Kinetic analysis of electron transfer across single water-microdroplet/oil and oil-microdroplet/water interfaces

Using techniques comprising laser trapping, microcapillary injection/manipulation, fluorescence microspectroscopy and electrochemistry of single microdroplets, we kinetically investigated the electron transfer (ET) reaction between decamethylferrocene in tributyl phosphate and hexacyanoferrate(III) in water. In the oil-microdroplet/water system, the overall ET reaction rate significantly depended on the droplet radius (r(d), 0.5 microm < r(d) < 10 microm) and on the potential-determining ion concentration in the oil phase. The interfacial ET reaction rate constant determined in the water-microdroplet (r(d) = 21 microm)/oil system agreed very well with that in the oil-microdroplet (r(d) > 2 microm)/water system. The rate constant values were extremely small in the Gibbs free energy (DeltaG) range of -10 to -25 kJ mol(-1), with DeltaG consisting of the Galvani potential difference between the water and oil phases and the redox potential difference of the solutes. The characteristic ET reaction was discussed in terms of the ion transfer and the ET across the interfacial mixed layer with nanometer-sized thickness.