Interfacial inner potential

The presence of an electrical potential drop, i.e., interfacial potential, across the boundary between two dissimilar phases, as well as at their surfaces exposed to a neutral gas phase, is the most characteristic feature of every interface and surface electrified due to the ion separation and dipole orientation. This charge separation is usually described as the formation of the ionic and dipolar double layers. The main interfacial potential is the Galvani potential (termed also by Trasatti the operative potential), which is the difference of inner potentials (p and of both phases. It is a function only of the chemical... 

An electrostatic potential difference, called the inner potential difference, arises across the interface of two contacting phases. This inner potential difference consists of a potential gA/ac) due to an interfacial charge (charge on both sides of the interface), Oa/b> aiid a potential gj /snip) due to an interfacial dipole, dipA/B, as shown in Eqn. 4-3 ... 

The inner potential difference between two contacting phases is cafied in electrochemistry the Galvani potential difference, and the outer potential difference is called the Volta potential difference. The outer potential difference corresponds to what is called the contact potential between the two phases. We call, in this test, the inner potential difference across an interface the interfacial potential. 

The outer potential difference between two contacting phases can be measured because it is a potential difference between two points in the same vacuum or gas phase outside the free surfaces of the two phases. On the other hand, the inner potential difference can not be measured, because the potential measuring probe introduces its interfacial potential that differs with the two phases and thus can not be canceled out this gives rise to an unknown potential in the potential measurement. 

The interface at which the interfacial charge,, is zero is called the interface of zero charge or the zero charge interface. The inner potential difference across the zero charge interface is determined by the interfacial dipole only, thus, it is characteristic of the contacting interface of the two phases as indicated in Eqn. 4-6 ... 

It follows from Eqn. 4—13 that the electron level o u/av) in the electrode is a function of the chemical potential p.(M) of electrons in the electrode, the interfacial potential (the inner potential difference) between the electrode and the electrolyte solution, and the surface potential Xs/v of the electrolyte solution. It appears that the electron level cx (ii/a/v) in the electrode depends on the interfacial potential of the electrode and the chemical potential of electron in the electrode but does not depend upon the chemical potential of electron in the electrolyte solution. Equation 4-13 is valid when no electrostatic potential gradient exists in the electrolyte solution. In the presence of a potential gradient, an additional electrostatic energy has to be included in Eqn. 4-13. 

Remember that the interfacial potential is a difference between the inner potential of the electrode material and the potential established in the bulk of the solution. An iR drop can cause the latter to change. One way to minimize this problem is to construct the cell with an auxiliary electrode(s) that is directly across from the working electrodes, resulting in a very short current path and, therefore, a very small iR drop. Multiple-electrode transducers constructed in this way minimize cross-talk between different electrodes. 

A distinguishing aspect in electrode kinetics is that the heterogeneous rate constants, kred and kox, can be controlled externally by the difference between the inner potential in the metal electrode (V/>M) and in solution (7/>so1) that is, through the interfacial potential difference E = electrode setup (typically, a three-electrode arrangement and a potentiostat), the E-value can be varied in order to distort the electrochemical equilibrium and favor the electro-oxidation or electro-reduction reactions. Thus, the molar electrochemical Gibbs energy of reaction Scheme (l.IV), as derived from the electrochemical potentials of the reactant and product species, can be written as (see Eqs. 1.32 and 1.33 with n = 1)... 

Outer Potential The potential just outside the interface bounding a specified phase. Also termed the Volta potential. The difference in outer (Volta) potentials between two phases in contact is equal to the surface or interfacial potential between them. See also Inner Potential. 

Sometimes it is useful to break the inner potential into two components called the outer (or Volta) potential, if/, and the surface potential, x- Thus, (f) = if/ + x- There is a large, detailed literature on the establishment, the meaning, and the measurement of interfacial potential differences and their components. See references 23-26. Although silver chloride is a separate phase, it does not contribute to the cell potential, because it does not physically separate silver from the electrolyte. In fact, it need not even be present one merely requires a solution saturated in silver chloride to measure the same cell potential. 

These two charge densities and the inner potential provide a complete description of the interfacial region according to DDL theory. 

Under separate headings, the nature and origin of other membrane potentials diffusion (concentration) potential adsorption (surface or interfacial) potential, distribution (outer) potential, Galvani (inner) potential, and Gouy (Dorman) potential will be considered. These potentials are... 

Petek and co-workers have investigated ultrafast interfacial inner sphere PCET dynamics, where the presence of strong potential gradients will subject electrons and protons to opposite forces within a spatial region.This is the case for water oxidation on semiconductor photoelectrodes, and therefore with potential impact in the context of artificial photosynthesis. 

Two types of EDL are distinguished superficial and interfacial. Superficial EDLs are located wholly within the surface layer of a single phase (e.g., an EDL caused by a nonuniform distribution of electrons in the metal, an EDL caused by orientation of the bipolar solvent molecules in the electrolyte solution, an EDL caused by specific adsorption of ions). Tfie potential drops developing in tfiese cases (the potential inside the phase relative to a point just outside) is called the surface potential of the given phase k. Interfacial EDLs have their two parts in dilferent phases the inner layer with the charge density in the metal (because of an excess or deficit of electrons in the surface layer), and the outer layer of counterions with the charge density = -Qs m in the solution (an excess of cations or anions) the potential drop caused by this double layer is called the interfacial potential... 

The description of the ion transfer process is closely related to the structure of the electrical double layer at the ITIES [50]. The most widely used approach is the combination of the BV equation and the modified Verwey-Niessen (MVN) model. In the MVN model, the electrical double layer at the ITIES is composed of two diffuse layers and one ion-free or inner layer (Fig. 8). The positions delimiting the inner layer are denoted by X2 and X2, and represent the positions of closest approach of the transferring ion to the ITIES from the organic and aqueous side, respectively. The total Galvani potential drop across the interfacial region, AgCp = cj) — [Pg.545]

Figure 22. Variations of the electrical potential and the interfacial tension. The aqueous phases in the inner cylinder are ...

Obviously, the interfacial charge differs from the initial (before contact) surface charges oa and ob (oa/b " Oa + ob) and the interfacial dipole dipA/s is not the same as the arithmetic sum of the initial surface dipoles igtjaibt) XA(dip)-Xadip)) Thus, it follows that both the inner and the outer potential differences, A( >a/b and Ai a , between the two contacting phases are not the same as those and Atp A/B before the contact. As a result, Eqn. 4-2 yields Eqn. 4-4 ... 

As the field intensity in the inner Helmholtz layer becomes extremely high, the field intensity E in the outer Helmholtz layer is reversed as shown in Fig. 5-29. Figure 5-30 illustrates the potential profile across the interfacial double layer of a mercury electrode in an aqueous chloride solution this result was obtained by calculations at various electrode potentials ranging fi om negative (cathodic) to positive (anodic) potentials. 

Since the electron transfer of the interfacial redox reaction, + cm = H.a> on electrodes takes place between the iimer Helmholtz plane (adsorption plane at distance d ) and the electrode metal, the ratio of adsorption coverages 0h,j/ in electron transfer equilibrium (hence, the charge transfer coefficient, 6z) is given in Eqn. 5-58 as a function of the potential vid /diOMn across the inner Helmholtz layer ... 

In electrode kinetic studies, reactant concentrations are, in general, in the millimolar range and double layer contributions for such low ionic concentrations may become very important. If excess of inert or supporting electrolyte is used, the relative variation in the ionic concentration at the double layer due to the electrochemical reaction is at a minimum at high concentration of an inert z z electrolyte, most of the interfacial potential drop corresponds to the Helmholtz inner layer and variations of A02 with electrode potential are small (Fig. 3). In addition, use of supporting electrolyte prevents the migration of electroactive ionic species from becoming important and also reduces the ohmic overpotential. 

The changes in the potential profile of the interfacial region because specific adsorption do indeed affect the electrode kinetics of charge transfer processes, particularly when these have an inner sphere character [13, 26] (see Fig. 1.12). When this influence leads to an improvement of the current response of these processes, the global effect is called electrocatalysis. ... 

For the outer-sphere Co(NH3)63+ reduction, the SERS and current-potential data are closely compatible in that the SERS intensities drop sharply at potentials towards the top of the voltammetric wave where the overall interfacial reactant concentration must decrease to zero. Some discrepancies between the SERS and electrochemical data were seen for the inner-sphere Cr(NH3)sBr2 and Cr(NH3)sNCS2+ reductions, in that the SERS intensities decrease sharply to zero at potentials closer to the foot of the voltammetric wave. This indicates that the inner-sphere reactant bound to SERS-active sites is reduced at significantly lower overpotentials than is the preponderant adsorbate. (15)This suggests that SERS-active surface sites might display unusual electrocatalytic activity in some cases. 

FIGURE 5.8 Theoretical analysis of an amphifunctionally electrified interface (a) effect of pH and externally applied potential on the interfacial double-layer potential and (b) potential necessary to apply across the interface to reach the isoelectric point as a function of pH. Electrolyte concentration, 0.01 M protolytic site density, 3 x 1018/m2 point of zero charge, pHPZC = 4.5 inner-layer capacitance, 0.05 F/m2 outer-layer capacitance, 0.30 F/m2. (Adapted from Duval, J., et al., Langmuir, 17, 7573, 2001.)... 

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