Proton-transfer kinetics at liquid-liquid interfaces

Proton transfer at electrochemical interfaces is fundamentally important for many areas of science and technology, yet kinetic measurements of this elementary step are often convoluted by inhomogeneous electrode surface structures. We show that facilitated proton transfer at the interface between two immiscible electrolyte solutions (ITIES) can serve as a model system to study proton transfer kinetics in the absence of defects found at solid|electrolyte interfaces. Diffusion-controlled micropipette voltammetry revealed that 2,6-diphenylpyridine (DPP) facilitated interfacial proton-transfer across the HCl(aq)|Trifluorotoluene interface and voltammetry at nanopipette-supported interfaces yielded activation-controlled ion transfer currents. Fitting quasireversible voltammograms to a mixed diffusive-kinetic model allow for the extraction of kinetic parameters k0 and α, which were equal to 3.0 +/- 1.8 cm/s and 0.3 +/- 0.2, respectively for DPP facilitated proton transfer. Finite element simulations highlighted regimes of direct proton transfer and sequential proton transfer, where the current divided between these two possible pathways was shown to favor direct PT when the neutral partitioning step DPP(org) to DPP(aq) was rate determining. Understanding the kinetics of ion transfer at the ITIES will be important in the development of general theories of ion transfer in electrochemical science and technology.