Electrocapillary measurement

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

In Ref 169, some peculiarities associated with adsorption of alkyne peroxides from DM F-water solutions onto the mercury electrode in the presence of tetraethylammonium cations have been described. Polarography and electrocapillary measurements were employed as the experimental techniques. It has been shown that interfacial activity of these peroxides was determined by the species generated as a result of associative interactions between peroxides and DMF and tetraethylammonium cations. 

What are the capabilities of this system Since the system consists of a polarizable interface coupled to a nonpolarizable interface, changes in the potential of the external source are almost equal to the changes of potential only at the polarizable interface, i.e., the changes in zl< ) across the mercuiy/solution interface are almost equal to changes in potential difference Vacross the terminals of the source. Hence, the system can be used to produce predetermined zl< ) changes at the mercuiy/solution interface (Section 6.3.11). Further, measurement of the surface tension of the mercuiy/solution interface is possible, and since this has been stated /Section 6.4.5) to be related to the surface excess, it becomes possible to measure this quantity for a given species in the interphase. In short, the system permits what are called electrocapillary measurements, i.e., the measurement of the surface tension of the... 

The experimental y versus V curves obtained by electrocapillary measurements demonstrate this variation of surface tension y with the potential difference V across the cell. What is informative, however, is the nature of the variation (Table 6.2). A typical electrocapillary curve is almost a parabola (Fig. 6.53). The potential at which... 

With liquid metals, the most convenient method of determining the pzc is by making electrocapillary measurements. From the y versus V curve, the qM versus V curve can be found and thus the value of 0 or E. The pzc, however, is such a fundamental characteristic of the interface that there is a considerable need to know its value for interfaces involving solid electrodes. Here, surface tensions cannot be determined with capillary electrodes, and one must resort to other methods of pzc determination. Some values of the pzc for solid metals are given in Table 6.4. 

Define the following terms used in Section 6.5 (a) electrocapillary measurements, (b) ecm, (c) Lippmann equation, (d) integral capacitance, (e) differential capacitance, and (1) potential of zero charge. (Gamboa-Aldeco)... 

The Lippmann equation gives the charge density of the electrode based on electrocapillary measurements. This equation can be approximate as (Ay/AV)const comp= —qM. Measurements of surface tension of Hg in contact with 1.0AHC1 gave the following data ... 

Electrocapillary measurements with the dropping mercury electrode... 

The total capacity C of the double layer has been determined from electrocapillary measurements for mercury-aqueous electrolyte interfaces89, and from potentiometric titration measurements for silver... 

The proportionality constant between the applied potential and the charge due to the species ordering in the solution interfacial region is the double layer capacity. The study of the double layer capacity at different applied potentials can be done by various methods. One much used is the impedance technique, which is applicable to any type of electrode, solid or liquid, and is described in Chapter 11. Another method uses electrocapillary measurements. It was developed for the mercury electrode, being only applicable to liquid electrodes, and is based on measurement of surface tension. 

The principle of electrocapillary measurements was described more than a century ago by Lippmann9. It is a null-point technique that counterbalances the force of gravity and surface tension, and highly accurate results can be obtained. 

For ideally polarizable electrodes - since as a whole, the double layer is electrically neutral - the absolute value of the -> surface charge on the metal (opposite charge accumulated at the solution phase near the metal (surface charge density and for the ideally polarizable electrode it is equal to the surface charge density (Q), i.e., potential of zero charge (pzc, Ea = Eq = 0) the - Galvani potential difference between the two phases is due to the orientation of dipoles (e.g., water molecules) [i.v]. 

The surface tension of solids has been the subject of several reviews. Theoretical advances are reviewed in a paper of Linford [ 16] and more recently by Rusanov (17). Also reviews about experimental techniques for determining the surface tension of solids in general I8] and of electrocapillary measurements 119], and a collection of experimental results 120], have appeared. Rusanov and Prokhorov provide a detailed review about the theoretical background of more classical experimental methods [2I. ... 

To perform the integration, it is necessary to know the value of E. This is readily obtained, in the case of mercury, from electrocapillary measurements, as will be shown. Note that we have used the potential E measured versus some reference electrode, instead of the potential < ) in the metal. This can be done because the difference between the two is a constant, which is eliminated from the final equations as long as all potentials are measured versus the same reference electrode. 

Measurement of the electrocapillary curve consists of changing the potential stepwise and determining the pressure required to return the mercury meniscus to the same location in the fine capillary. A plot of this pressure as a function of potential is nothing but the electro-capillary curve, within a constant, as seen from Eq. 53H. The best way to determine the magnitude of this constant is by calibration with a known system. This requires, of course, one accurate determination of y by an independent method. Very careful experiments were performed by Gouy around the turn of the century, and these results are used even now as the primary standard for electrocapillary measurements. 

Let us make a small detour here to discuss a minor point, which appears to be purely technical but may help us to better understand the physics behind electrocapillary measurements. The question we want to answer is Should one make an effort to use a perfectly cylindrical capillary (which is not easy to come by), or is a slightly tapered, conical capillary satisfactory The answer cannot be found in Eq. 53H, which relates the height of the mercury column, or the pressure difference, to the radius. Consider, however, the situation in a perfectly... 

There is one important point in favor of making electrocapillary measurements. 

The mechanism of reduction at mercury apparently changes dramatically when As is replaced by Sb. This is seen in the shift from one two-electron process to two one-electron processes, where the potential of the first reduction is shifted at least 0.5 V in the positive direction relative to the single reduction process for the analogous arsonium salt. The potential of the second reduction process remains close to that for the arsonium compound cf Table 1. The main difference between the arsonium and the stibonium salts is their tendency to adsorb on the mercury electrode. From electrocapillary measurements, in which the drop time (t) of the mercury electrode is measured as a function of potential ( ) in the absence and in the presence of the stibonium ions, it appears that the t/ relationship is severely effected by the presence of the stibonium ions the effect is the greater, the larger... 

The acetone solvent system was studied by Frumkin. Electrocapillary measurements showed that NOj ions are much less strongly adsorbed from acetone than from water whereas for Cl" ions the opposite is true. This again illustrates the importance of specific solvation effects in ionic adsorption. 

Frumkin has also reported electrocapillary measurements for NH4CNS and NHJ solutions in pyridine which show that CNS and I are less strongly adsorbed from pyridine than from water. Pyridine itself is evidently strongly adsorbed on mercury due to interaction of the aromatic nucleus with the metal. However, the relative importance of ionic solvation and solvent co-adsorption to the anion adsorption process is unknown. 

A large number of electrocapillary measurements in amyl alcohol, phenol, furfural, ethyl acetate, aniline, chloroform, propanol, wobutanol and diethylether, and in binary and tertiary mixtures of these solvents with each other and with water are given in an early paper by Wild. ... 

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