Probing Ion Transfer at the Liquid/Liquid Interface by Scanning Electrochemical Microscopy (SECM)

A new way of using the scanning electrochemical microscope (SECM) to probe ion transfer (IT) reactivity of the liquid/liquid and liquid/membrane interfaces is described. A micropipet filled with solvent (e.g., water) immiscible with the outer solution (e.g., organic) serves as an SECM tip. The tip current is due to transfer of an ion between the pipet and the outer solution. When such a tip approaches a macroscopic liquid/liquid or liquid/membrane interface, the IT process depletes the concentration of the transferred ion near the phase boundary. If the second phase (e.g., another aqueous solution) contains the same ion, the depletion results in the IT across the interface. The tip current depends on the rate of the interfacial IT reaction, which can be extracted from the tip current vs distance curves. By scanning the pipet tip over the phase boundary the topographic images and maps of IT reactivity of the interface can be obtained, e.g., high-resolution feedback mode images of micrometer-sized pores in polycarbonate membranes. The feedback-mode SECM theory previously developed for metal tips surrounded by a relatively thick insulating glass sheath is not fully applicable to the pipet tips. A family of theoretical tip current vs distance curves for pipet tips with different insulator thicknesses is obtained by numerical solution of partial differential equations using a commercial adaptive-grid program, PDEase2. The developed theory is presented in the form of two analytical approximations suitable for diffusion-controlled heterogeneous reaction and for a blocking interface. The possibilities of measuring fast IT kinetics are also discussed.