Chemical migration, driving force

The most important driving forces for the motion of ionic defects and electrons in solids are the migration in an electric field and the diffusion under the influence of a chemical potential gradient. Other forces, such as magnetic fields and temperature gradients, are commonly much less important in battery-type applications. It is assumed that the fluxes under the influence of an electric field and a concentration gradient are linearly superimposed, which... 

Chemical potential is analogous to the temperature gradient that drives heat flow or the cell emf potential that drives electrical current flow, in that it provides the driving force for diffusive migration of chemical species from one region of the system to another. 

Donnan dialysis is a membrane separation process that uses ion-selective membranes to prevent the flow of certain ions from one solution to another. A schematic of the process is presented in Figure 29.8. When a salt solution is separated from its corresponding acid by a cation-exchange membrane, the anions are excluded from the membrane, whUe the cations are redistributed across the membrane to attain Donnan equilibrium. By changing the salt solution periodically, it would be possible to shift the equilibrium favorably to effect simultaneous neutralization of acid on one side of the membrane (feed compartment) and acid recovery on the other side (receiver compartment). The driving force for ion migration is the chemical potential gradient for the cation across the membranes. 

Electrodes apply the driving force for ion migration and are therefore critical components of the system. They serve as the bridge between the electric circuit and the two reservoirs, and perform both electrical and chemical functions. During iontophoretic therapy, electrodes undergo sustained electrochemical reactions and thus the migration of reactants is a critical functional consideration. In this aspect, electrodes used in iontophoretic devices are different from those of most other medical electrodes. For example, medical... 

Electrochemical transport in clay-electrolyte media is complicated because of the coupling between multiple physical and chemical processes, where bulk liquid and surface processes occur simultaneously. In the bulk, the driving force for ion motion comes from two fundamentally different sources electroosmosis and ion migration. At electrochemical surfaces, such as that of a colloid or clay particle, double layer charging, electrochemical reactions, and surface conduction can take place simultaneously (Kortiim and Bockris, 1951). 

If voltages Vp and V2 simulate the chemical driving forces acting on the ions during scaling, the resistors R3 and R2 simulate the scale s resistance to cation and anion migration. Its impedance to electron flow is represented by the single resistor R3 which is shown connected all the way across the scale thickness. 

Being charged particles, ions will respond to both chemical and electrical-potential gradients, which together provide the net driving force for ion migration. 

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