Ion transfer processes at 4-(3-phenylpropyl)-pyridine | aqueous electrolyte | electrode triple phase boundary systems supported by graphite and by mesoporous TiO2

Abstract

Biphasic electrode systems allow electrochemical reactions to be driven in a microphase of organic liquid (typically 1–100 nL), which is coupled via ion transfer processes to the surrounding aqueous electrolyte medium. Microdroplet deposits on basal plane pyrolytic graphite as well as thin film deposits of the organic phase within a mesoporous titanium oxide host film are investigated. Cobalt tetraphenylporphyrin (CoTPP) is dissolved in the organic liquid 4-(3-phenylpropyl)-pyridine (PPP) and deposited in the form of microphases at suitable electrode surfaces. The electrode is immersed in aqueous electrolyte environments. It is shown that two stable and highly reversible one-electron metal-centred redox processes occur consistent with Co(III/II)TPP and Co(II/I)TPP in the presence of axial pyridine ligands. The electrochemical characteristics for both processes are strongly affected by the liquid | liquid ion exchange accompanying the redox processes. The potential for both the Co(III/II)TPP and the Co(II/I)TPP redox processes can be adjusted independently by the choice of the nature and concentration of the aqueous electrolyte. The reversible potential observed for the CoTPP metal complex is dominated by the Gibbs energy of transfer for the ‘spectator ions’. Conditions can be chosen to eliminate ion transfer effects on the potential scale for biphasic oxidation and reduction processes.