Probing Thermodynamic Aspects of Electrochemically Driven Ion-Transfer Processes Across Liquid|Liquid Interfaces: Pure versus Diluted Redox Liquids

Abstract

The voltammetry of the water-insoluble redox liquids para-N,N,N‘,N‘-tetraalkylphenylenediamines (TAPDs; A = butyl, hexyl, heptyl, octyl, and nonyl) at the TAPD|electrode|electrolyte interface is studied. The ion-transfer processes between the aqueous and the organic phaseconsequent of the necessity to maintain charge neutrality during electrochemical oxidation and reduction reactions in the organic phaseare elucidated as a function of the Gibbs free energy of transfer of the electrolyte anions, of the concentration of TAPD in the organic phase, and of the length of the alkyl chain of the phenylenediamines. Two conditions are considered: the voltammetry of microdroplets of the pure redox liquids and the voltammetry of the TAPDs dissolved in nitrobenzene, either deposited as microdroplets at an electrode surface. The oxidation of the neutral redox liquids, accompanied by the uptake of anions from the adjacent aqueous solution leads to the formation of ionic liquid phases, TAPD+X- and TAPD2+X-2 (X- = monovalent electrolyte anions). Despite the high concentration of redox centers in the microdroplets, the redox potentials of the TAPDs are exclusively determined by the Gibbs free energy of the electrochemically driven transfer of anions between the aqueous and the redox liquid phase. There is no evidence for ion pair formation of the TAPD+ and TAPD2+ with the transferred anions X- either in the pure redox liquid phase or in the nitrobenzene matrix. Depending on the Gibbs energy of transfer of the electrolyte anions two different reaction mechanisms are observed: At low ΔG values, a reversible transfer of electrolyte anions across the liquid/liquid interface as a consequence of the redox process takes place. However, anions with high transfer energies do not enter the organic phase. Instead, upon oxidation TAPD mono- and dications are transferred from the organic into the aqueous phase, which leads to the diminution of the voltammetric signals due to the loss of material from the droplets. From the voltammetric data, the Gibbs energy of transfer of the different TAPD cations and of the hexafluoroarsenate anion were derived for the water|nitrobenzene interface. Whereas the oxidation of the TAPD in the nitrobenzene droplets due to its low concentration does not measurably affect the properties of the nitrobenzene|water interface of the droplet, the oxidation of the pure redox liquids leads to a considerable change of the liquid|liquid interface upon ionic liquid formation. This change depends on the nature of the inserted anion: the more hydrophilic the inserted anion is, the less hydrophobic the ionic liquid becomes.