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 (H2O) 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+).