Multiscale dynamics in ionic media

Transport coefficients in charged media exhibit strong variations, accordin g to the conditions of displacement of the particles. Electrical transport, characterized by the simultaneous displacement of positive and negative ch arges in opposite directions obeys Ohm's law, but its variation with concen tration (non-ideality), depends on several types of interactions, whose tim e of establishment varies from picosecond to nanosecond. Several diffusion processes can occur: mutual diffusion, where ions move simultaneously in th e same direction, keeping local electroneutrality, and self diffusion where individual ionic particles move separately. The variation of diffusion coe fficients with concentration depends on non-ideality factors analogous to t hose occurring in conductance, and their experimental evidence is facilitat ed by the availability of experimental techniques owing different character istic times of observation. This phenomenon is particularly noticeable for self-diffusion coefficients, where the dynamical processes can be observed from the picosecond range (neutron quasi-elastic scattering), to millisecon d (NMR) and to hour scale (radioactive tracers). The results are especially enhanced for porous charged media like ion exchanging membranes (nafions). Those results are be explained here theoretically in the framework of conti nuous solvent model theories (brownian dynamics) and experimentally in the study of self-diffusion in nafion membranes. (C) 2001 Elsevier Science B.V. All rights reserved.