Ionic potential, positional variation

Where ions are disordered over a partially occupied array of energetically equivalent sites, their motion is diffusive. Fig. 3.6 illustrates the variation in ionic potential with intersite position for three possible situations in which a set of energetically equivalent sites are partially occupied by mobile ions. The partially occupied sites may (a) share common faces in a continuously connected network through the structure, (b) be separated from one another by an array of empty sites, or (c) be separated from one another by an array of sites that are occupied by the mobile ions. 

Implications of Mass Spectrometric Data for Radiation Chemistry. Apart from the positive identification of the ion-molecule reactions in ethyl chloride, the most significant observation from the mass spectrometric studies which has direct application to the radiolysis of this compound is the fact that at pressures greater than ca. 100 /a, essentially the only stable ion in this system is C4Hi0C1+. Therefore, the neutralization of ions as a potential contributor to radiolysis products will be important only for this ion. Moreover, this will hold true even if there are variations in the extent of primary fragmentation with increasing pressure. The radiolysis studies which will now be described assess the contribution of ionic processes to radiolytic yields and provide some indications as to the mode of neutralization of the stable ionic species in the ethyl chloride system. 

The local ohmic behavior is not in contradiction with the fact that Eq. (41) shows a nonlinear variation of 7 with the potential drop A >. At low potentials, fA4> 1, the DBL is practically nonpolarized (i.e. the ionic concentrations take their bulk values throughout the DBL) and the slope of the current-voltage curve is When increasing A, the slope of the current-voltage curve decreases owing to the development of concentration polarization. The effective conductivity varies then with position and this makes the overall system behavior to be nonohmic. 

Other potential applications are based on the general band schema of an ionic conductor. As recalled in another paper, under open circuit conditions, the band edges tend to be "horizontal" and the material can be regarded as a rigid framework, unaltered by variations in the position of the Fermi level (within the limits which correspond to a predominant ionic conductivity). In an electrochemical cell, the measured voltage is then simply related to the Fermi level positions with respect to the band edges Pej in the vicinity of the electrodes, by the expression... 

Some models try to explain the variations in surface charge and potential on the basis of a specific model of the interface [53-55], but some do not use such models [56]. Various models provide good fits of experimental adsorption curves as a function of pH and ionic strength. These models are not described here because of some of the remarks expressed above their ability to fit experimental data does not necessarily reflect their ability fully to describe the mechanisms involved in adsorption phenomena. The models usually do not take into account any structural specificity. The type of complexes formed, the position of ions and the characteristics of the interface are usually imposed by the parameters used to fit the data. 

The advantage of tliis method is that it takes into account the influence of the ionic strength on the charge of the particles. The model also simultaneously accounts for the variations in charge and electrokinetic potential with pH, for reasonable values of system parameters. Nevertheless, the position of the ions through the interface must be considered carefully. 

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