Imaging Molecular Transport in Porous Membranes. Observation and Analysis of Electroosmotic Flow in Individual Pores Using the Scanning Electrochemical Microscope

A method of determining absolute rates of diffusion and electroosmotic convective flow through individual pores in porous ion-selective membranes is described. The method is based on positioning a scanning electrochemical microscope (SECM) tip directly above a membrane pore and detecting electroactive molecules as they emerge from the pore. Absolute diffusive and electroosmotic fluxes, electroosmotic drag coefficient, convective velocity, and pore radius can be evaluated in a single experiment by measuring the faradaic current at the SECM tip as a function of the iontophoretic current passed across the membrane. Electroosmotic transport of hydroquinone through a permselective polymer (Nafion), contained within ∼50-μm-radius pores of a 200-μm-thick mica membrane, is used as a model system to demonstrate the analytical method. Analysis of electroosmotic transport parameters obtained by SECM suggests that the average electroosmotic velocities of solvent (H 2 O) and solute (hydroquinone) in the Nafion are significantly different, a consequence of the differences in their chemical interactions with the current-carrying mobile cations (Na + ).