Potential-dependent adsorption and partitioning of ionic components at a liquid ∣ liquid interface

Abstract

Abstract In adsorption at the interface between two immiscible solutions, the adsorption of a substance from one phase is related to its adsorption from the other, when the substance is soluble in both solution phases, because of the presence of the simultaneous partition equilibrium of the substance. This interdependence of the adsorption and partition gives unique features to the effect of the phase-boundary potential on these three processes, particularly when the surface-active substance is ionic. The electrical part of the adsorption Gibbs energy is a fraction of the electrical driving force for the ion transfer from one phase to the other. A simple yet general model proposed for the dependence of the adsorption Gibbs energy on the phase-boundary potential takes a form similar to the Butler–Volmer equation in electrode kinetics and demonstrates that the adsorption becomes a maximum at the potential around the standard ion-transfer potential of the surface active ions. The link between the adsorption and partition has been illustrated for the partitioning of an ionic component between two immiscible solutions having a certain volume ratio. The implications of this link in current–potential curves of ion transfer across the interface in the presence of adsorption have been elucidated.