Transfer of highly hydrophilic ions from water to nitrobenzene, studied by three-phase and thin-film modified electrodes.
Résumé
The study by voltammetry of hydrophilic ion transfers across the interface between an aqueous solution and an immiscible organic solvent is limited by the presence of supporting electrolytes in both phases. Such a study is impossible for ions having for water a higher affinity than ions of the electrolytes. Indirectly, methods based on modified solid electrodes can be used ; these are obtained by the deposition of an organic phase containing a molecule having redox properties, the modified electrode being in contact with an aqueous solution of the appropriate electrolyte. The three-phase electrode is very convenient for that purpose. However, this experimental tool also has its own limitation, due mainly to the redox species produced in the organic phase. The oxidized, or reduced, form of the redox molecule must have a very low affinity for water, as otherwise its transfer masks that of the ion under study. Ferrocene is almost useless because of the affinity of the ferrocenium cation for water, decamethylferrocene being a better choice. The present work illustrates how the use of lutetium bisphthalocyanines expands widely the possibilities, as these molecular sandwich complexes can be reduced as well as oxidized, the products of the reactions having a very low affinity for water. This made possible the determination of the Gibbs energy for the transfers of highly hydrophilic ions from water to nitrobenzene: Cl- (40 kJ mol-1), F- (57 kJ mol-1), H2PO4- (64 kJ mol-1). Nothing being really known about the transfer of F- or H2PO4- from water to organic solvents, these are the first values ever published. H+, OH- and HSO4- have also been studied, showing that these species, which have a poor affinity for nitrobenzene, are prone to association reactions with the reduced or oxidized forms of the lutetium bisphthalocyanine.