Specifically adsorbed species

Equation (17) expresses the cell potential difference in a general way, irrespective of the nature of the electrodes. Therefore, it is in particular valid also for nonpolarizable electrodes. However, since

interfacial structure, only polarizable electrodes at their potential of zero charge will be discussed here. It was shown earlier that the structural details are not different for nonpolarizable electrodes, provided no specifically adsorbed species are present. 

Figure 3 also contains an example of an ISER-flrel plot for a simple specifically adsorbed species, bromide on silver (solid curve). This plot was extracted from bromide coverage-potential data, obtained from differential capacitance measurements, along with the corresponding potential-dependent intensity of the SERS bromide-surface stretching mode at ca. 160 cm"1 (19.). In this case, the maximum (i.e. unity) value of 0r>1 corresponds to a close-packed bromide monolayer, ca. 1.4 x 10"9 mol cm 2. Again, the ISER-0t 1... 

V Specifically adsorbed species are those that are bound by interactions other than electrostatic ones. To what extent SO and Ca2+ can form inner-sphere complexes is not yet well established. SO2 is able to shift the point of zero proton condition of many oxides. 

By contrast, the charge of the solution, qs, is distributed in a number of layers. The layer in contact with the electrode, called the internal layer, is largely composed of solvent molecules and in a small part by molecules or anions of other species, that are said to be specifically adsorbed on the electrode. As a consequence of the particular bonds that these molecules or anions form with the metal surface, they are able to resist the repulsive forces that develop between charges of the same sign. This most internal layer is also defined as the compact layer. The distance, xj, between the nucleus of the specifically adsorbed species and the metallic electrode is called the internal Helmholtz plane (IHP). The ions of opposite charge to that of the electrode, that are obviously solvated, can approach the electrode up to a distance of x2, defined as the outer Helmholtz plane (OHP). 

While surface charge is by no means the only factor responsible for electrolyte adsorption (94), particularly organic electrolytes (9, 27), the extent of adsorption of the less specifically adsorbed species, such as the simple aquo ions and, for example, primary amines and alkyl sulfonates, decreases rapidly when the sign of the oxides surface charge is changed to that of the sorbing species (6, 10). 

To date, very little experimental and modeling work has been done on the adsorption of oxygen in the presence of water and polymer electrolyte although it is strongly anticipated that water, polymer, and specifically adsorbed species have a great impact on adsorption of oxygen. Most experimental and simulation studies were carried out in so-called vapor phase. In these simulations, only a few gas molecules and a small-size lattice of the catalyst were considered. 

The actual interface is comprised of only the top layer of atoms on the electrode and the layer of adsorbed solvent and specifically adsorbed species. However, an... 

In the presence of a large quantity of inert electrolyte, all the potential drop is confined to within the compact layer and is zero. By application of an appropriate potential at an electrode we can also arrange for = 0 - this value of the potential is called the isoelectric point. This is, in general, not equal to the point of zero charge, as the value of the latter is affected by the presence of specifically adsorbed species (Section 3.4). 

Pulse electrolysis, which is often used to improve the quality of electrodeposits (see below), was found to have a strong effect on the structure of nickel deposits by modifying the interfacial sorption properties [6.62]. Molecular species such as H2 or Ni(OH)2 are easily desorbed during the relaxation time, improving the electrocrystallization process. Conversely, specifically adsorbed species such as Hads or anions inhibit the electrodeposition process of nickel. 

Figure 10.18 shows H and OH" specifically adsorbed onto surface sites in the zero plane in both models. Other specifically adsorbed species are also assumed to occupy the zero plane. With protonated and deprotonated surface sites symbolized as SOHJ, SOH and SO", a specifically adsorbed cation M + and anion A may be represented as SOM+, and SOH2A . The surface charge, which of course neglects uncharged SOH, then equals... 

Suspensions of the colloidal-sized oxyhydroxides can be destabilized by increases in ionic strength (strong electrolyte concentrations) such as occur when a stream enters an estuary (Fig. 12.5). Small amounts of specifically adsorbed species at well-defined concentations can also destabilize the suspended oxyhydroxides. 

The course of add-base titrations conducted in the presence of specifically adsorbing species is different from that observed at pristine conditions. The experimental limitations and difficulties encountered by potentiometric titration have been discussed in Section 3.I.B.1 and 3.LB.2. Cancellation of some errors simplifies the assessment of the effect of spedfic adsorption on surface charging (comparison of titrations in the presence and absence of specific adsoiption), but the compensation of errors is not complete, e.g. the solubility of materials (adsorbents) is strongly affected by specific adsorption. 

The examples shown is Section D indicate that the shape of calculated uptake curves (slope, ionic strength effect) can be to some degree adjusted by the choice of the model of specific adsorption (electrostatic position of the specifically adsorbed species and the number of protons released per one adsorbed cation or coadsorbed with one adsorbed anion) on the one hand, and by the choice of the model of primary surface charging on the other. Indeed, in some systems, models with one surface species involving only the surface site(s) and the specifically adsorbed ion successfully explain the experimental results. For example, Rietra et al. [103] interpreted uptake, proton stoichiometry and electrokinetic data for sulfate sorption on goethite in terms of one surface species, Monodentate character of this species is supported by the spectroscopic data and by the best-fit charge distribution (/si0,18, vide infra). 

Methods used in studies of adsorption of ions, their advantages and limitations, the meaning of results, and possibilities to combine results obtained by different methods are discussed. A large compilation of adsorption data is presented. The results obtained in simple adsorption systems (with one specifically adsorbed species) are sorted by the adsorbent and then by the adsorbate. Then, more complex systems are discussed with many specifically adsorbing species. 

Electrocapillary methods, described in Sections 13.2 and 13.3, are very useful in the determination of relative surface excesses of specifically adsorbed species on mercury. As discussed in Section 13.4, such methods are less straightforward with solid electrodes. For electroactive species and products of electrode reactions, the faradaic response can frequently be used to determine the amount of adsorbed species (Section 14.3). Nonelectro-chemical methods can also be applied to both electroactive and electroinactive species. For example, the change in concentration of an adsorbable solution species after immersion of a large-area electrode and application of different potentials can be monitored by a sensitive analytical technique (e.g., spectrophotometry, fluorimetry, chemiluminescence) that can provide a direct measurement of the amount of substance that has left the bulk solution upon adsorption (7, 44). Radioactive tracers can be employed to determine the change in adsorbate concentration in solution (45). Radioactivity measurements can also be applied to electrodes removed from the solution, with suitable corrections applied for bulk solution still wetting the electrode (45). A general problem with such direct methods is the sensitivity and precision required for accurate determinations, since the bulk concentration changes caused by adsorption are usually rather small (see Problem 13.7). 

Molecular organi2ation of solvents and specifically adsorbed species,... 

Adsorption on Bare Metal Surfaces The adsorption of ions or neutral molecules on bare metal surfaces immersed in solution is determined by the mutual interactions of all species present at the phase boundary. These include electrostatic and chemical interactions of the adsorbate with the surface, adsorbate-adsorbate, and adsorbate-solvent interactions. Inhibitors are usually specifically adsorbed species that adsorb directly on the metal surface in a process involving (partial) desolvation of the adsorbate species and replacement of solvent molecules from the electrode surface. Consequently, the interaction of the adsorbate with the surface has to exceed that of the solvent. Commonly, one distinguishes chemisorption, in which the adsorbate chemically interacts with the surface, and physisorption, caused by much weaker van der Waals or (hydrophobic) adsorbate-solvent interactions. 

In simple cases where there are no specifically adsorbed species at the electrochemical interface, the various flux densities of the electroactive species at this interface are all proportional to each other. This link looks like a mass balance taking into account the stoichiometric numbers as in any chemical reaction. Therefore, the amount of charge flowing through the system, for instance via the current density, can then be linked to the mass balance. 

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