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 now through individual pores in porous ion-selective membr anes is described, The method is based on positioning a scanning elect rochemical microscope (SECM) tip directly above a membrane pore and de tecting 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 exp eriment by measuring the faradaic current at the SECM tip as a functio n of the iontophoretic current passed across the membrane. Electroosmo tic transport of hydroquinone through a permselective polymer (Nafion) , contained within similar to 50-mu m-radius pores of a 200-mu m-thick mica membrane, is used as a model system to demonstrate the analytica l method. Analysis of electroosmotic transport parameters obtained by SECM suggests that the average electroosmotic velocities of solvent (H 2O) and solute (hydroquinone) in the Nafion are significantly differen t, a consequence of the differences in their chemical interactions wit h the current-carrying mobile cations (Na+).