Electrocapillarity curves

The results obtained by the method of Sato come in the form of plots of u and (j> against potential. Since u ac (dy/dE), then integrating the plot of u vs. E around the pzc (shown by the 180" change in ), gives the electrocapillarity curve of Ay vs. E, providing the proportionality constant between a and (dy/dE) can be determined. This latter is normally straightforward since dy/dE = a, which can be determined independently. 

Fundamental knowledge about the behavior of charged surfaces comes from experiments with mercury. How can an electrocapillarity curve of mercury be measured A usual arrangement, the so-called dropping mercury electrode, is shown in Fig. 5.2 [70], A capillary filled with mercury and a counter electrode are placed into an electrolyte solution. A voltage is applied between both. The surface tension of mercury is determined by the maximum bubble pressure method. Mercury is thereby pressed into the electrolyte solution under constant pressure P. The number of drops per unit time is measured as a function of the applied voltage. 

If we measure electrocapillary curves of mercury in an aqueous medium which contains KF, NaF, or CsF, then we observe that the typical parabolas become narrower with increasing concentration. Explanation With increasing salt concentration the Debye-length becomes shorter, the capacity of the double layer increases. The maximum of the electrocapillarity curve, and thus the point of zero charge (pzc), remains constant, i.e., neither the cations nor fluoride adsorb strongly to mercury. 

X 10" respectively. When pilocarpine is dissolved in excess of acid and back titrated with sodium hydroxide, a sharp break in the titration curve is observed at pH 4.31 (99). The pH of a solution of pilocarpine hydrochloride, measured electrometrically, is 4.44 at 18° (100). The influence of pilocarpine on the electrocapillarity curve of mercury has been determined by M. Gouy (101). 

Electrocapillarity curves have been obtained for interfaces between molten metals, and either molten salts or molten slags, which are ionic in nature [6]. It has been established that the metal side of the interface has a net positive charge and the slag a net negative charge. In this case there are no solvent molecules to separate the ions from one another. It is believed that there is an excess charge at the interface (+ for metal and - for the slag) and this excess falls to zero over 3-4 ionic, or metal, molecule layers. 

For the electrochemical zero charge potential (point of zero charge), which can be obtained from the electrocapillarity curve, Ee disappears (cf. page 148, cf. footnote 69). 

Figure 2.1 (a) A schematic representation of the apparatus employed in an electrocapillarity experiment, (b) A schematic representation of the mercury /electrolyte interface in an electro-capillarity experiment. The height of the mercury column, of mass m and density p. is h, the radius of the capillary is r, and the contact angle between the mercury and the capillary wall is 0. (c) A simplified schematic representation of the potential distribution across the metal/ electrolyte interface and across the platinum/electrolyte interface of an NHE reference electrode, (d) A plot of the surface tension of a mercury drop electrode in contact with I M HCI as a function of potential. The surface charge density, pM, on the mercury at any potential can be obtained as the slope of the curve at that potential. After Modern Electrochemistry, J O M. 

See also - electrocapillarity, - electrocapillary curve, -r Gibbs-Lippmann equation, - Wilhelmy plate (slide) method, - ring method, - Lippmann capillary electrometer. 

Ring method — Method to determine the - interfacial tension in liquid-gas systems introduced by Lecomte du Noiiy [i]. It is based on measuring the force to detach a ring or loop of a wire from the surface of a liquid. The method is similar to the -> Wilhelmyplate method when used in the detachment mode [ii]. See also -> electrocapillarity, -r electrocapillary curve, -> Gibbs-Lippmann equation, - Wilhelmy plate (slide) method, - drop weight method, - Lippmann capillary electrometer. 

The values of surface charge density obtained by numerical differentiation of the electrocapillary curve agreed well with those obtained by numerical integration of the differential capacity curve [17,29] (Fig. 3). These results indicate that the interface between a nitrobenzene solution of TBATPB and an aqueous solution of LiCl actually behaves as an ideal-polarized interface in a certain potential range and also that the differential capacity measurements should give essentially the same information on the electrocapillarity and the double layer structure of nitrobenzene/water interfaces as the electrocapillary curve measurements, provided that their electrocapillary maximum potential which is now equal to the potential of zero charge (pzc) and interfacial tension at the pzc (y J known. 

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