Adsorption of Electrolytes

There are several variations to this technique. The negative adsorption method is based on the exclusion of co-ions from the electrical double layer surrounding charged particles [76]. 

The ion exchange method is based on replacing of loosely held ions by others of the same sign [30, p. 593]. A large amount of work has been done on the adsorption of electrolytes by ionic crystals and the adsorption of ions from solution on to metals. Since the adsorption tends to be very small and the measurements rather tedious, these are not suitable as routine methods. 

---

A logical division is made for the adsorption of nonelectrolytes according to whether they are in dilute or concentrated solution. In dilute solutions, the treatment is very similar to that for gas adsorption, whereas in concentrated binary mixtures the role of the solvent becomes more explicit. An important class of adsorbed materials, self-assembling monolayers, are briefly reviewed along with an overview of the essential features of polymer adsorption. The adsorption of electrolytes is treated briefly, mainly in terms of the exchange of components in an electrical double layer. 

Stem layer adsorption was involved in the discussion of the effect of ions on f potentials (Section V-6), electrocapillary behavior (Section V-7), and electrode potentials (Section V-8) and enters into the effect of electrolytes on charged monolayers (Section XV-6). More speciflcally, this type of behavior occurs in the adsorption of electrolytes by ionic crystals. A large amount of wotk of this type has been done, partly because of the importance of such effects on the purity of precipitates of analytical interest and partly because of the role of such adsorption in coagulation and other colloid chemical processes. Early studies include those by Weiser [157], by Paneth, Hahn, and Fajans [158], and by Kolthoff and co-workers [159], A recent calorimetric study of proton adsorption by Lyklema and co-workers [160] supports a new thermodynamic analysis of double-layer formation. A recent example of this is found in a study... 

Actually, it is recognized that two different mechanisms may be involved in the above process. One is related to the reaction of a first deposited metal layer with chalcogen molecules diffusing through the double layer at the interface. The other is related to the precipitation of metal ions on the electrode during the reduction of sulfur. In the first case, after a monolayer of the compound has been plated, the deposition proceeds further according to the second mechanism. However, several factors affect the mechanism of the process, hence the corresponding composition and quality of the produced films. These factors are associated mainly to the com-plexation effect of the metal ions by the solvent, probable adsorption of electrolyte anions on the electrode surface, and solvent electrolysis. 

Hence, the question arose whether the lifting of reconstruction is due to surface charging (as a result of the electrode potential), adsorption of electrolyte ions (or molecules), or both. Although various experimental and theoretical investigations have been carried out, the driving force for the lifting of reconstruction is not yet clear. 

As was mentioned in the introduction, Frumkin and his group [Kuchinsky et al. (107) and Frumkin et al. (10 )] proposed an electrochemical theory, according to which the adsorption of electrolytes by carbon would be determined by the electrical potential at the carbon-solution interface, and by the capacity of the double layer. At higher concentrations some physical adsorption of acid might occur in addition. 

The electrochemical properties of metals in an electrolyte solution are strongly connected to their surface properties on an atomic or molecular scale such as the structure of the surface and the presence of adsorbates. In this section, we consider how the interfacial potential is linked to the structure of the surface and the adsorption of electrolyte ions and solvent molecules... 

Let us evaluate how much the surface potential affects the surface adsorption of electrolyte ions. When only the short-range hydration interactions of ions with the... 

The electrical double layer at the metal oxide/electrolyte solution interface can be described by characteristic parameters such as surface charge and electrokinetic potential. Metal oxide surface charge is created by the adsorption of electrolyte ions and potential determining ions (H+ and OH-).9 This phenomenon is described by ionization and complexation reactions of surface hydroxyl groups, and each of these reactions can be characterized by suitable constants such as pKa , pKa2, pKAn and pKct. The values of the point of zero charge (pHpzc), the isoelectric point (pH ep), and all surface reaction constants for the measured oxides are collected in Table 1. 

Since the particle surface composition is determined by the adsorption of electrolyte ions, changes in surface composition of the particles are expected to play a role in the effect of the bath constituents. Kariapper and Foster67 found that the amount of metal ions adsorbed on a particle increases with increasing metal ion concentration in the electrolyte. For SiC particles this was again related to the -potential of the particles, because it was found68 that the -potential increases with... 

In the absence of specific adsorption of electrolyte ions, surface charge is considered to originate from acid-base dissociation of ionizable groups. In terms of acid groups (AH) and basic groups (B), the respective pH-dependent equilibrium between surface sites and solution at the interface can be represented as... 

Many more-sophisticated models have been put forth to describe electrokinetic phenomena at surfaces. Considerations have included distance of closest approach of counterions, conduction behind the shear plane, specific adsorption of electrolyte ions, variability of permittivity and viscosity in the electrical double layer, discreteness of charge on the surface, surface roughness, surface porosity, and surface-bound water [7], Perhaps the most commonly used model has been the Gouy-Chapman-Stem-Grahame model 8]. This model separates the counterion region into a compact, surface-bound Stern" layer, wherein potential decays linearly, and a diffuse region that obeys the Poisson-Boltzmann relation. 

The r.h.s. s only contain measurable variables. The ionic components of charge and or are obtainable except for a constant, by Integration of the Esln-Markov coefficient with respect to a°, see 13.4.16). Here, no single ionic excesses are counted but sums of electroneutral combinations, including the negative adsorption of electrolyte,, see 13.4.8). Therefore, dy is also... 

For Isolated spheres of water, the value to be taken for the equilibrium electrolyte concentration in a bulk phase, c( ), may be a problem. In the situation of fig. 3.16b, and c do not approach this value. When there is equilibrium with an external bulk phase, as with vesicles, this is no problem, because c (r = 0) and c.(r = 0) and c(oo) are simply related via the Boltzmann equation. When Boltzmann s law applies, the equilibrium concentration c(=o) in a (virtual) bulk phase can be written as c( o) = (c (r = 0) c (r = 0)). However, if there is no such equilibrium (say, for microdrops of water formed in a nonconducting oil. under highly dynamic conditions) c(o ) may differ between one drop and the other and nothing can be said in general. Alternatively, the negative adsorption of electrolyte can be computed if y is known (this is the Donnan effect). 

The conclusion is that the thermodynamic analysis contributes to framing models changes of mechanism as a function of coverage can be detected and the positive or negative) adsorption of electrolyte quantified. Similar remarks can be made about the temperature influence, either using (3.12.1] to obtain changes in surface entropy or by obtaining the isosteric enthalpy from adsorption isosters. 

By a similar mechanism, in other double layer parts where there is a deficit of positive charge (to the left in figs. 3.86a2 and 3.88), a local shortage of electrolyte is built up, which has to be replenished by diffusion from beyond the double layer. In fig. 4.25 these salt fluxes are sketched. Recall that for equilibrated double layers the equilibrium electrolyte concentration is higher than in the absence of a double layer because of the negative adsorption of electrolyte the present asymmetrical disturbance develops in addition to this. The excess of electrolyte on the one side and the deficit to the other try to annihilate each other by diffusion in the far field. 

Adsorption from solution is discussed by Adamson, but with emphasis on the equilibrium aspects, rather than the kinetics. The subject can conveniently be divided into adsorption of non-electrolytes and adsorption of electrolytes. The former can be treated for dilute solutions, in a similar manner to adsorption of gases on solid surfaces. Multilayer adsorption has been observed however, so that... 

Many properties of disperse systems are related to the distribution of charges in the vicinity of the interface due to the adsorption of electrolytes. The adsorption of molecules is driven by the van der Waals attraction, while the driving force for the adsorption of electrolytes is the longer-range electrostatic (Coulomb) interaction. Because of this, the adsorption layers in the latter case are less compact than in the case of molecular adsorption (i.e., they are somewhat extended into the bulk of the solution), and the discontinuity surface acquires noticeable, and sometimes even macroscopic thickness. This diffuse nature of the ionized adsorption layer is responsible for such important features of disperse systems as the appearance of electrokinetic phenomena (see Chapter V) and colloid stability (Chapters VII, VIII). Another peculiar feature of the adsorption phenomena in electrolyte solutions is the competitive nature of the adsorption in addition to the solvent there are at least two types of ions (even three or four, if one considers the dissociation of the solvent) present in the system. Competition between these ions predetermines the structure of the discontinuity surface in such systems -i.e. the formation of spatial charge distribution, which is referred to as the electrical double layer (EDL). The structure and theory of the electrical double layer is described in detail in textbooks on electrochemistry. Below we will primarily focus on those features of the EDL, which are important in colloid... 

Double-Layer Effects in the Absence of Specific Adsorption of Electrolyte... 

Corrections for the (j)2 effect can be made most readily for the mercury electrode, since the variation of with E and electrolyte concentration can be obtained from electrocapillary curves, as discussed in Section 13.2. In the absence of specific adsorption of electrolyte, (f)2 can then be calculated by assuming that the GCS model applies [from (13.3.26)]. Such corrections are less frequently attempted at solid electrodes because data about the doublelayer structure at them is often lacking. Typical results showing such corrections for the reduction of Zn(II) at a Zn(Hg) electrode in aqueous solution (58) and for the reduction of several aromatic compounds in MA -dimethylformamide solution (65) are given in Table... 

While these results involving double-layer corrections are very useful in explaining supporting electrolyte effects on rate constants, we must be aware of several limitations in this treatment. The absence of specific adsorption of electrolyte, reactants, and products is a rather rare occurrence. The limitations of the GCS model, as well as the general lack of... 

To account for the apparent strong adsorption of electrolyte ions at the ceramic powder/water interface, Yates et al. [10,21] proposed the formation of ion pairs at charged surface sites (the site-binding model for the metal oxide/aqueous solution interface). This model and its modifications have been successfully applied to many oxide/aqueous solution interfaces in the presence of simple monovalent inorganic ions [21-23]. For a ceramic powder surface in a simple electrolyte solution (e.g., KNO3 and NaCl), the formation of ion pairs can be represented as... 

---