Thermodynamics of Ion Transfer Across the Liquid|Liquid Interface at a Solid Electrode Shielded with a Thin Layer of Organic Solvent

Abstract

Thermodynamics of ion transfer across the liquid|liquid interface at a solid electrode shielded with a thin layer of water-immiscible organic solvent is investigated. The shielded electrode has been made by using a commercial hydrophobic carbon filament material (LSG) as an electrode support and a specially synthesized water-insoluble electroactive polymer (polyphenotiazine, PPTA) as a mediator. Oxidation of PPTA dissolved in the organic layer is accomplished with anion transfer from the aqueous phase to compensate for the positive charge of the mediator. The formal potential of the electrode shielded with PPTA solution in nitrobenzene (NB) is proportional to the Gibbs free energy of anion transfer across the liquid|liquid interface. An enhanced potential window as compared to a “classic” four-electrode setup allowed direct determination of Gibbs free energy for fluoride ion transfer from water to NB. For less polar organic solvents the formal potential of a shielded electrode is mainly determined by the enthalpy of anion transfer across the liquid|liquid interface, whereas the entropy term of Gibbs free energy causes only a minor effect.