Diffusive-convective transport into a porous membrane. A comparison of theory and experiment using scanning electrochemical microscopy operated in reverse imaging mode.

Molecule transfer at the interface between a single ion-selective micropore and aqueous solutions is quantitatively investigated using scanning electrochemical microscopy operated in reverse imaging mode (SECM-RIM). Accumulation of two electroactive solute molecules, acetaminophen and ferrocenylmethyltrimethylammonium, at the pore/solution interface is observed when an electrical current is passed through the pore. Slow interfacial transfer of solute relative to the solvent as the solution is driven across the membrane by electroosmosis is responsible for solute accumulation. A theoretical expression for the concentration distribution of solute molecules above an individual pore opening is obtained by analytical solution of the convective-diffusive flux equation. The fluid velocity through the pore at constant electroosmotic force is determined by fitting the theoretical expression to SECM-RIM concentration profiles and is found, as anticipated, to be independent of the solute species and the bulk solute concentration. The results provide a theoretical basis for the SECM-RIM imaging of biological membranes as well as a general method for characterizing interfacial molecule/ion transfer kinetics.