Adsorption of ions specific

Only at extremely high electric fields are the water molecules fiilly aligned at the electrode surface. For electric fields of the size normally encountered, a distribution of dipole directions is found, whose half-widtli is strongly dependent on whether specific adsorption of ions takes place. In tlie absence of such adsorption the distribution fiinction steadily narrows, but in the presence of adsorption the distribution may show little change from that found at the PZC an example is shown in figure A2.4.10 [30]. 

Vitanov and Popov etc//.151,377 have found on quasi-perfect surfaces that 1) and have suggested that weak specific adsorption of ions... 

It will be assumed that the interactions between each of metals (1) and (2) and the corresponding surface layers of the electrolyte solution are approximately identical, and also that specific adsorption of ions does not occur in the system being considered. In this case the values of the expressions in the last two sets of brackets in Eq. (9.10) become zero, and from (9.10) and (9.11) an important relation is obtained which links the OCV of galvanic cells with the Volta potential ... 

Two types of EDL are distinguished superficial and interfacial. Superficial EDLs are located wholly within the surface layer of a single phase (e.g., an EDL caused by a nonuniform distribution of electrons in the metal, an EDL caused by orientation of the bipolar solvent molecules in the electrolyte solution, an EDL caused by specific adsorption of ions). Tfie potential drops developing in tfiese cases (the potential inside the phase relative to a point just outside) is called the surface potential of the given phase k. Interfacial EDLs have their two parts in dilferent phases the inner layer with the charge density in the metal (because of an excess or deficit of electrons in the surface layer), and the outer layer of counterions with the charge density = -Qs m in the solution (an excess of cations or anions) the potential drop caused by this double layer is called the interfacial potential... 

Consider first the situation when specific adsorption of ions is absent. In this case the ions cannot penetrate to the inner Hehnhoftz pfane, the charge density i is zero, and hence = — Qg. Since no charges exist in the compact EDL part, JC2 x > 0, the vafue of d /dx wiU be constant and the potentiaf wiU vary Unearfy from /q to /2- For the Hefmhoitz fayer we can write... 

Specific adsorption of ions changes the value of E, hence, one distinguishes the notion of a point of zero charge, in solutions of surface-inactive electrolytes, which depends on the metal, from that of a point of zero charge, in solutions of surface-active ions, which in addition depends on the nature and concentration of these ions. The difference between these quantities. 

Partial Charge Transfer Specific adsorption of ions is often attended by a partial transfer of their charge to the metal surface for instance, in the specific adsorption of cations M + on platinum... 

The introduction of the concept of the micropotential permits derivation of various expressions for the potential difference produced by the adsorbed anions, i.e. for the potential difference between the electrode and the solution during specific adsorption of ions. It has been found that, with small coverage of the surface by adsorbed species, the micropotential depends almost linearly on the distance from the surface. The distance between the inner and outer Helmholtz planes is denoted as xx 2 and the distance between the surface of the metal and the outer Helmholtz plane as jc2. The micropotential, i.e. the potential difference between the inner and... 

Specific adsorption of ions (probably anions) of the electrolyte phase on the metal also should depend on the metal. Assuming a Langmuir-type equilibrium, one has22 for ions of charge qt and solution concentration c,... 

In addition to the specific adsorption of ions, it is also possible to adsorb neutral organic molecules at the interface and both Butler and Frumkin independently arrived at equivalent expressions for the free energy of adsorption of a neutral dipolar species, A, replacing the solvent, S. The free energy change takes the form ... 

The electrode roughness factor can be determined by using the capacitance measurements and one of the models of the double layer. In the absence of specific adsorption of ions, the inner layer capacitance is independent of the electrolyte concentration, in contrast to the capacitance of the diffuse layer Q, which is concentration dependent. The real surface area can be obtained by measuring the total capacitance C and plotting C against Cj, calculated at pzc from the Gouy-Chapman theory for different electrolyte concentrations. Such plots, called Parsons-Zobel plots, were found to be linear at several charges of the mercury electrode. ... 

Specific adsorption of ions other than protons causes the pzc and the iep to shift along the pH scale (Stumm, 1992). Specifically adsorbed cations (anions) shift the titration curve and the point of zero proton condition at the surface (pznpc) to lower (higher) pH values, whereas the iep is moved to higher (lower) pH values. The shift of the iep of hematite to a lower pH by adsorbed EDTA and Cl" is shown in Figure 10.8. 

Gouy—Chapman theory and involves coulombic and possibly specific interactions due to weak electron orbital overlapping. The amount of specifically adsorbed ions at the electrode generally varies linearly with the charge at the electrode and logarithmically with the ion concentration in the solution. Further evidence of specific adsorption of ions at electrodes is the Esin—Markov effect, i.e. the shift in the pzc due to specific adsorption of ions [6]. 

As the electrode surface will, in general, be electrically charged, there will be a surplus of ionic charge with opposite sign in the electrolyte phase in a layer of a certain thickness. The distribution of jons in the electrical double layer so formed is usually described by the Gouy— Chapman—Stern theory [20], which essentially considers the electrostatic interaction between the smeared-out charge on the surface and the positive and negative ions (non-specific adsorption). An extension to this theory is necessary when ions have a more specific interaction with the electrode, i.e. when there is specific adsorption of ions. 

At the next level we also take specific adsorption of ions into account (Fig. 4.6). Specifically adsorbed ions bind tightly at a short distance. This distance characterizes the inner Helmholtz plane. In reality all models can only describe certain aspects of the electric double layer. A good model for the structure of many metallic surfaces in an aqueous medium is shown in Fig. 4.6. The metal itself is negatively charged. This can be due to an applied potential or due to the dissolution of metal cations. Often anions bind relatively strongly, and with a certain specificity, to metal surfaces. Water molecules show a distinct preferential orientation and thus a strongly reduced permittivity. They determine the inner Helmholtz plane. 

Surface functional groups Specific adsorption of ions or molecules, in-situ spectroscopic techniques... 

Finally, the Schulze-Hardy provides a very useful rule of thumb, but it is limited in that it only addresses first-order effects. Secondary effects that will have a bearing on aggregation and CCCs include specific adsorption of ions in the Stem layer,... 

If surfactant ions adsorb only through electrostatic interactions, then AGa(js = AG , = v.F. ipi- During specific adsorption of ions it is necessary to calculate with the additional term AG p<.c. An estimation of this term can be made for ips = 0. 

The degree of adsorption depends on electrolyte concentration. The degree of coverage of a surface by specific adsorption of ions can be described by monolayer adsorption isotherms (Fig. 3.11). Three types of isotherm are generally considered ... 

The specific adsorption of ions may led to the formation inner- or outer-sphere complexes. In the first case, the adsorbed ion occupies position in outer Helmholtz plane (IHP) and it is separated from the surface plate by the water molecule which, is in its salvation layer. In the case of... 

TABLE 6 Specific Adsorption of Ions at the TiC>2-Electrolyte Interface... 

TABLE 8 Specific Adsorption of Ion at the Ferrous (Hydr)oxide-Electrolyte Interface... 

Price and Halley (PH) [136] and Halley, Johnson, Price and Schwalm (HJPS) [137] have described a different theory of electron overspill into the layer between the solvent and metal-ion cores at a metal-electrolyte interface in the absence of specific adsorption of ions. Previous authors avoided the use of Schrodinger s equation altogether by introducing trial functions for the electron density function n(x). In contrast Halley and co-workers (HQ [138-141] used the Kohn-Sham version [122] of the variational principle of Hohenberg and Kohn [121] in which n(x) was described in terms of wave functions obeying Hartree-like equations. An effective one-electron Schrodinger equation is solved... 

The double-layer capacity depends strongly on the nature of the electrode material, even in cases where there is no specific adsorption of ions and solvent. It was therefore suggested, first by O.K. Rice, that the metal makes a direct contribution to the double-layer capacity. This idea was quantitatively pursued within the -> jel-lium model, in which the distribution of the electrons at the surface is affected by the double-layer field. In essence, the surface electrons form a highly polarizable medium, which enhances the capacity. In combination with the hard sphere electrolyte model, it gives the correct order of magnitude for the capacity at the -> point of zero charge also, it predicts correctly that the capacity of simple sp-metals should increase with the electronic density. An extension of the jellium-hard sphere elec-... 

As shown in the Figure, electro capillary curves are affected by - specific adsorption of ions (here, anions) at the electrode surface. Additionally, they are influenced by the - space charge region of the electrical double layer. Thus, electrocapillary curves as well as capacitance curves provide useful information on the electrical double-layer structure of electrode surfaces. 

At the interface between O and W, the presence of the electrical double layers on both sides of the interface also causes the variation of y with Aq<. In the absence of the specific adsorption of ions at the interface, the Gouy-Chapman theory satisfactorily describes the double-layer structure at the interface between two immiscible electrolyte soultions [20,21]. For the diffuse part of the double layer for a z z electrolyte of concentration c in the phase W whose permittivity is e, the Gouy-Chapman theory [22,23] gives an expression... 

For showing the presence of the instability window, we here employ a simlified assumption that the variation in y due to the specific adsorption of ions (Equation (9)) and that... 

When there are special surface states and specific adsorption of ions or dipoles, a shift occurs in die energy scale of the surface states normally the shift will be of minor importance as long as no degeneracy of states occurs in the surface. 

Diffuse Part of the Double Layer and Specific Adsorption of Ions. 309... 

---