On the non-ideal behaviour of polarised liquid-liquid interfaces

Interpretation of electrochemical data generated at the interface between two immiscible electrolyte solutions (ITIES), and realisation of the ITIES for technological applications, requires comprehensive knowledge of the origin of the observed currents (i.e., capacitive, ion or electron transfer currents) and the factors influencing the electrical double layer. Upon formation, the ITIES is away from equilibrium and therefore is a close approximation, but not a perfect realisation, of an ideally polarisable interface. Nevertheless, the formalism of equilibrium thermodynamics, e.g., the Nernst equation, are universally applied to interpret electrochemical processes at the ITIES. In this study, electrochemical impedance spectroscopy (EIS), cyclic and AC voltammetry were applied to probe electrochemical processes at an ITIES formed between aqueous and α,α,α-trifluorotoluene electrolyte solutions. A significant contribution from faradaic currents is observed across the whole polarisable potential window and the electrolyte solution is not an ideal resistor (especially at high electric field frequencies). The electrical double-layer at the interface is influenced by the nature of the ions adsorbed. Small inorganic ions, such as sulfate anions and aluminium cations, are shown to absorb at the interface, with methanesulfonic acid absorbing strongly. The nature of ions adsorbed at the interface shifts the potential of zero charge (PZC) at the ITIES, which we propose in turn influences the kinetics of ion transfer.