Surfactants electrocapillary curves

Electrocapillary Curves in the Presence of the Adsorption of Ionic Surfactant... 

The width of the instability window depends on three factors, the magnitude of Ay, due to the surfactant adsorption, the curvature of an electrocapillary curve, and the relative location of Ao f with respect to Aq pzc- The quantity Ay,- is a function of, i.e., AG, and the amount of ionic surfactant, and the form of the adsorption isotherm, in addition to r. The effect of the relative location of Aq pzc and on... 

In agreement with the Lippmann equation, in the absence of surfactants the curve showing the surface tension as a function of the potential difference between phases (the electrocapillary curve) contains a maximum at some particular value of (p (Fig. Ill-17). This potential, which corresponds to the... 

The cations are smaller in size than the anions and, as a rule, have less influence on the shape of the electrocapillary curve. Cationic surfactants, however, adsorb strongly and cause a shift of the maximum into the anodic region. 

Nonionic surfactants are also capable of influencing the shape of the electrocapillary curve at moderate applied potentials these compounds adsorb in both the anodic and cathodic regions. At high positive and negative potentials the attraction of dipolar water molecules to the surface is so strong... 

Fig. Ill-18. Change in the shape of the electrocapillary curve due to the adsorption of nonionic surfactants...

In 2002, Kakiuchi et al. proposed a thermodynamic theory that showed that, upon coupled adsorption and transfer of surfactants, the electrocapillary curve may have a positive curvature, as opposed to a negative curvature in classical electrocapillary theory. This implies that the double-layer capacitance would be... 

The effect of adsorption of surfactants at the mercury electrode on electrocapillary curves, drop times, maximum suppression have been discussed by Malik et al [227] and by Barradas and Kimmerle [228], the former attempting to measure the CMCs of non-ionic surfactants by changes in their effects on electrocapillary curves. By tagging micelles with a cation which is reducible at the dropping mercury electrode, Novodoff et al. [229] have developed a method of measuring the diffusion coefficient of the surfactant system using the Ilkovic equation,... 

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. 

The effect of Triton X-100 on the current-voltage curve of the iron (III)-diethylenetriaminepentaacetic acid complex has been explained as follows [226] at low levels of surfactant the mercury is only partly covered by surfactant and the reversible reduction proceeds at the uncovered portions of the undisturbed surface and at the covered areas a penetration-controlled reaction occurs. As the Triton concentration is increased so the penetration-controlled reaction contribution increases. When the coverage is complete only the second wave is observed on the polarogram. The shift in the half-wave potential with increasing surfactant concentration is probably due to increased adsorbed layer thickness and is shown along with the polarograms and electrocapillary capillary curves in the absence and presence of Triton X-100 in Fig. 11.35. 

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