Analytical Sciences

Electroneutrality Coupling of Electron Transfer at an Electrode Surface and Ion Transfer across the Interface between Thin-layer of 1-Octyl-3-methylimidazolium Bis(perfluoroalkylsulfonyl)imide Covering the Electrode Surface and an Outer Electrolyte Solution

Kazuyuki TANAKA, Naoya NISHI, and Takashi KAKIUCHI

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyoku, Kyoto 615-8510, Japan

The electrode reaction of decamethylferrocene (DMFc) dissolved in a thin layer of a room-temperature molten salt (RTMS), 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C₈mimC₁C₁N) or 1-octyl-3-methylimidazolium bis(pentafluoroethylsulfonyl)imide (C₈mimC₂C₂N), on a self-assembled monolayer-modified gold electrode is coupled with the ion transfer across the interface between the RTMS and the outer aqueous solution (W) to give a voltammogram whose shape resembles a voltammogram of a simple one-electron transfer process. The electroneutrality of the RTMS layer during the oxidation of DMFc to decamethylferricenium ion is maintained by the concomitant dissolution of C₈mim⁺ ion from the RTMS phase to the W phase, and the reduction of decamethylferricenium ion to DMFc is accompanied by the transfer of either C₁C₁N^- or C₂C₂N^- from RTMS to W. The midpoint potential of the voltammogram varies with the concentration of the salt in the aqueous phase, C₈mimCl or LiC_nC_nN (n = 1 or 2), in a Nernstian manner, showing that the phase-boundary potential between the RTMS and the W is controlled by the partition of these ions. Although the phase-boundary potential across the RTMS | W interface is Nernstian with respect to the ions common to both phases at the equilibrium, the polarization at the RTMS | W interface under current flow distorts the shape of the voltammograms, resulting in a wider peak separation in the voltammogram.

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