On the theory of ion transfer rates across the interface of two immiscibleliquids

Ion transfer across the interface of two immiscible liquids involves a mech anism for initiating desolvation from the first liquid, A, and concerted so lvation 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 ef fects: 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 hydrody namics is noted, namely, the more convex the surface the less the frictiona l 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. Exis ting data on ion transfer rates are discussed, including the question of di ffusion vs kinetic control. Computer simulations that correspond to the exp erimental conditions in realistic liquids for measurement of the electroche mical exchange current rate constant k(0) are suggested. They can be used t o test specific theoretical features. With a suitable choice of systems the need (and a major barrier to the simulations) for having a base electrolyt e in such simulations can be bypassed. An experiment for the real-time obse rvation of an ion leaving the interface is also suggested. (C) 2000 America n Institute of Physics. [S0021-9606(00)50628-X].