Electrochemical study of the thermodynamics and kinetics of hydrophilic ion transfers across water | n-octanol interface

Abstract

Thermodynamics and kinetics of hydrophilic ion transfers across water|n-octanol (W|OCT) interface have been electrochemically studied by means of novel three-phase and thin-film electrodes. Three-phase electrodes used for thermodynamics measurements comprise edge plane pyrolytic graphite, the surface of which was partly modified with an ultrathin film of OCT, containing hydrophobic lutetium bis(tetra-tert-butylphthalocyaninato) (Lu[tBu₄Pc]₂) as a redox probe. The transfers of anions and cations from W to OCT were electrochemically driven by reversible redox transformations of Lu[tBu₄Pc]₂ to chemically stable lipophilic monovalent cation ${\left( {{\text{Lu}}{\left[ {t{\text{Bu}}_{4} {\text{Pc}}} \right]}^{ + }_{2} } \right)}$ and anion ${\left( {{\text{Lu}}{\left[ {t{\text{Bu}}_{{\text{4}}} {\text{Pc}}} \right]}^{{\,\, - }}_{2} } \right)}$, respectively. Upon reduction of Lu[tBu₄Pc]₂, the transfers of alkali metal cations from W to OCT have been studied for the first time, enabling estimation of their Gibbs transfer energies. For kinetic measurements, a thin-film electrode configuration has been used, consisting of the same electrode covered completely with a thin layer of OCT that contained the redox probe and a suitable electrolyte. Combining the fast and sensitive square-wave voltammetry with thin-film electrodes, the kinetics of ${\text{ClO}}^{{\,\, - }}_{4}$, ${\text{NO}}^{{\,\,{\text{ - }}}}_{{\text{3}}}$, and Cl⁻ transfers have been estimated.