Point mobile ions

Assign A value of 1 or 0 to each grid point to identify points that are or are not accessible to mobile ions, respectively. 

It will be recalled that in Fig. 28 we found that for the most mobile ions the mobility has the smallest temperature coefficient. If any species of ion in aqueous solution at room temperature causes a local loosening of the water structure, the solvent in the co-sphere of each ion will have a viscosity smaller than that of the normal solvent. A solute in which both anions and cations are of this type will have in (160) a negative viscosity //-coefficient. At the same time the local loosening of the water structure will permit a more lively Brownian motion than the ion would otherwise have at this temperature. Normally a certain rise of temperature would be needed to produce an equal loosening of the water structure. If, in the co-sphere of any species of ion, there exists already at a low temperature a certain loosening of the water structure, the mobility of this ion is likely to have an abnormally small temperature coefficient, as pointed out in Sec. 34. 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

We may point out parenthetically that it is usually customary to attribute the expansion to electrostatic repulsions between the net (positive) charges on the polymer molecule which are uncompensated due to loss of counter-ions to the outer solution. It may be shown that the osmotic force owing to the excess of mobile ions within the molecule must be equal to the force of electrostatic repulsion when the molecule is in equilibrium with its surroundings. Hence either point of view is equally satisfactory in principle. The two are, of course, mutually related no net charge would develop in the molecule were it not for the mobile counter-ions, and no excess of mobile ions would be retained to exert an osmotic pressure if it were not for the charges on them. 

A low AH for a cooperative cluster rotation allows excitation of a cluster of atoms from normal to saddle-point positions. Such an excitation may, in turn, lower the energy of the saddle-point sites relative to the normal sites, thus effectively introducing a AHg(T) that collapses in a smooth transition. At temperatures T> T, the mobile ions become disordered over the normal and saddle-point sites. Such a situation appears to be illustrated by stoichiometric LijN and PbFj (Goodenough, 1984). 

Stern combined the ideas of Helmholtz and that of a diffuse layer [64], In Stern theory we take a pragmatic, though somewhat artificial, approach and divide the double layer into two parts an inner part, the Stern layer, and an outer part, the Gouy or diffuse layer. Essentially the Stern layer is a layer of ions which is directly adsorbed to the surface and which is immobile. In contrast, the Gouy-Chapman layer consists of mobile ions, which obey Poisson-Boltzmann statistics. The potential at the point where the bound Stern layer ends and the mobile diffuse layer begins is the zeta potential (C potential). The zeta potential will be discussed in detail in Section 5.4. 

Although it is not as severe in PEVD systems as in aqueous electrochemical systems in which various kinds of mobile ions are present in the electrolytes, it should be pointed out that, in the presence of reactants at the sink electrode surface, other electrochemical reactions might also take place in parallel with the desired one at the sink side. If side reactions exist, usually such parallel reactions contributions to the measured current are not easy to quantify. If it is desired to use current to monitor the reaction and product formation in PEVD, side reactions should be eliminated or at least controlled. Fortunately, only one ionic species is usually mobile in a solid electrochemical cell because of the nature of the solid electrolyte. As long as the vapor phase is properly controlled, usually one electrode reaction is predominant over a wide range of PEVD applied potentials. Virtually 100% current efficiency for product formation can be expected. 

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