The potential of zero charge

The potential of zero charge (pzc) is a characteristic potential for a given interface, and hence is of obvious interest. In the absence of specific adsorption, it can be measured as the potential at which the Gouy-Chapman capacity obtains its minimum this value must be independent of the electrolyte concentration, otherwise there is specific adsorption. For liquid metals the pzc coincides with the maximum of the surface tension (see Section 3.5). 

An interesting correlation exists between the work function of a metal and its pzc in a particular solvent. Consider a metal M at the pzc in contact with a solution of an inert, nonadsorbing electrolyte containing a standard platinum/hydrogen reference electrode. We connect a platinum wire (label I) to the metal, and label the platinum reference electrode with II. This setup is very similar to that considered in Section 2.4, but this time the metal-solution interface is not in electronic equilibrium. The derivation is simplified if we assume that the two platinum wires have the same work function, so that their surface potentials are equal. The electrode potential is then  

The first and the last term can again be expressed through the work function differences, but not the second term, since this interface is not in electronic equilibrium  

To evaluate the last term we go through a cycle taking a test charge (not an electron ) from outside the metal first into the bulk of the metal, then through the metal-solution interface, then to a position just outside the solution, and finally back to outside the metal. This 

The changes in the dipole potentials are typically small, of the order of a few tenths of a volt, while work functions are of the order of a few volts. If we keep the solvent, and hence 3 ref, fixed and vary the metal, the potential of zero charge will be roughly proportional to the work function of the metal. This is illustrated in Fig. 3.6. A more detailed consideration of the dipole potentials leads to a subdivision into separate correlations for sp, sd, and transition metals [3]. 

The potential E of metal electrodes at which the interfacial charge o is zero (hence, = 0) is the potential of zero charge (the zero charge potential), It foUows from Eqn. 5-12 that the potential, 4 pic, across the compact layer at the potential of zero charge is composed of M dip nd gs,dip as shown in Eqn. 5-13  

Obviously, is not zero but maintains a certain potential due to the interfacial dipoles. Table 5-1 shows the potential of zero charge for various metals in aqueous solution. The potenti d of zero charge appears to depend to some extent on the crystal plane of metal surfaces. 

At the potential of zero charge, the difference in electrostatic inner potential,. d c, across the electrode interface is related to the difference in outer potential, /lippK, between the free surface of metal electrode and the fi surface of aqueous solution as formulated in Eqn. 5-14 and shown in Fig. 5-14  

Fi p 6-14. Inner and outer potential differences, AApk and at the zero charge interface between metal M and aqueous solution S. 

As illustrated in Fig. 5-15, the relationship is obtained between the electrode potential of zero charge p (= - cieiwsfvy/e ), the work function I of electrode metal, and the difference in outer potential between the free surface of electrode metal and the free surface of aqueous solution. Thus, taking Eqns. 5-14 and 5-15 into account, we obtain Eqn. 5-16  

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An important point of the electrocapillary curve is its maximum. Such maximum value of y, obtained when q = 0, corresponds to the potential of zero charge (E ). The surface tension is a maximum because on the uncharged surface there is no repulsion between like charges. The charge on the electrode changes its sign after the... 

If the concentration of the metal ion is not negligible at the potential of zero charge, the electrode potential varies linearly with log c according to Eq. (2) and there is no distinctive sign of the situation where the charge at the interface vanishes. The Nemst approach is obviously unsuitable for defining the nature and the amount of the charge at an interface. If the concentration of the metal ion at the pzc is small or very small, the behavior of the interface becomes that of a polarizable electrode. 

Since a metal is immersed in a solution of an inactive electrolyte and no charge transfer across the interface is possible, the only phenomena occurring are the reorientation of solvent molecules at the metal surface and the redistribution of surface metal electrons.6,7 The potential drop thus consists only of dipolar contributions, so that Eq. (5) applies. Therefore the potential of zero charge is directly established at such an interface.3,8-10 Experimentally, difficulties may arise because of impurities and local microreactions,9 but this is irrelevant from the ideal point of view. 

The notion of pzc is absent in early textbooks. A table with pzc values for about 10 metals (but for only 5 are reliable values claimed) was given by Parsons in 1954 in the first volume of this series.4 After a more complete attempt by Frumkin in 196520 to compare work function, extensive work on pzc was reported by Perkins and Andersen9 in this series and by Frumkin etal.8 in another series. Compilations of pzc values were also made by Campanella, Trasatti, Frumkin et al., and Frumkin and Petrii14 up to 1979. A book by Frumkin10 devoted entirely to the potential of zero charge was published posthumously in 1979. 

If M and R are in the same solvent S containing only an inert, surface-inactive supporting electrolyte, AE equals the difference in the potentials of zero charge between the two metals ... 

On the other hand, surface physicists often measure 0 which represents the work function of metals as modified by adsorption of polar (water) molecules.35-39 What they are measuring (although they may not realize it) is precisely the potential of zero charge of the given metal in the UHV scale. The value of 0 is exactly known in that case, but the relevance of the value of A0 is in doubt.32,33 In fact, only a few layers of a solvent... 

The contact potential difference between Hg and water (actually a dilute aqueous solution of a surface-inactive electrolyte) has been measured42,43 to be -0.25 V. The negative sign means that the work function of Hg decreases upon contact with water. Since 4.50( 0.02) cV is the currently accepted5 value for 0 of Hg, the value of 0 for the uncharged metal (at the potential of zero charge) is 4.25 eV. 

On the other hand, if the 0 of Hg in the stream is modified by contamination in the cpd measurement, this should not be the case during the measurement of the potential-of-zero charge. If the value of 4.8 eV is accepted for the SHE in the UHV scale, the value of 4.61 eV for < of Hg at the pzc would imply that for 0 to decrease upon water adsorption, the 0 of clean Hg should be substantially higher than 4.61 eV. No experimental evidence exists for this for the time being. 

Relation of the Potential of Zero Charge to Other Quantities... 

For an electrochemical cell consisting of a metal at the potential of zero charge in a solution of surface-inactive electrolyte and a reference electrode (let us assume that any liquid junction potential can be neglected), the electrode potential is given by (cf. Eq. (20)]... 

The temperature coefficient of the potential of zero charge has often been suggested to indicate the orientation of solvent molecules at the met-al/solution interface. However, this view is based only on the response of a simple two-state model for the interfacial solvent, and on neglecting any contribution from the electronic entropy.76,77 This is in fact not the case. The temperature coefficient of 0in many instances is negative and of the... 

The potential of zero charge depends on the composition of the solution if adsorption takes place. If partial or total charge transfer occurs, the situation becomes more complex than in a perfect condenser,82 as discussed in Section I.l(iii). 

As ionic adsorption takes place, normally the potential of zero charge varies linearly with the amount adsorbed.83 Such a variation is used84,85 as a means of extrapolating to zero concentration of the adsorbing sub-... 

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