Kinetics of solute partitioning into ultrathin Nafion films on electrode surfaces: Theory and experimental measurement

A simple voltammetric procedure for measuring the kinetics of slow partitioning processes into thin films is described and applied to determine the transport of trimethylammonium ferrocinium, FA(+), into ultrathin Nafion Langmuir-Schaefer films. By recording cyclic voltammograms for the oxidation of FA(+) as a function of time of a Nafion modified electrode placed in a dilute solution containing FA(+), one can readily measure the uptake of FA(+) into a film. An analytical model for the charge accumulated in the film has been derived for the cases of (i) transport-controlled partitioning; (ii) transport with a kinetic barrier at the film/solution (f/s) interface; and (iii) pure kinetic control. Analysis of experimental data indicates that, in contrast to dropcast Nafion films, there is a significant kinetic barrier at the f/s interface to FA(+) partitioning. The methodology and theory described are generic and could be applied to other chemical systems and detection methods.