The interface of zero charge

The interface at which the interfacial charge,, is zero is called the interface of zero charge or the zero charge interface. The inner potential difference across the zero charge interface is determined by the interfacial dipole only, thus, it is characteristic of the contacting interface of the two phases as indicated in Eqn. 4-6  

For the interface of zero charge the inner potential difference is given by 

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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 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... 

Japaridze et al.m 323 have studied the interface between Hg and a number of vicinal and nonvicinal diols such as 1,2-, 1,3-, 2,3- and 1,4-butanediol (BD), ethanediol (ED), and 1,3-propanediol. KF and LiC104 were used as surface-inactive electrolytes. The potential of zero charge was measured by the capacitance method against an SCE in water without correction for the liquid junction potential at the solvent/H20 contact (such a potential drop is estimated to be in the range of 20 to 30 mV). The potential of the capacitance minimum was found to be independent of the electrolyte concentration while capacitance decreased with dilution. Therefore, Emin was taken to measure E . These values are reported in Table 4. 

Guidelli and co-workers336-338 measured the potential of zero charge by chronocoulometry. They found that the pzc was independent of the electrolyte concentration in both NaC104 and Na2S04. However, Ea=0 in the presence of sulfates was ca. 40 mV more negative. These authors have explained this apparent discrepancy in terms of the perturbation of the solvent structure at the interface by the ions at the electrode surface, which are, however, nonspecifically adsorbed. 

The analysis in this chapter has shown that during the past 10-15 years there have been only marginal modifications in our understanding of the structure of metal/solution interfaces based on the potential of zero charge. The general picture for the relative behavior of the various metals seems well established. In particular, new, more reliable data, where available, have confirmed trends already identifiable in a more ambiguous situation. 

Adsorption processes diagrammed, 266 Adsorption spectra of electrochromic polypyrrole, 363 Affinity for metal-water, 177 Air-solution interface, Nikitas on the potential of zero charge at, 30 Albury and Mount, interpretation of the semi-circle, 584 Alloys, potential of zero charge gold and silver, 142 tin and lead, 142 Kukk and Puttsepp on, 145 metals alloys, 141... 

Kolb, reconstruction of, 85 Gold-solution interface (Clavilier and Nguyen Van Huong), 77 Guidelli, and the methods for the determination of the potential of zero charge, 63... 

CO adsorption on electrochemically facetted (Clavilier), 135 Hamm etal, 134 surfaces (Hamm etal), 134 Platinum group metals in aqueous solutions, 132 and Frumkin s work on the potential of zero charge thereon, 129 Iwasita and Xia, 133 and non-aqueous solutions, 137 potentials of zero charge, 132, 137 preparation of platinum single crystals (Iwasita and Xia), 133 Platinum-DMSO interfaces, double layer structure, 141 Polarization time, 328 Polarons, 310... 

At a definite value of the electrode potential E, the charge of the electrode s surface and hence the value of drop to zero. This potential is called the point of zero charge (PZC). The metal surface is positively charged at potentials more positive than the PZC and is negatively charged at potentials more negative than the PZC. The point of zero charge is a characteristic parameter for any electrode-electrolyte interface. The concept of PZC is of exceptional importance in electrochemistry. 

If the interface is in the zero charge state, named also the point of zero charge (pzc), the Galvani potential should be equal to the dipolar term [19-21] ... 

The other is AG g, at the potential of zero charge (PZC), where no direct electrostatic effect is expected. The former reflects the affinity to the interfacial region when the driving forces toward the interface from W and from O are balanced, notwithstanding that the surface activity at Aq phase-boundary potential as Aq 4>f is usually different from the PZC. AG g, values at the PZC are, however, useful in comparing the intrinsic or chemical surface activities of ionic compounds. 

After addition of lipid DSPC into the organic phase a monolayer is formed at the interface, and the steady-state current increased at all potentials. On expansion, the time constant of the charging current is reduced to ca. 5 ms and a shift of ca. 100 mV is observed in the potential of zero charge. From the video image of the droplet a highly distorted and heterogeneous interface is seen which relaxes after the fast stage (a few... 

In fact, the orientation of water at the potential of zero charge is expected to depend approximately linearly on the electronegativity of the metal.9 This orientation (see below) may be deduced from analysis of the variation of the potential drop across the interface with surface charge for different metals and electrolytes. Such analysis leads to the establishment of a hydrophilicity scale of the metals ( solvophilicity for nonaqueous solvents) which expresses the relative strengths of metal-solvent interaction, as well as the relative reactivities of the different metals to oxygen.23... 

According to Eq. (2), the work function for a metal M is related to the potential of zero charge for the M/aqueous solution interface by... 

Gratifyingly (see below), various theories for the metal in the interface have found values close to this one for this quantity.] From a number of experiments, it is suggested that xHl° is between 0.08 and 0.13 V, which means that gH2°(dip) is between 0.02 and 0.07 V on a mercury electrode at the point of zero charge (oxygen end of the water molecules toward the metal). 

The experimental data bearing on the question of the effect of different metals and different crystal orientations on the properties of the metal-electrolyte interface have been discussed by Hamelin et al.27 The results of capacitance measurements for seven sp metals (Ag, Au, Cu, Zn, Pb, Sn, and Bi) in aqueous electrolytes are reviewed. The potential of zero charge is derived from the maximum of the capacitance. Subtracting the diffuse-layer capacitance, one derives the inner-layer capacitance, which, when plotted against surface charge, shows a maximum close to qM = 0. This maximum, which is almost independent of crystal orientation, is explained in terms of the reorientation of water molecules adjacent to the metal surface. Interaction of different faces of metal with water, ions, and organic molecules inside the outer Helmholtz plane are discussed, as well as adsorption. 

For the metal in the interface, the surface potential of the electrolyte phase is nearly the same for all crystal orientations.29 Therefore, referring to Eq. (2), the potential of zero charge varies with the surface potential or the work funtion and is larger for the most densely packed faces. Correspondingly, atomic irregularities... 

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