IMAGING MOLECULAR TRANSPORT ACROSS MEMBRANES
I. INTRODUCTION Traditional models of molecular transport in porous membranes are based on measurements of spatially averaged fluxes, incorporating details of the membrane structure obtained from independent experimentation and theory (1). While such transport models are successful in predicting the overall flux across a membrane, the microscopic features incorporated within the models are often left untested by direct measurements. In this chapter, we describe spatially resolved measurements of the transport of molecular species across porous membranes using scanning electrochemical microscopy (SECM) (26). Absolute values of the rate of transport of a chemical species through a single pore can be directly measured using SECM, and "images of the molecular flux" across a porous membrane can be obtained with submicrometer resolution. SECM provides a means to identify microscopic structures in the membrane that are associated with molecular transport paths and mechanisms.