Electrocapillary curve layer from

Fig. V-12. Variation of the integral capacity of the double layer with potential for 1 N sodium sulfate , from differential capacity measurements 0, from the electrocapillary curves O, from direct measurements. (From Ref. 113.)...

The interaction between the adsorbed molecules and a chemical species present in the opposite side of the interface is clearly seen in the effect of the counterion species on the HTMA adsorption. Electrocapillary curves in Fig. 6 show that the interfacial tension at a given potential in the presence of the HTMA ion adsorption depends on the anionic species in the aqueous side of the interface and decreases in the order, F, CP, and Br [40]. By changing the counterions from F to CP or Br, the adsorption free energy of HTMA increase by 1.2 or 4.6 kJmoP. This greater effect of Br ions is in harmony with the results obtained at the air-water interface [43]. We note that this effect of the counterion species from the opposite side of the interface does not necessarily mean the interfacial ion-pair formation, which seems to suppose the presence of salt formation at the boundary layer [44-46]. A thermodynamic criterion of the interfacial ion-pair formation has been discussed in detail [40]. 

In situation (b) the anion adsorption is compensated by the negative overall potential of the dme. In situation (c), with a further increase in the negative potential, an electric double layer will now be formed with cations from the solution, so that the apparent <7Hg is lowered again. Hence crHg as a function of the negative dme potential, yielding the so-called electrocapillary curve, shows a maximum at about -0.52 V (see Fig. 3.18). 

The capacity C of the double layer per square centimetre, or the rate of change of the charge with the applied potential E, can be found from the electrocapillary curve, for it is equal to the second differential coefficient of the surface tension with respect to the potential. This follows at once from (13), since... 

There are two reports that determined the double-layer capacitance of ionic liquids [31, 40]. By an electrocapillary curve measurement using dropping mercury electrode (DME), the integral double-layer capacitances of ionic liquids were shown to be smaller than those of aqueous solutions and larger than those of non-aqueous solutions, as summarized in Table 17.2 [31]. This behavior can be explained by the thinner double-layer being due to the higher ionic concentration than that of nonaqueous solutions. However, the correlation between the doublelayer capacitance and anion size [41] observed in PC solutions [8] is not clear. It was further shown that the double-layer capacitance of the ionic liquid was not dependent on the choice of electrode from among DME, GC, and activated carbon fiber [31]. 

Potentials of zero charge of the interface can be found reliably by the same independent methods that are used at the metal-water interface. These include finding the differential capacitance minimum of the electric double layer, from electrocapillary curves, with a flowing-electrolyte electrode, with the vibrating boundary method, with radiotracers, or by measuring the second harmonic... 

These equations clearly show that the the slope of the electrocapillary curve of nonpolarized interface does not give the surface charge density but the relative surface excess of ionic components, as defined by Eq. (18) for case Ilb. In other words, the electrocapillary maximum potential does not correspond to the potential of zero charge . An approach to investigate the surface charge density and the double layer structure may be predicted as follows. When the values of the second terms of the right-hand sides of Eq. (18) (that is, the and Tnb values), are known or estimated on reasonable argument, Fd and F(so that by Eq. (19)) can be found from the slope... 

It follows from Eq. (9.9) that at the maximum of the electrocapillary curve, qM = 0. In other words, the potential of zero charge, p2Cf coincides with the potential of the electrocapillary maximum. The double-layer capacitance can be obtained by double differentiation of the surface tension with respect to potential, and the surface tension can be obtained by double integration of the dependence of Cdi on E. The situation is not entirely symmetrical, however. For double differentiation, all one needs is very accurate data of y versus For double integration, one also needs two constants of integration. These are the coordinates of the electrocapillary maximum, namely Epzc and For liquid electrodes (e.g., mercury and... 

At this stage it may be interesting to compare the capacities of the double layer according to the different theories with the experimental values from the electrocapillary curve. 

Electroneutral substances that are less polar than the solvent and also those that exhibit a tendency to interact chemically with the electrode surface, e.g. substances containing sulphur (thiourea, etc.), are adsorbed on the electrode. During adsorption, solvent molecules in the compact layer are replaced by molecules of the adsorbed substance, called surface-active substance (surfactant).t The effect of adsorption on the individual electrocapillary terms can best be expressed in terms of the difference of these quantities for the original (base) electrolyte and for the same electrolyte in the presence of surfactants. Figure 4.7 schematically depicts this dependence for the interfacial tension, surface electrode charge and differential capacity and also the dependence of the surface excess on the potential. It can be seen that, at sufficiently positive or negative potentials, the surfactant is completely desorbed from the electrode. The strong electric field leads to replacement of the less polar particles of the surface-active substance by polar solvent molecules. The desorption potentials are characterized by sharp peaks on the differential capacity curves. 

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