Self-Diffusion of Ions at Different Time Scales in a Porous and Charged Medium: The Nafion Membrane

The self-diffusion coefficients of N(CH 3 ) 4 + ions in a cation-exchange membrane, the Nafion, have been measured at different time scales. The time range extends from the picosecond with neutron quasi-elastic scattering experiments to the second with NMR experiments and tens of seconds with radiotracer experiments. At short times, the values of self-diffusion coefficient in the membrane are equal to those in nonconfined solution; the diffusion process is governed by ion−solvent collisions. The NMR results present two different time scales: for short times (<1 s), the self-diffusion coefficient depends both on time and on electrolyte concentration; for long times, these two dependencies disappear. The radiotracer method confirms the NMR results at long time scales: the diffusion does not depend on electrolyte concentration.