Participation in Self-Emulsification by Oil-Thin Film Voltammetry

When an oil phase comes in contact with an aqueous phase, emulsions are formed spontaneously in each phase even without surfactant. The self-emulsification seems inconsistent with the electron transfer model proposed by Anson, in which ferricyanide in the aqueous phase is reduced at the oil/water interface by decamethylferrocene of the thin nitrobenzene phase. Anson’s experimental data were here reproduced at slow scan voltammetry. However, the electron transfer model did not agree with our experimental results at high scan rates, in that the reduction wave of ferricyanide appeared without decamethylferrocene. Ferricyanide was demonstrated to pass through the nitrobenzene film in the form of aqueous droplets, which were adsorbed on the electrode surface. Formation of aqueous droplets can explain electrode reactions at carbon paste electrodes without including redox species in paste.


Introduction
Thin-layer cell voltammetry with an oil film is an electrochemical technique of detecting hydrophilic redox species with a help of chemical selectivity of oil-dissolved species (Shi & Anson, 1998a, 1998b, 1999).The principle suggested by Anson, as illustrated in Figure 1(A), is composed of basically two concepts.(i) The hydrophilic reactant (Fe(CN) 6 3-) cannot reach the electrode surface by penetration of the oil phase.(ii) The hydrophobic reactant (decamethylferrocene (DMFc)) in the thin organic layer is oxidized by the electron transfer reaction with the hydrophilic species (Fe(CN) 6 3-), and the oxidized one is reduced by the electrode reaction.The second step is a redox cycling, including diffusion back and forth in the oil film.The electron transfer mechanism at oil/water interface has been demonstrated through in-situ spectro-electrochemical technique (Ding et al., 1998).It has also been shown by scanning electrochemical microscopy, in which redox species generated at the oil/water interface is detected by the probe electrode close to the interface (Wei, Bard, & Mirkin, 1995;Tsionsky, Bard, & Mirkin, 1996, 1997).Various applications have been reported in the light of electron transfer mechanisms (Zhang, Barker, & Unwin, 2000;Sun et al., 2003;Liu et al., 2005;Xu et al., 2004;Solomont & Bard, 1995;Wang et al., 2003;Li et al., 2006;Michael et al., 2008;Quentel et al., 2007).The recent progress, the theory, the data analysis and applications have been reviewed (Lu et al., 2011), especially emphasizing electron transfer rates.
The concept of oil/water interface voltammetry assumes that the oil phase and the aqueous phase are separated unequivocally.The clear phase separation is, however, not guaranteed partly because of mutual dissolution (Samec & Kakiuchi, 1990;Kakiuchi et al., 2003;Freire et al., 2008) and partly because of self-emulsification (Shchipunov & Schmiedel, 1996;Pautot et al., 2003;Gonzalez-Ochoa, Ibarra-Bracamontes, & Arauz-Lara, 2003;Sacanna, Kegel, & Philipse, 2007).The latter occurs by mixing entropy (Aoki, 2011) even under quiescent conditions without including surfactants.Water droplets were found near the oil/water interface by an optical microscope (Aoki et al., 2009), while oil droplets were detected by dynamic light scattering and voltammetry (Li et al., 2011).Thin layer-voltammograms may be influenced by formation of droplets in the oil film, and can be explained from a view point of self-emulsification rather than the electron transfer reactions.The emulsified aqueous droplets should contain Fe(CN) 6 3-, which can be reduced with DMFc in the oil phase, as is illustrated in Figure 1(B).This mechanism is close to the penetration mechanisms by Osakai (Hotta et al., 2003;Osakai et al., 2004).

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The optica (Ivium, Ne polished w and the co deaerated NB films w injected N The thickn Thermogra mg.Temp of droplets

Conclusions
The electrocatalysis by thin-oil films works efficiently at slow scan rates, as Anson et al did.Voltammograms at high scan rates, however, make the influence of self-emulsification remarkable.When a NB-coated electrode without any redox species is immersed in the aqueous solution including hydrophilic redox species and supporting electrolyte, the electrode reaction occurs by penetration of the hydrophilic species into the NB phase.The penetration is caused by diffusion, of which value is much larger than the conventional value.Consequently, the electron exchange reaction at the NB/water interface is not necessarily a rate-determining step but the reaction within the NB film is responsible for the current.The reaction at the interface is noticeable as the catalytic process at very slow scan rates, whereas the reaction within the NB film is remarkable at fast scan rates.The latter case may be used for be one of mechanisms of voltammetry at carbon paste electrodes.

Figure
Figure 8. V

Figure 10 .
Figure 10.Voltammograms at v = 30 mV s -1 immediately after the transfer of the electrode to {W} without Fe(CN) 6 3-at (a) the first, (b) the second and (c) tenth scan