Adsorption of ions

The adsorption and ordering of ions at the electrode/electrolyte interface to form the electrochemical double layer is a fundamental concept of electrochemistry. In-situ SXS has been used to study both anion adsorption [19] and cation adsorption [20]. 

Magnussen et al. studied the adsorption of bromide adlayers on Au lll electrodes in 0.1 mol dm HCIO4 + 0.001 to 0.1 mol dm NaBr. In this case, Br ions were selected as a model system, in contrast to the more usually studied I , as Br ions are less strongly adsorbed but are still classed as specifically adsorbed ions [19]. At potentials more positive than a critical potential, which would depend on the NaBr concentration, it was found that Br ions would form a hexagonal adlayer rotated relative to the 3 direction of the Au l 11 substrate, with the extent of the rotation and adlayer density being dependent on the potential and NaBr concentration. The Br -Br spacing within the layer was found to vary continuously, from 4.24 A at the critical potential to 4.03 A at a potential 0.3 V more positive. 

Oxide film formation is important in a number of areas in electrochemistry, including corrosion, where the oxide film frequently offers protection to the surface, and in electrocatalysis, where oxide formation may limit the rate of the desired reaction or even inactivate the electrocatalyst all together. In 2003, Scherer et al. reported a study of passive oxide formation on the Ni surface [21]. In contrast to Fleischmann s earlier study in KOH electrolyte on polycrystalline Ni [3], this more recent investigation was on Ni l 11 in air and 0.05 mol dm H2SO4 solutions. The SXS measurements were collected in-situ, with the window of the cell deflated after the electrochemical reduction or passivation had been completed with 

For the adsorption of charged species (i.e., ions) an electrical term must be included in the criterion for adsorption equilibrium Equation 14.15 

Equation 14.39 is an implicit equation, because / is a function of o, which, in turn, is directly related to the degree of surface coverage, 0. Furthermore, y is influenced by the ionic strength of the medium. Evaluation of /(a) requires a model for the charge distribution in the electrical double layer. The models most currently applied are presented in Section 9.4. The diffuse double layer according to Gony and Chapman gives 

For the definition of K, please refer to Eqnation 9.29. The interfacial charge density a is determined by ion adsorption 

Oniax is the maximum charge density, that is, the difference between the charge densities corresponding to 0 = 0 and 0 = 1, respectively 9 is the surface coverage for which a = 0 

It is obvious that for surfaces containing only one type of ion-adsorbing group 0 = 0 or 0 = 1. For amphoteric surfaces, 0 0 1. Combining Equations 14.39,14.41, and 14.42 gives 

The interaction of an electrol3rte with an adsorbent may take one of several forms. The electrolyte may adsorb in total, in which case the situation is similar to that for molecular adsorption described earlier. It is more often observed that ions of one sign (+ or -) are held more 

FIGURE 9,14 Schematic representation of molecular arrangement dose to a solid surface showing the inner (IHP) and outer (OHP) Helmholtz planes, the stem layer, difAise double layer, also called the Gouy layer, and the slip plane where the zeta potential is measured. Also shown is the potential for various distances from the surface. 

This equation works for most simple metal oxides dispersed in HCl or KOH or KCl. Table 9.7 gives the values of pK, pK2, and PZC for various oxides. 

For some oxides other PDIs are important. For example, when a small amount of barium ion (i.e., 10 BaCl2) is added to a dispersion 

TABLE 9.7 Acid-Base Properties of Various Oxides 

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At the electrocapillary maximum, dyIdE is zero and hence a is zero. There may still be adsorption of ions, but in equal amounts, that is, T+ = F (for a 1 1 electrolyte). 

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]. 

Similarly, adsorption of ions (n+) onto a metal surface leads to a heat of adsorption of Q,. Generally, Q is about 2-3 eV and is greater than Q, which itself is about 1 eV. The difference between Q, and is the energy required to ionize neutrals (n ) on a metal surface so as to give ions (n+) or vice versa. This difference, Q - Q, can be equal to, greater than, or less than the difference, I - ( ), between the ionization energy (1) of the neutral and the ease with which a metal can donate or accept an electron (the work function, ( )). Where Q, - Q, > I - ( ), the adsorbed... 

The rate of evaporation of ions from a heated surface is given by Equation 7.3, in which Q, is the energy of adsorption of ions on the filament surface (usually about 2-3 eV) and Cj is the surface density of ions on the surface (a complete monolayer of ions on a filament surface would have a surface density of about 10 ions/cm" ). 

Electrostatic Interaction. Similarly charged particles repel one another. The charges on a particle surface may be due to hydrolysis of surface groups or adsorption of ions from solution. The surface charge density can be converted to an effective surface potential, /, when the potential is <30 mV, using the foUowing equation, where -Np represents the Faraday constant and Ai the gas law constant. 

In the same way that potential differences can occur due to different mobility, they can also occur due to different adsorption of ions. There are therefore a large number of possibilities for potential errors in the field of reference electrodes [2], which, however, are generally less than 30 mV. Such potential errors can be neglected in the application of protection potential criteria, but they can lead to increased error in the evaluation of voltage cones (see Section 3.3.1). Equation (3-4) can be used for their evaluation in this case. It explains, for example, the increased... 

Efficiency The effectiveness of the operational performance of an ion ex- changer. Efficiency in the adsorption of ions is expressed as the quantity of regenerant required to effect the removal of a specified unit weight of adsorbed material, for example, pounds of acid per kilogram of salt removed. 

Although the p.z.c. is difficult to determine experimentally, and although the values obtained vary with the method used, it is of fundamental significance in electrochemistry, since it provides information on adsorption of ions and molecules, i.e. if the potential is negative with respect to the p.z.c. cations will tend to be adsorbed and anions repelled, and vice versa. The p.z.c. appears to be a natural reference point for a rational scale of potentials defined by... 

The adsorption of ions due to an electrified interface can be evaluated by considering a series of laminae of the solution at various distances from the metal surface and assessing the number of ions present as compared with those that would have been present if the electrified interface had been... 

Figure 7.15 shows the adsorption of ion exchangers, downflow pattern. The bed has an adsorption zone with respect to time and is saturated with solute. 

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

Many precipitates, such as Fe(OH)3, form initially as colloidal suspensions. The tiny particles are kept from settling out by Brownian motion, the motion of small particles resulting from constant bombardment by solvent molecules. The sol is further stabilized by the adsorption of ions on the surfaces of the particles. The ions attract a layer of water molecules that prevents the particles from adhering to one another. 

Table 4.1. Free enthalpies of adsorption of ions and organie moleeules at metal eleetrodes.

Althongh van der Waals forces are present in every system, they dominate the disjoining pressnre in only a few simple cases, such as interactions of nonpolar and inert atoms and molecnles. It is common for surfaces to be charged, particularly when exposed to water or a liquid with a high dielectric constant, due to the dissociation of surface ionic groups or adsorption of ions from solution, hi these cases, repulsive double-layer forces originating from electrostatic and entropic interactions may dominate the disjoining pressure. These forces decay exponentially [5,6] ... 

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 ... 

Adsorption of ions from the solution. There are two types of ionic adsorption from solutions onto electrode surfaces an electrostatic (physical) adsorption under the effect of the charge on the metal surface, and a specific adsorption (chemisorption) under the effect of chemical (nonelectrostatic) forces. Specifically adsorbing ions are called surface active. Specific adsorption is more pronounced with anions. 

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... 

Grahame introdnced the idea that electrostatic and chemical adsorption of ions are different in character. In the former, the adsorption forces are weak, and the ions are not deformed dnring adsorption and continne to participate in thermal motion. Their distance of closest approach to the electrode surface is called the outer Helmholtz plane (coordinate x, potential /2, charge of the diffuse EDL part When the more intense (and localized) chemical forces are operative, the ions are deformed, undergo partial dehydration, and lose mobility. The centers of the specifically adsorbed ions constituting the charge are at the inner Helmholtz plane with the potential /i and coordinate JCj < Xj. 

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... 

In situ infrared spectroscopy allows one to obtain stracture-specific information at the electrode-solution interface. It is particularly useful in the study of electrocat-alytic reactions, molecular adsorption, and the adsorption of ions at metal surfaces. 

The properties of these systems depend strongly on the interfacial potentials created at the interface. They arise from oriented molecular dipoles, from ionization of the surfactant hydrophylic groups, and from the partition and adsorption of ions presented in the environment. 

In the case of the adsorption of ions, the adsorption free energy is proportional to the excess surface charge density [37],... 

Capacitance and surface tension measurements have provided a wealth of data about the adsorption of ions and molecules at electrified liquid-liquid interfaces. In order to reach an understanding on the molecular level, suitable microscopic models have had to be considered. Interpretation of the capacitance measurements has been often complicated by various instrumental artifacts. Nevertheless, the results of both experimental approaches represent the reference basis for the application of other techniques of surface analysis. 

Whereas in many instances potentiometric non-aqueous titrations of acids can show anomalies24 depending on the type of solvents and/or electrodes (owing to preferential adsorption of ions, ion pairs or complexes on the highly polar surface of the indicator electrode, or even adherence of precipitates on the latter), conductometric non-aqueous titrations, in contrast, although often accompanied by precipitate formation30, are not hindered by such phenomena sometimes, just as in aqueous titrations, the conductometric end-point can even be based on precipitate formation34. 

It is assumed that the quantity Cc is not a function of the electrolyte concentration c, and changes only with the charge cr, while Cd depends both on o and on c, according to the diffuse layer theory (see below). The validity of this relationship is a necessary condition for the case where the adsorption of ions in the double layer is purely electrostatic in nature. Experiments have demonstrated that the concept of the electrical double layer without specific adsorption is applicable to a very limited number of systems. Specific adsorption apparently does not occur in LiF, NaF and KF solutions (except at high concentrations, where anomalous phenomena occur). At potentials that are appropriately more negative than Epzc, where adsorption of anions is absent, no specific adsorption occurs for the salts of... 

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... 

The adsorption of ions is determined by the potential of the inner Helmholtz plane 0n while the shift of Epzc to more negative values with increasing concentration of adsorbed anions is identical with the shift in 0(m). Thus, the electrocapillary maximum is shifted to more negative values on an increase in the anion concentration more rapidly than would follow from earlier theories based on concepts of a continuously distributed charge of adsorbed anions over the electrode surface (Stern, 1925). Under Stern s assumption, it would hold that 0(m) = 0X (where, of course, 0X no longer has the significance of the potential at the inner Helmholtz plane). 

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