Proton-Transfer Kinetics at Liquid–Liquid Interfaces

Proton transfer at electrochemical interfaces is fundamentally important across science and technology, yet kinetic measurements of this elementary step at electrode|electrolyte interfaces are convoluted with other electron-transfer steps and by inhomogeneous electrode surfaces. We use facilitated proton transfer at the interface between two immiscible electrolyte solutions (ITIES) as a platform to study proton-transfer kinetics in the absence of interfacial electron transfer and without the defects at solid|electrolyte interfaces. Diffusion-controlled micropipette voltammetry revealed that 2,6-diphenylpyridine (DPP) facilitates proton transfer across the HCl(aq)|trifluorotoluene interface, while voltammetry at nanopipette-supported interfaces yielded activation-controlled ion-transfer currents. We extract kinetic parameters <i>k</i>_app⁰ and α_app, 3.0 ± 1.8 cm/s and 0.3 ± 0.2, respectively, for DPP-facilitated proton transfer by fitting quasi-reversible voltammograms to a mixed diffusive-kinetic model. 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 proton transfer when the neutral partitioning step DPP(org) → DPP(aq) was rate-determining. Atomistic molecular-dynamics simulations were used to compute the free energy change to move DPP and its protonated analogue within, and across, the liquid|liquid interface. The most-likely location for proton transfer is predicted to be in the surface region where significant interpenetration of the two liquids occurs. Understanding the kinetics of ion transfer at the ITIES illustrated here is important in the development of general theories of ion transfer in electrochemical science and technology.