Surface excess charge

In this example, we have focused on the surface excess charge term in (5.18) and (5.19) the next example wUl show that the potential is able to modify not only the electrode structure, but also its composition. 

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

The extensive variable Q associated with the electrical potential + in Eqs. (15), (17), and (21) is the thermodynamic surface excess charge density, which is defined by... 

Provided that Ag

P2 are constant, and Tjjx is proportional to (c "). The observed nonlinearity at higher electrolyte concentrations [2] is probably due to a change in the inner-layer potential difference A"y>, with the surface excess charge density. The inner-layer potential difference (< 50 mV) was evaluated from the linear part of the Tjj vs. plot, and was found to depend on the nature of the... 

Kakiuchi and Senda [36] measured the electrocapillary curves of the ideally polarized water nitrobenzene interface by the drop time method using the electrolyte dropping electrode [37] at various concentrations of the aqueous (LiCl) and the organic solvent (tetrabutylammonium tetraphenylborate) electrolytes. An example of the electrocapillary curve for this system is shown in Fig. 2. The surface excess charge density Q, and the relative surface excess concentrations T " and rppg of the Li cation and the tetraphenylborate anion respectively, were evaluated from the surface tension data by using Eq. (21). The relative surface excess concentrations and of the d anion and the... 

When ions are present in a system that contains an interface, there will be a variation in the ion density near that interface that is described by a profile like that shown in Figure 7.13. The boundary we identify as the surface defines the surface excess charge, as explained in Chapter 7. Suppose that it was possible to separate the two bulk phases at this boundary in the manner shown in Figure 6.8. Then, each of the separated phases would carry an equal and opposite charge. These two charged portions of the interfacial region are called the electrical double layer. 

Let us return now to the main course of our discussion. It is of interest to consider just how accurately we need to determine y versus E. This depends on the purpose of the experiment. For example, if all we need to know is the PZC, within say, 5 mV, a simple measurement of drop time, described in Section 18.2 may suffice. In most cases, however, the purpose is to obtain surface excess, charge density, and even double-layer capacitance, as a function of potential. This task. 

When an electric field is applied to a system consisting of droplets of liquid phase B present in liquid A, electrocapillary forces can bring about the movement of these droplets. These forces can be used to recover trace metals and metal mattes from waste pyrometallurgical slags [47]. The driving force in thermocapillary flows is (dy/dT) i.e. the temperature dependence of the surface tension. Whereas in electrocapillarity the driving force is (dy/dE) where E is the electrical potential at constant chemical potential X, and (dy/dE) is equal [47] to the surface excess charge density (qi) at the droplet interface (Equation 18). 

Here Q is the thermodynamic surface excess charge density ... 

Figure 2.4. Schematic of a droplet with two separate phases, surface excess charge and internal electrically neutral. Analytes (A and B ) compete with electrolyte (E ) for the surface excess charge phase of the droplet. A postulate of the equilibrium partitioning model is that the analytes that are part of the surface excess charge phase are most likely to become gas-phase (analyzable) ions. Those ions that reside in the droplet interior will be paired with counterions and consequently will not be detectable by the mass spectrometer. (Reprinted fiom Ref. 77, with permission.)...

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