Electrocapillary maximum

It should be noted that the capacity as given by C, = a/E, where a is obtained from the current flow at the dropping electrode or from Eq. V-49, is an integral capacity (E is the potential relative to the electrocapillary maximum (ecm), and an assumption is involved here in identifying this with the potential difference across the interface). The differential capacity C given by Eq. V-50 is also then given by... 

Properties of the Electrical Double Layer at the Electrocapillary Maximum... 

At the electrocapillary maximum, dyIdE is zero and hence a is zero. There may still be adsorption of ions, but in equal amounts, that is, T+ = F (for a 1 1 electrolyte). 

Fig. V-13. Composite x/ln a curve for 3-pentanol. The various data points are for different E values each curve for a given E has been shifted horizontally to give the optimum match to a reference curve for an E near the electrocapillary maximum. (From Ref. 134.)...

The electrical double layer in 1-PrOH + NaC104 has been studied by Protskaya etal,320 and the value of E from the potential of the electrocapillary maximum was equal to -0.31 V (SCE in H20). 

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

The reduction wave of peroxydisulphate at dme starts at the potential of the anodic dissolution of mercury. The current-potential curve exhibits certain anomalous characteristics under various conditions. At potentials more negative than the electrocapillary maximum, a current minimum can be observed this is due to the electrostatic repulsion of the peroxydisulphate ion by the negatively charged electrode surface. The current minimum depends on the concentration and nature of the supporting electrolyte, and can be eliminated by the adsorption of capillary active cations of the type NR4. ... 

Girault and Schiffrin [6] and Samec et al. [39] used the pendant drop video-image method to measure the surface tension of the ideally polarized water-1,2-dichloroethane interface in the presence of KCl [6] or LiCl [39] in water and tetrabutylammonium tetraphenylborate in 1,2-dichloroethane. Electrocapillary curves of a shape resembling that for the water-nitrobenzene interface were obtained, but a detailed analysis of the surface tension data was not undertaken. An independent measurement of the zero-charge potential difference by the streaming-jet electrode technique [40] in the same system provided the value identical with the potential of the electrocapillary maximum. On the basis of the standard potential difference of —0.225 V for the tetrabutylammonium ion transfer, the zero-charge potential difference was estimated as equal to 8 10 mV [41]. 

Samec et al. [15] used the AC polarographic method to study the potential dependence of the differential capacity of the ideally polarized water-nitrobenzene interface at various concentrations of the aqueous (LiCl) and the organic solvent (tetrabutylammonium tetra-phenylborate) electrolytes. The capacity showed a single minimum at an interfacial potential difference, which is close to that for the electrocapillary maximum. The experimental capacity was found to agree well with the capacity calculated from Eq. (28) for 1 /C,- = 0 and for the capacities of the space charge regions calculated using the GC theory,... 

Within the low-capacity region between the adsorption/desorption peaks around the electrocapillary maximum (ecm see Fig. 3.18), the depression of the base current is greatest because of maximum adsorption of the surfactant in that area. 

Double integration with respect to EA yields the surface excess rB+ however, the calculation requires that the value of this excess be known, along with the value of the first differential 3TB+/3EA for a definite potential. This value can be found, for example, by measuring the interfacial tension, especially at the potential of the electrocapillary maximum. The surface excess is often found for solutions of the alkali metals on the basis of the assumption that, at potentials sufficiently more negative than the zero-charge potential, the electrode double layer has a diffuse character without specific adsorption of any component of the electrolyte. The theory of diffuse electrical double layer is then used to determine TB+ and dTB+/3EA (see Section 4.3.1). 

The quantity dyl3 In a2 at the potential of the electrocapillary maximum is of basic importance. As the surface charge of the electrode is here equal to zero, the electrostatic effect of the electrode on the ions ceases. Thus, if no specific ion adsorption occurs, this differential quotient is equal to zero and no surface excess of ions is formed at the electrode. This is especially true for ions of the alkali metals and alkaline earths and, of the anions, fluoride at low concentrations and hydroxide. Sulphate, nitrate and perchlorate ions are very weakly surface active. The remaining ions decrease the surface tension at the maximum on the electrocapillary curve to a greater or lesser degree. 

The adsorption of ions is determined by the potential of the inner Helmholtz plane 0n while the shift of Epzc to more negative values with increasing concentration of adsorbed anions is identical with the shift in 0(m). Thus, the electrocapillary maximum is shifted to more negative values on an increase in the anion concentration more rapidly than would follow from earlier theories based on concepts of a continuously distributed charge of adsorbed anions over the electrode surface (Stern, 1925). Under Stern s assumption, it would hold that 0(m) = 0X (where, of course, 0X no longer has the significance of the potential at the inner Helmholtz plane). 

As mentioned above, the quantity 0(m) is identified with the shift in the potential of the electrocapillary maximum during adsorption of surface-active anions. For large values of this shift (0(m) RT/F)... 

Indeed, for adsorption of iodide, Essin and Markov found a shift in the electrocapillary maximum d

[Pg.234]

For molecules with small dipoles, the adsorption region is distributed symmetrically around the potential of the electrocapillary maximum. However, if chemisorption interaction occurs between one end of the dipole (e.g. sulphur in thiourea) and the electrode, the adsorption region is shifted to the negative or positive side of the electrocapillary maximum. 

Of the quantities connected with the electrical double layer, the interfacial tension y, the potential of the electrocapillary maximum Epzc, the differential capacity C of the double layer and the surface charge density q(m) can be measured directly. The latter quantity can be measured only in extremely pure solutions. The great majority of measurements has been carried out at mercury electrodes. 

The quantity ys, is a function of the electrode potential, the quantity y,g is independent of the potential and ysg should not depend on the potential, provided that the surface is dry below the bubble. As there is always a trace of moisture below the bubble in this arrangement, the value of ysg changes slightly with potential, but far less than ysl. The further the electrode potential is from the potential of the electrocapillary maximum, the more ys, decreases, i.e. cos 6 increases and the wetting angle 6 decreases. Obviously,... 

The potential of the electrocapillary maximum can be found from the electrocapillary curve while the Paschen method is an alternative. In this... 

Figure 2.13 pH dependence of the electrocapillary maximum, Emill(. The solutions were 0 5 M Na2S0 /H2S04, except for the pH 14 electrolyte which was 1 M NaOH, The open circles represent data obtained from the anodic scan and the filled circles from the cathodic scan. From... 

At the electrocapillary maximum, the charge density, a, is zero (point of zero charge) (Fig. A.4.5c). By definition, the differential capacity of the double layer, Cd, is equal (Second Lippmann Equation). 

The potential of zero charge, ac, can be obtained from the condition at which Om = - (dy/d.E)=O.This is the potential at which the interfacial tension is maximum in an electrocapillary curve (yvs.E) and is called the electrocapillary maximum. Figure 5-17 illustrates the electrocapillary curves observed for a liquid mercury electrode in aqueous solutions of varioxis anions. It is found that the greater the adsorption affinity of the anions (Cl" < Br" < I") on mercury, the more negative is the potential of zero charge (the potential of electrocapillary maximum). 

Would Eq. (6.261) for the total differential capacity be able to reproduce the experimental capacity curves Let us have a look again at one of the complete capacity-potential curves shown in Fig. 6.65(b) and illustrated in Fig. 6.106 in this section. This is not a simple curve. It breaks out into breaks and flats, and it has a complicated fine structure that depends upon the ions that populate the interphasial region. Whereas there is a region of constant capacity at potentials more negative than the electrocapillary maximum, there is also a "hump in the capacity-potential... 

The general shape of the curves is roughly parabolic. The coordinates of the maximum in the electrocapillary curve depend on the electrolyte content of the system. Since 7 decreases on both sides of the electrocapillary maximum and since reductions in 7 are associated with adsorption, we conclude that adsorption increases as we move in either direction from the maximum that is, the electrocapillary maximum seems to be a point of minimum adsorption. 

This result shows that the vertical displacements (at fixed potential) of the electrocapillary curve with changes in electrolyte concentration measure the sum of the surface excesses at the solution surface. Curves such as those in Figure 7.23b may be interpreted by this result. We have already seen that T+ = T at the electrocapillary maximum (where E = Emax) therefore... 

Use these results to estimate T, the surface excess of KI at the electrocapillary maximum, for 1.0 and 0.1 M KI. Express your results as moles of KI adsorbed per square centimeter and as total charge qT per square meter. 

The interfacial tension at the electrocapillary maximum for several electrolytes in dimethyl-formamide (DMF) solutions has been measured as a function of the electrolyte concentration ... 

In agreement with Eq. (1.72), at the potential of the electrocapillary maximum qM = qs = 0, i.e., the free net charge on the interface is null. For this reason, this potential is called the zero charge potential (PZC, Ez), and its determination is of great interest in the thermodynamic study of the interface (see Figs. 1.7 and 1.8). 

Fig. 1.7 Surface tension of mercury in contact with aqueous solutions of the salt named. T = 291 K. Abscissas are measured relative to a rational scale in which the potential difference between the mercury and a capillary-inactive electrolyte is arbitrarily set equal to zero at the electrocapillary maximum. Taken from [19] with permission...

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