Transient Ion-Pair Dynamics Induced by External Electric Field in Electrolytic Cell with Blocking Electrodes

Abstract

The conventional equations describing the transport of free ions in electrolytes have been extended to take into account the behavior of the ion pairs. The space and time dependencies of the ion-pair density and ion-pair fluxes in the electrolytic cell with blocking electrodes, when disturbed by a stepwise, external electric field, have been examined by numerical calculations. Solutions are obtained for different regimes: weak and strong electric fields, symmetrical electrolytes (equal ion valence and electrically neutral ion pairs), unsymmetrical electrolytes (different ion valence and charged ion pairs), different diffusivity (mobility) of the free ions, etc. The nonlinear, competitive processes defining the time evolution of the ion pairs have been analyzed and discussed. For symmetrical electrolytes, the ion-pair densities have a minimum at the middle of the cell and maxima at the two electrodes, whereas the neutral ion-pair flux is dominated by diffusion processes. For unsymmetrical electrolytes, the ion-pair density has a maximum at the electrode with higher polarization, whereas the charged ion-pair flux is dominated by migration. The ion-pair fluxes in a strong electric field tend to reverse direction in space and time. The oscillations occur in a short time interval, slightly delayed from the initial moment when the electric field is switched on.