Curved interfaces, double layer

The electrical double layer at a pc-Ag/aqueous solution interface has been discussed by LeiMs et al., Valette and Hamelin, and Beck et al. in many papers.24 63 67146 223 272363-368 Detailed reviews have been given by Hamelin63 and Vorotyntsev74 in this work only a few comments will be added. First, the diffuse-layer minimum in the C,E curve was obtained363... 

The electrical double layer at pc-Zn/fyO interfaces has been studied in many works,154 190 613-629 but the situation is somewhat ambiguous and complex. The polycrystalline Zn electrode was found to be ideally polarizable for sufficiently wide negative polarizations.622"627 With pc-Zn/H20, the value of Eg was found at -1.15 V (SCE)615 628 (Table 14). The values of nun are in reasonable agreement with the data of Caswell et al.623,624 Practically the same value of Eff was obtained by the scrape method in NaC104 + HjO solution (pH = 7.0).190 Later it was shown154,259,625,628 that the determination of Eo=0 by direct observation of Emin on C,E curves in dilute surface-inactive electrolyte solutions is not possible in the case of Zn because Zn belongs to the group of metals for which E -o is close to the reversible standard potential in aqueous solution. 

In the region of a very good correspondence has been found between experimental and calculated C,E curves and this has been taken to indicate that the electrical double-layer structure conforms to the GCSG theory. Comparison of the ChE curves for Hg/TMU and Fe/TMU shows that the dependence of Cf on E is less pronounced for an Fe electrode than for Hg/TMU, and the values of Cf for Fe are remarkably lower than for Hg. The same is the case for Fe/DMF, DMAA, MPF, and HMPA interfaces.732-736... 

Measurement of the differential capacitance C = d /dE of the electrode/solution interface as a function of the electrode potential E results in a curve representing the influence of E on the value of C. The curves show an absolute minimum at E indicating a maximum in the effective thickness of the double layer as assumed in the simple model of a condenser [39Fru]. C is related to the electrocapillary curve and the surface tension according to C = d y/dE. Certain conditions have to be met in order to allow the measured capacity of the electrochemical double to be identified with the differential capacity (see [69Per]). In dilute electrolyte solutions this is generally the case. 

For the same reason, Ru(OOOl) modihcation by Pt monolayer islands results in a pronounced promotion of the CO oxidation reaction at potentials above 0.55 V, which on unmodified Ru(OOOl) electrodes proceeds only with very low reaction rates. The onset potential for the CO oxidation reaction, however, is not measurably affected by the presence of the Pt islands, indicating that they do not modify the inherent reactivity of the O/OH adlayer on the Ru sites adjacent to the Pt islands. At potentials between the onset potential and a bending point in the j-E curves, COad oxidation proceeds mainly by dissociative H2O formation/ OHad formation at the interface between the Ru(OOOl) substrate and Pt islands, and subsequent reaction between OHad and COad- The Pt islands promote homo-lytic H2O dissociation, and thus accelerate the reaction. At potentials anodic of the bending point, where the current increases steeply, H2O adsorption/OHad formation and COad oxidation are proposed to proceed on the Pt monolayer islands. The lower onset potential for CO oxidation in the presence of second-layer Pt islands compared with monolayer island-modified Ru(OOOl) is assigned to the stronger bonding of a double-layer Pt film (more facile OHad formation). 

The tip current depends on the rate of the interfacial IT reaction, which can be extracted from the tip current vs. distance curves. One should notice that the interface between the top and the bottom layers is nonpolarizable, and the potential drop is determined by the ratio of concentrations of the common ion (i.e., M ) in two phases. Probing kinetics of IT at a nonpolarized ITIES under steady-state conditions should minimize resistive potential drop and double-layer charging effects, which greatly complicate vol-tammetric studies of IT kinetics. 

Since the equations of state of the system are summarized by the curves in Figure 2, all interesting thermodynamic properties of the interface will have a simple representation in such a diagram. We shall consider the free energy of formation of a single charged surface and the interaction free energy due to the overlap of two identical planar double layers. 

The reaction path from the initial state to the final state of an elementary step is represented by the potential eneigy curves of the initial and final states of a reacting particle as shown in Fig. 7-6, where the reaction coordinate x denotes the position of a reaction particle moving across a compact double layer on the electrode interface. 

The surface tension was stated (Section 6.4.5), on general grounds, to be related to the surface excess of species in the interphase. The surface excess in turn represents in some way the structure of the interface. It follows therefore that electrocapillaiy curves must contain many interesting messages about the double layer at the electrode/ electrolyte interface. To understand such messages, one must learn to decode the electrocapillary data. It is necessary to derive quantitative relations among surface tension, excess charge on the metal, cell potential, surface excess, and solution composition. 

It appears that an electrified interface does not behave like a simple double layer. The parallel-plate condenser model is too naive an approach. Evidently some crucial secrets about electrified interfaces are contained in those asymmetric electrocapillaiy curves and the differential capacities that vary with potential. One has to think again. 

The vertical shift has arisen from the application of an absolute potential difference of d to a hypothetical interface, initially with zero potential difference across it, i.e., zhj) = 0. But the argument is valid for any change of potential across the interface. Thus, if the double-layer potential is initially Atye (i.e., the interface is at equilibrium) and then the potential is change to zf< ), the Morse curve for the initial state is shifted vertically through an energy F(Aty — Atye), or Ft],... 

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