The Specific Adsorption of Ions

In the following discussion some important features of the experimental results for anion adsorption are presented. Emphasis is placed on data obtained at mercury electrodes in aqueous solutions. This simply reflects the fact that a large fraction of the existing data were obtained for these systems. This is followed by a presentation of the theory applied to ion adsorption at polarizable interfaces. 

The presence of ionic specific adsorption may be confirmed by examining the dependence of the PZC on electrolyte concentration. From the data presented in fig. 10.6, it is seen that the PZC measured with respect to a constant reference electrode is approximately independent of electrolyte concentration when ionic adsorption is absent. The classic example of such a system is NaF in water at an Hg electrode. However, when anion adsorption occurs the PZC shifts in the negative direction with increase in electrolyte concentration. This is most pronounced for the I anion in the case of the halides. When cations are adsorbed, the PZC shifts in the positive direction. The interpretation of the shift of the PZC with electrolyte concentration can be put on a sound thermodynamic basis when the electrode potential is measured with a reference electrode reversible to one of 

In order to determine the extent of ionic specific adsorption, several assumptions must be made in analyzing thermodynamic data for the polarizable interface. If anions are adsorbed, it is customary to assume that cations are not adsorbed in the same potential range. Eurthermore, it is assumed that the surface excess of cations is equal to that estimated by Gouy-Chapman theory. In order to illustrate the method of data analysis, the cell 

It is often more convenient to analyze the data at constant electrode charge density c rather than constant potential E. Introducing the Parsons function = y -I- a E, one may write 

Noting that dpg = RT din a, the charge density due to cations is given by 

---

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 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... 

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... 

The proposed thermodynamic treatment has two main advantages. It keeps the modelistic assumptions to a minimum and it is relatively simple. The first advantage increases the applicability and reliability of the treatment, since there is no need for questionable structural assumptions and / or drastical approximations, like those involved in statistical mechanical treatments. The second advantage allows many interesting adsorption phenomena, like co-adsorption and re-orientation processes as well as the effect of the specific adsorption of ions and the heterogeneity of the adsorbing surface on the equilibrium properties of adsorbed layers, to be readily taken into account. 

Also the choice of the electrostatic model for the interpretation of primary surface charging plays a key role in the modeling of specific adsorption. It is generally believed that the specific adsorption occurs at the distance from the surface shorter than the closest approach of the ions of inert electrolyte. In this respect only the electric potential in the inner part of the interfacial region is used in the modeling of specific adsorption. The surface potential can be estimated from Nernst equation, but this approach was seldom used In studies of specific adsorption. Diffuse layer model offers one well defined electrostatic position for specific adsorption, namely the surface potential calculated in this model can be used as the potential experienced by specifically adsorbed ions. The Stern model and TLM offer two different electrostatic positions each, namely, the specific adsorption of ions can be assumed to occur at the surface or in the -plane. 

Is the smeared stop-spike for ATh (Fig. 14) what we see for a hump on the measured capacitance curve, or is it screened by molecular reorientations. We don t know that. Moreover, the specific adsorption of ions, which was assumed absent, can give rise to a hump too. Somehow this feature is interesting because of its quantum-mechanical origin and direct dependence on the nature of the metal. The more densely packed the metal surface, the more abrupt the stop, the sharper the spike. Open surfaces should favor soft landing and the hump would... 

In contrast to y, the value of C can be measured not only for liquid electrodes but also for solid ones. Usually, the specific adsorption of ions increases the capacitance of the EDL, whereas the adsorption of organic molecules decreases it. In the framework of a simplified model of the double layer as a capacitor, this corresponds to varying the distance between the capacitor plates and decreasing the permittivity. In dilute solutions of a surface-inactive 1,1-electrolyte, a capacitance minimum appears in the vicinity of pzc (see curve 1 in Fig. 4). The position of this minimum is determined by the potential Eq = 0. In solutions with relatively high (>0.1 M) concentrations of surface-inactive electrolytes, this minimum of C, Fo-curve disappears, but the pzc position stiU corresponds to Fq = 0 (curve 2 in Fig. 4). Numerical integration of these C, Fo-curves... 

The direct in situ study of the specific adsorption of ions and organic species was dictated by the desire to obtain more and more direct information on the processes occurring on the surface of the electrode. This was one of the most important factors leading to the elaboration of nonelectrochemical methods for the study of specific adsorption. For instance, radio-tracer method, ellipsometry, and various spectroscopic methods (such as Raman spectroscopy) should be mentioned here. 

We discuss now how the morphology of a hydrophobic polymer can be modified by the contact with water. For the reasons described in previous section, this transformation depends on the amount of gases dissolved and the ionic species present. The typical size and nature of formed pattern can then be tuned if these variables are precisely controlled. Two processes must be considered. First, the GNs have a transforming effect, as described in next chapter. Second, the specific adsorption of ions may induce spontaneous surface deformation. 

The effects of ionic strength, ion valence and pH value on the streaming potential of the fiber membrane were investigated. It was demonstrated that the PVB membrane accumulates a weak negative charge due to the specific adsorption of ions. 

An attempt to describe the response of the EQCM in the double-layer region only on the basis of the properties of the diffuse double layer was undertaken in Ref. 61. In doing so, the specific adsorption of ions and the potential-dependent specific interactions of the solvent with the metal were entirely ignored. Under these assumptions one could think of three reasons leading to the observed dependence of frequency on potential (1) dependence of the surface tension on potential, (2) electrostatic adsorption of charged species, and (3) a local change in viscosity in the diffuse double layer. 

The specific adsorptions of ions can affect the Helmholtz layer. For example, if the total charge of anions adsorbed on the electrode surface is more than the... 

As early as 1901, Wulff described a thermodynamic treatment of the crystal shape changes based on an energy minimized total surface area [39]. It is nowadays well known that this purely thermodynamic treatment cannot always predict the crystal shape, because crystalhzation and crystal shape often also rely on kinetic effects and defect structures Hke screw dislocations or kinks etc. The specific adsorption of ions or organic additives to particu-... 

The specific adsorption of ions at the ITIES has been studied by Su et al. who have simulated the capacitance response for different adsorption isotherm conditions [77]. Figure 1.6 illustrates the effect of adsorption on the capacitances and potential distribution in the case of a potential dependent Langmuir isotherm for which the surface coverage is given by... 

The exponential nature of the accumulation of oppositely charged ions means that Gouy-Chapman theory predicts local concentrations which are much larger than that in bulk if the potential difference applied between the electrode and the solution greatly exceeds RT/F. In practice, the Stern layer of adsorbed solvent molecules and the specific adsorption of ions mediates the potential perceived by the diffuse component of the double layer to a great extent. 

Parsons, R. (1955). The specific adsorption of ions at the metal-electrolyte interphase. Transactions of the Faraday Society, Vol. 51,1518-1529. 

---