, Girault and coworkers. A typical cell apparatus is composed of the oil droplet including only a redox species, which is located on an electrode in an aqueous solution including supporting electrolyte, as is shown in the photograph. The use of supporting electrolyte, sodium sulfate, insoluble to the oil phase has allowed us to keep the steady-state current for TPB reactions. This concept is based on the restriction of forming a double layer only to the water|electrode interface, which penetrates the oil phase to cause the electrode reaction. However, it is not easy to obtain the obvious evidence because of local motion of the boundary during the redox reaction, formation of emulsions around TPB, and convection caused by the redox reaction. Here we use an air bubble to be a measure of the environment of TPB. Air bubbles are always located on the O|W interface near the top of the hemispherical droplet, as shown in the photograph. They do not float to be expelled from the solutions owing to their buoyancy. We have found motion of an air bubble depending on surfactants and electrode potentials, probably because of response to a subtle change in the surface tension of the O|W interface. This report is devoted to investigating the static and dynamic behavior of the air bubble on the O|W interface during TPB reaction of ferrocene. The force balance of the air bubble at the O|W interface will be formulated in terms of the interfacial tensions and the buoyancy. It will be demonstrated that the electrode reaction varies the surface tension at the O|W interface to cause surface convection. References 1. Girault, H.H. Modern Aspects of Electrochemistry, Eds. J.O. Bockris et al, Plenum Press, New York, Vol. 25, (1993) pp.1-62. 2. M. Lovric, F. Scholtz, J. Solid State Electrochem., 1997, 1, 108. F. Scholz, S. Komorsky-Lovric, M. Lovric, Electrochem. Commun., 2000, 2, 112. 3. P.Tasakorn, J.Chen, K.Aoki, J. Electroanal. Chem., 2002, 533, 119. 4. J.Chen, M.Sato, J. Electroanal. Chem., 2004, 572, 153. 5. M.Satoh, K.Aoki, J.Chen, Langmuir, 2008, 24(8), 4364. air bubble