On the theory of ion transfer rates across the interface of two immiscible liquids

Ion transfer across the interface of two immiscible liquids involves a mechanism for initiating desolvation from the first liquid, A, and concerted solvation by the second, B. In the present article a mechanism is considered in which this initiation is facilitated by the ion attaching itself to the tip of a solvent protrusion of B into A. (Protrusions have been observed in computer simulations and termed fingers or cones.) It is presumed that the most effective protrusion represents a balance between two opposing effects: the more convex the protrusion the less probable the ion/protrusion formation but also the less the resistance to extrusion of the intervening liquid between the ion and the surface. An analogy of the latter to hydrodynamics is noted, namely, the more convex the surface the less the frictional force it exerts on the approaching ion. After diffusion in coordinate and solvation space across the interfacial region, the final detachment of the ion from solvent A is assumed to occur from a protrusion of A into B. Existing data on ion transfer rates are discussed, including the question of diffusion vs kinetic control. Computer simulations that correspond to the experimental conditions in realistic liquids for measurement of the electrochemical exchange current rate constant k(0) are suggested. They can be used to test specific theoretical features. With a suitable choice of systems the need (and a major barrier to the simulations) for having a base electrolyte in such simulations can be bypassed. An experiment for the real-time observation of an ion leaving the interface is also suggested. (C) 2000 American Institute of Physics. [S0021-9606(00)50628-X].