Multiscale dynamics in ionic media

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Abstract

Transport coefficients in charged media exhibit strong variations, according to the conditions to displacement of the particles. Electrical transport, characterized by the simultaneous displacement of positive and negative charges in opposite directions obeys Ohm's law, but its variation with concentration (non-ideality), depends on several types of interactions, whose time of establishment varies from picosecond to nanosecond. Several diffusion processes can occur: mutual diffusion, where ions move simultaneously in the same direction, keeping local electroneutrality, and self diffusion where individual ionic particles move separately. The variation of diffusion coefficients with concentration depends on non-ideality factors analogous to those occurring in conductance, and their experimental evidence is facilitated by the availability of experimental techniques owing different characteristic 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 millisecond (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 continuous solvent model theories (brownian dynamics) and experimentally in the study of self-diffusion in nafion membranes.

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