Interfacial outer potential

Furthermore, as shown in Fig. 9.3, there arises between the two phases a difference in the outer potential Aipm - zpt — ip2, which is equivalent to what in physics called the contact potential. In electrochemistry we may call Arj>m the interfacial outer potential. The relation of Afj) to AipIJ2 is given by Eq. 9.6 ... 

Besides the Galvani potential, another important interfacial potential is the Volta potential, Aj I, sometimes ealled the eontaet potential. Aj I is the differenee of the outer potentials of the phases, whieh are in eleetroehemieal equilibrium with regard to the eharged speeies, i.e., ions or eleetrons. As for any two-phase eleetroehemieal system, ineluding the w/s system, it may be eharaeterized by the eommonly known relation ... 

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 ... 

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. 

Since u,(M) and xsat are characteristic of specific combinations of electrodes and electrolyte solutions, they are constant. For an electrode S3 tem, thereby, the electrode potential is a function of the interfacial potential A u/s only. The electrode potential, E, defined in Eqn. 4—14 corresponds to what is called the absolute electrode potential. The reference zero level of the absolute electrode potential is set at the outer potential of the electrolyte solution in which the electrode is immersed. 

Fig. 9-1. Potential energy profile for transferring metal ions across an interface of metal electrode M/S py. = metal ion level (electrochemical potential) x = distance fiom an interface au. = real potential of interfacial metal ions = real potential of hydrated metal ions - compact layer (Helmholtz layer) V = outer potential of solution S, curve 1 = potential energy of interfadal metallic ions curve 2 = potential energy of hydrated metal ions.

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. 

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... 

IHP) (the Helmholtz condenser formula is used in connection with it), located at the surface of the layer of Stem adsorbed ions, and an outer Helmholtz plane (OHP), located on the plane of centers of the next layer of ions marking the beginning of the diffuse layer. These planes, marked IHP and OHP in Fig. V-3 are merely planes of average electrical property the actual local potentials, if they could be measured, must vary wildly between locations where there is an adsorbed ion and places where only water resides on the surface. For liquid surfaces, discussed in Section V-7C, the interface will not be smooth due to thermal waves (Section IV-3). Sweeney and co-workers applied gradient theory (see Chapter III) to model the electric double layer and interfacial tension of a hydrocarbon-aqueous electrolyte interface [27]. 

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... 

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. 

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.)... 

To conclude this section, it should be emphasized that the steady state vol-tammograms described above are quite different from the first scan of the cyclic voltammetry of these systems. During the first polarization of these electrodes to low potentials, pronounced reduction processes of solution components are observed. As a result of these processes, a stable precipitate forms on the electrodes as insoluble films, and hence the above steady state voltammetric behavior reflects electrochemistry which is surface film controlled. The outer, solution side of these films is probably porous, leading to the high interfacial capacity which is reflected by the relatively high non-Faradaic currents which characterize these voltammograms. The next section describes in detail the initial voltammetric behavior of these systems and the surface film formation on the electrodes. 

Figure 14. Schematic representation of the electrode-solution interfacial region, (a) Helmholtz model (b) structured layer model (c) thermally disorganized layers and (d) resulting potential variations with distance of the electrode electrode potential Ojoi, solution potential OHP, outer Helmholtz plane (few A) Xq, extremity of the diffuse layer (few tens of A) x < xohP compact layer xqhp < x < x, diffuse layer.

The local interfacial impedance involves the potential of the electrode measured relative to a reference electrode o( ) located at the outer limit of the diffuse double layer. Thus, the local interfacial impedance is given by... 

Following Hueing et al., ° a notation is presented in Section 7.5.2 that addresses the concepts of a global impedance, which involved quantities averaged over the electrode surface a local interfacial imgedance, which involved both a local current density and the local potential drop V — Oo(r) across the diffuse double layer a local impedance, which involved a local current density and the potential of the electrode V referenced to a distant electrode and a local Ohmic impedance, which involved a local current density and potential drop Oo(r) from the outer region of the diffuse double layer to the distant electrode. The corresponding list of symbols is provided in Table 7.2. 

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. 

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