Mercury electrolyte interface

Kakiuehi et al. [84] studied the adsorption properties of two types of nonionic surfactants, sorbitan fatty acid esters and sucrose alkanoate, at the water-nitrobenzene interface. These surfactants lower the interfacial capacity in the range of the applied potential with no sign of desorption. On the other hand, the remarkable adsorption-desorption capacity peak analogous to the adsorption peak seen for organic molecules at the mercury-electrolyte interface can be observed in the presence of ionic surfactants, such as triazine dye ligands for proteins [85]. 

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. 

Thus, we have a relationship between y and the potential drop across the mercury/electrolyte interface, as desired in addition, the relationship is parabolic. However, a link must now be established between the unmeasurable A, and the measured applied potential. 

It would seem, at first glance, that the place of electrocapillarity ties in the history of electrochemistry rather than its future, since its application appears limited only to the mercury/electrolyte interface. However, the work of Sato and colleagues (1986, 1987, 1991) has very definitely placed the technique, or at least a development of it, very firmly at the frontiers again. 

In terms of understanding the mercury/electrolyte interface, it is clear from the above discussion that the measurement of the surface free energy (in terms of the surface tension), is central. If the clectrocapillarity technique could be applied to solid electrodes, then it is capable of supplying information extremely difficult to obtain by any other technique. Sato has indeed developed a technique to measure the surface tension of a metal electrode which he terms piezoelectric surface stress measurement and is based upon the previous work of Gokhshtein (1970). 

One other caveat concerning the approach used here must be made. This discussion, and the studies to which it relates, are based on some version of the Stern model for the oxide-electrolyte interface. Oxide surfaces are rough and heterogeneous. Even for the mercury-electrolyte interface, or single crystal metal-electrolyte interfaces, the success of some form of the Stern model has been less than satisfactory. It is important to bear in mind the operational nature of these models and not to attach too much significance to the physical picture of the planar interface. 

For the familiar dropping mercury electrode, the electrical potential 1J1 at the metal surface relative to the bulk region of the electrolyte is controlled by an external potential source - a constant voltage source. In this case, can be set to any value (within reasonable physical limits) as the mercury/electrolyte interface does not allow charge transfer or chemical reactions to occur (at least to a good approximation for the case of NaF). Therefore, we can say that the equation of state of the mercury surface is... 

Lippmanns classical derivation was simpler. He treated the mercury-electrolyte interface as a capacitor. The capacitance is assumed to increase with the surface area A much like a plate capacitor. One plate is the metal, the other the layer of counterions in the electrolyte. The potential difference between the two plates is U. A change in the Gibbs free energy of the system is equal to the reversible work upon a change of the surface A or of the charge Q ... 

The surface tension depends on the potential (the excess charge on the surface) and the composition (chemical potentials of the species) of the contacting phases. For the relation between y and the potential see - Lipp-mann equation. For the composition dependence see -> Gibbs adsorption equation. Since in these equations y is considered being independent of A, they can be used only for fluids, e.g., liquid liquid such as liquid mercury electrolyte, interfaces. By measuring the surface tension of a mercury drop in contact with an electrolyte solution as a function of potential important quantities, such as surface charge density, surface excess of ions, differential capacitance (subentry of... 

Currently no adequate quantitative theory of the discrete-ion potentials for adsorbed counterions at ionized monolayers exists although work on this problem is in progress. These potentials are more difficult to determine than those for the mercury/electrolyte interface because the non-aqueous phase is a dielectric medium and the distribution of counterions in the monolayer region is more complicated. However the physical nature of discrete-ion potentials for the adsorbed counterions can be described qualitatively. This paper investigates the experimental evidence for the discrete-ion effect at ionized monolayers by testing our model on the results of Mingins and Pethica (9, 10) for SODS. The simultaneous use of the Esin-Markov coefficient (Equation 3) and the surface potential AV as functions of A at the same electrolyte concentration c yields the specific adsorption potentials for both types of adsorbed Na+ ions—bound and mobile. Two parameters which need to be chosen are the density of sites available to the adsorbed mobile Na+ ions and the capacity per unit area of the monolayer region. The present work illustrates the value... 

Theories more recent than the simple Langmuir (random-mixing) adsorption statistics and usually regarded as more realistic can handle the entropic terms in Equations 10 and 12. Two of these, which have been used for adsorbed ions at the mercury/electrolyte interface, are based on Flory-Huggins (12,13,25, 26) and scaled-particle statistics (31,... 

It is not obvious why (13.1.31) is called an electrocapillary equation. The name is a historic artifact derived from the early application of this equation to the interpretation of measurements of surface tension at mercury-electrolyte interfaces (1-4, 6-8). The earliest measurements of this sort were carried out by Lippmann, who invented a device called a capillary electrometer for the purpose (9). Its principle involves null balance. The downward pressure created by a mercury column is controlled so that the mercury-solution interface, which is confined to a capillary, does not move. In this balanced condition, the upward force exerted by the surface tension exactly equals the downward mechanical force. Because the method relies on null detection, it is capable of great precision. Elaborated approaches are still used. These instruments yield electrocapillary curves, which are simply plots of surface tension versus potential. 

Elliott and Murray performed chronocoulometric experiments to measure the surface excess of T1 at a mercury-electrolyte interface. Of interest was the influence of bromide on the adsorption. Explain how such measurements would be carried out. The results are summarized in Figure 14.8.1. Explain the results in terms of chemical processes. 

Autocatalytically controlled surface reactions Michaihk and coworkers [98] proposed an autocatalytic surface process representing the time dependence of formation of a 2D condensed film of triphenylethyl phospho-nium sulfate ((TPEP)2S04) deposited at a mercury-electrolyte interface [99] ... 

The adsorption behavior of coumarin (2H-l-benzopyran-2-one) at the mercury-electrolyte interfaces was investigated by electrocapillary, capacitance, and transient experiments [162-164, 540-543]. 

Case Study III - 2,2 -Bipyridine (2,2 -BI on Au(hld) 2,2 -bipyridine is a bidentate ligand used in coordination chemistry and a typical representative of aromatic, nitrogen-containing heterocycles, which act as basic building blocks in highly specific and functionalized host lattices at defined surfaces [548, 549]. Two planar pyridine rings are connected via a C—C bond with 10% double-bond character [550]. Electrochemical studies revealed that n, n -bipyridine [551-553] and several of its Co " " and Ni " " complexes [554] form 2D condensed films at the atomically smooth mercury-electrolyte interface. The adsorption of 2,2 -bipyridine on solid electrodes, such as Ag(poly), Au(hkl), and Cu(lll) was studied by... 

The film is gradually transformed into an amalgam. For the Au(Hg) species, the platinum mercury interface, which is an electronic junction, is non-reactive while the mercury electrolyte interface is reactive. 

The two most common methods available to determine the interfacial tension at the mercury-electrolyte interface are the capillary rise method and the maximum... 

Determinations of the interfacial surface tension between mercury and electrolyte solution can be made with a relatively simple apparatus. All that are needed are (1) a mercury-solution interface which is polarizable, (2) a nonpolarizable interface as reference potential, (3) an external source of variable potential, and (4) an arrangement to measure the surface tension of the mercury-electrolyte interface. An experimental system which will fulfill these requirements is shown in Fig. 2.7. The interfacial surface tension is measured by applying pressure to the mercury-electrolyte interface by raising the mercury head. At the interface, the forces are balanced, as shown in Fig. 2.8. If the angle of contact at the capillary wall is zero (typically the case for clean surfaces and clean electrolyte), then it is a relatively simple arithmetic exercise to show that the interfacial surface tension is given by... 

FIGURE 2.7 Experimental arrangement to measure interfadal surface tension at mercury-electrolyte interface. 

Construction of an apparatus for measuring electrocapillary curves of the mercury-electrolyte interface based on the maximum... 

Shielding of charged surface sites is described quantitatively by the Esin-Markov coefficient, P [18,22]. This coefficient was introduced initially in the case of the mercury/electrolyte interface. It represents the variation in applied potential required to maintain a constant surface charge when electrolyte acitivity increases ... 

The mercury/electrolyte interface played a major role in the early studies of the structure of metal/solution interfaces, and electrode kinetics in general. The surface of the liquid metal is highly reproducible and the low catalytic activity of Hg towards hydrogen evolution provided a rather wide range of potentials where the thermodynamic properties of the interface coidd be determined experimentally, allowing theories to be verified or discarded. However, mercury is of little industrial interest, and its use has been all but eliminated in recent decades because of its high toxicity and devastating influence on the environment. 

When one studies an (almost) ideally polarizable interface, such as the mercury electrode in pure deaerated acids, the equivalent circuit is a resistor Rs and a capacitor Cdi in series. The high accuracy and resolution offered by modern instrumentation allows measurement in such cases in very dilute solutions or in poorly conducting, non-aqueous media, which could not have been performed in the early days of studying the mercury/electrolyte interface. 

---