Understanding liquid|liquid ion transfer processes is important in particular for naturally occurring species such as carboxylates. In this study electrochemically driven mono-, di-, and tri-carboxylate anion transfer at the 4-(3-phenylpropyl)pyridine|aqueous electrolyte interface is investigated experimentally for a triple phase boundary system at graphite electrodes. The tetraphenylporphyrinato-Mn(III/II) redox system (Mn(III/II)TPP) dissolved in the water-immiscible organic phase (4-(3-phenylpropyl)pyridine) is employed for the quantitative study of the structure-Gibbs transfer energy correlation and the effects of the solution pH on the carboxylate transfer process. For di- and tri-carboxylates the partially protonated anions are always transferred preferentially even at a pH higher than the corresponding pK(a). COSMO-RS computer simulations are shown to provide a quantitative rationalisation as well as a powerful tool for predicting Gibbs free energy of transfer data for more complex functionalised carboxylate anions. It is shown that the presence of water in the organic phase has a major effect on the calculated Gibbs free energies.
A microfluidic double channel device is employed to study reactions at flowing liquid–liquid junctions in contact with a boron-doped diamond (BDD) working electrode. The rectangular flow cell is calibrated for both single-phase liquid flow and biphasic liquid–liquid flow for the case of (i) the immiscible N-octyl-2-pyrrolidone (NOP)–aqueous electrolyte system and (ii) the immiscible acetonitrile–aqueous electrolyte system. The influence of flow speed and liquid viscosity on the position of the phase boundary and mass transport-controlled limiting currents are examined. In contrast to the NOP–aqueous electrolyte case, the acetonitrile–aqueous electrolyte system is shown to behave close to ideal without ‘undercutting’ of the organic phase under the aqueous phase. The limiting current for three-phase boundary reactions is only weakly dependent on flow rate but directly proportional to the concentration and the diffusion coefficient in the organic phase. Acetonitrile as a commonly employed synthetic solvent is shown here to allow effective three-phase boundary processes to occur due to a lower viscosity enabling faster diffusion. N-butylferrocene is shown to be oxidised at the acetonitrile–aqueous electrolyte interface about 12 times faster when compared with the same process at the NOP–aqueous electrolyte interface. Conditions suitable for clean two-phase electrosynthetic processes without intentionally added supporting electrolyte in the organic phase are proposed
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