Kinetics of ferrocenium cation transfer across ionic liquid/water interface using recessed microelectrode

Coupled Electron- and Phase-Transfer Reactions at a Three-Phase Interface

Coupled electron- and phase-transfer reactions are fundamentally important in electrochemical energy conversion and storage, e.g., intercalation of Li⁺ in batteries and electrochemistry at the three-phase boundary in fuel cells. The mechanism, energetics, and kinetics of these complex reactions play an important role in device performance. Herein, we describe experimental methodology to quantitatively investigate coupled electron- and phase-transfer reactions at an individual, geometrically well-defined, three-phase interface. Specifically, a Pt-Ir wire electrode is placed across a H₂O/1,2-dichloroethane (DCE) interface, creating a Pt-Ir/H₂O/DCE boundary that is defined mathematically by a line around the surface of the wire. We investigated the oxidation of ferrocene (Fc), initially present in DCE (but essentially insoluble in water), at the three-phase boundary, and the transfer of its charged reaction product ferrocenium (Fc⁺) across the interface into the aqueous phase. In cyclic voltammetry, a reversible wave at E_1/2 ∼ 0.58 V is observed for Fc oxidation in DCE on the first scan. The Fc⁺ produced near the H₂O/DCE interface transfers into the aqueous phase. On the second and subsequent cycles, a second reversible wave at more negative potentials, E_1/2 ∼ 0.33 V, is observed, corresponding to the reduction of Fc⁺ (and reoxidation back to Fc) in the aqueous phase. Finite-element simulations quantitatively capture the voltammetric response of coupled electron and phase transfer at the three-phase interface and indicate that the electrochemical response observed in the aqueous phase occurs within ∼200 μm of the Pt-Ir/H₂O/DCE boundary. Finally, we demonstrate that the rate of transfer of Fc⁺ is strongly dependent on the concentration of supporting electrolyte, reaching a maximum at an intermediate electrolyte concentration, suggesting a critical role of the electric field distribution in determining the reaction rates at the three-phase interface.

Kinetic Analysis of the Solvent Extraction of Cadmium(II) Complexes Using a Cadmium-deposited Recessed Microelectrode and Confocal Fluorescence Microscope

Using a Cd-deposited microelectrode, an electrochemically generated Cd(II) ion in a micro-water phase was reacted with 5-octyloxymethyl-8-quinolinol (HC₈Q) in 1,6-dichlorohexane. The fluorescence intensity of Cd(C₈Q)₂ near the water/oil interface (I_F) was analyzed under a confocal fluorescence microscope. The rate of decrease of I_F was independent of the HC₈Q concentration and pH, but was influenced by the phase-boundary potential between the water and oil phases, suggesting that Cd(II) extraction is governed by Cd(C₈Q)⁺ desorption at the interface.

Simple Ion Transfer at Liquid|Liquid Interfaces

The main aspects related to the charge transfer reactions occurring at the interface between two immiscible electrolyte solutions (ITIES) are described. The particular topics to be discussed involve simple ion transfer. Focus is given on theoretical approaches, numerical simulations, and experimental methodologies. Concerning the theoretical procedures, different computational simulations related to simple ion transfer are reviewed. The main conclusions drawn from the most accepted models are described and analyzed in regard to their relevance for explaining different aspects of ion transfer. We describe numerical simulations implementing different approaches for solving the differential equations associated with the mass transport and charge transfer. These numerical simulations are correlated with selected experimental results; their usefulness in designing new experiments is summarized. Finally, many practical applications can be envisaged regarding the determination of physicochemical properties, electroanalysis, drug lipophilicity, and phase-transfer catalysis.