Thermodynamic aspects of the electron transfer across the interface between water and a hydrophobic redox-active ionic liquid

Ion and electron transfer reactions at the interface between a redox-active ionic liquid (IL), (ferrocenylmethyl)dodecyldimethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (FcMDDATFPB), and water were investigated by cyclic voltammetry (CV) using the FcMDDATFPB membrane cell. The redox-active IL was shown to exhibit the same ion selectivity as the structurally similar but redox-inactive tridodecylmethylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate. Ion transfer potentials evaluated from CV measurements allowed establishing the scale of absolute potential differences for the interfacial oxidation of FcMDDA+ with the aqueous redox species including IrCl62−, Fe(CN)63−, Mo(CN)83−, ICl2− or IBr2−. In order to rationalize experimental data, the theory of CV was developed for an electron transfer at one membrane interface that is coupled with the ion transfer at the other membrane interface. Electron transfer mechanism was supported by the linear correlation between the reversible half-wave potentials of the electron transfer across the water IL interface and the standard electrode potentials for the aqueous redox couples. A notable difference between the formal electron transfer potentials for the water IL and water 1,2-dichloroethane interfaces pointed to strong ion–ion interactions within IL affecting the ion activity coefficient term.