Nitrobenzene (NB) including ferrocene was mixed vigorously with the sodium
dodecylsulfate aqueous solution to yield an oil-in-water emulsion. Voltammo
grams of the emulsion showed an anodic wave at approximately 0.90 V versus
Ag \ AgCl, which was discerned from the oxidation wave of micellized ferroc
ene in water (at 0.29 V), that of directly soluble ferrocene (at 0.22 V) an
d also that of ferrocene in NB (at 0.55 V). From dependence of the anodic c
urrent on the scan rate, the anodic wave was due to the adsorption on oil d
roplets including ferrocene. In contrast, a cathodic wave was only the redu
ction of the soluble ferrocinium ion at 0.15 V. Consequently, ferrocene, wh
ich is known as an electrochemically reversible couple, behaves as an irrev
ersible system in the emulsion. When a potential of more than 1.2 V was app
lied, the emulsion was gradually separated into the oil and the aqueous pha
ses, regardless of the presence or absence of ferrocene. A possible reactio
n mechanism is adsorption of oil droplets containing ferrocene at the elect
rode, an increase in the interfacial energy with the application of the pos
itive potential, coalescence of the droplets, dispersion of ferrocene in th
e water phase, and the oxidation of the ferrocene at the electrode. An opti
cal microscopic view showed spheres of oil less than 7 mum in diameter. The
emulsions were stable for a few weeks without agitation. (C) 2001 Elsevier
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