Electrocapillary curve substances

FIGURE 10.8 Influence of the adsorption of organic substances (a) on the electrocapillary curve, (b) on the capacitance curve, and (c) on the plot of surface charge against potential (1) 0.1 M H2SO4 solution (2) the same, with 0.1 MC4H9OH. 

A schematic example is given in Fig. A.4.5. The slope of the electrocapillary curve depends on the nature of the solution or the equilibrium structure of the double layer and on the specific sorbability of dissolved substances. In line with the Gibbs equation (Eq. 4.3), sorbable species depress the interfacial tension. 

Effect of neutral organic compounds upon the electrocapillary curves. In the presence of organic substances the electrocapillary curve always lies below the curve obtained with the solution of the supporting electrolyte. The decrease in the interfacial tension reaches its maximum in the vicinity of the potential of zero charge E of the solution of the supporting inorganic electrolyte and becomes smaller with a pro-... 

A comparison of electrocapillary curves measured in solutions of organic substances forming a homologous series shows that the decrease in interfacial tension and, consequently, adsorption at identical concentrations rise with an increase in chain length [3]. 

Curves for different inorganic cations are nearly coincident. Indicating almost complete absence of specific adsorption of the cations. More strongly adsorbable organic cations, however, depress the cathodic branch of the curye as is demonstrated by Fig. 17 The addition of non-ionic organic substances often results in a more or less symmetric truncation of the electrocapillary curve. See Fig, 18... 

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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