Specific-Ion Adsorption

The idea in these papers67,223,224 was to identify the potential of the capacitance minimum in dilute electrolyte solutions with the actual value of Ea=o (i.e., <7ge0m( min) = Ofor the whole surface) and to obtain the value of R as the inverse slope of the Parsons-Zobel plot at min.72 Extrapolation of Cwom vs- to Cgg0m = 0 provides the inner-layer capacitance in the / C geom, and not C ea as assumed in several papers.67,68,223,224 In the absence of ion-specific adsorption and for ideally smooth surfaces, these plots are expected to be linear with unit slope. However, data for Hg and single-crystal face electrodes have shown that the test is somewhat more complicated.63,74,219,247-249 More specifically,247,248 PZ plots for Hg/... 

Schmidder and Henderson282 have studied several solvents and metals, using the jellium model for the metal and the MSA for the solution. Deviations of the Parsons-Zobel plot from linearity in the experimental results72,286-288 at the highest concentration have been attributed to the onset of ion-specific adsorption. However, data at other electrode charges... 

Soils and vadose zone information, including soil characteristics (type, holding capacity, temperature, biological activity, and engineering properties), soil chemical characteristics (solubility, ion specification, adsorption, leachability, cation exchange capacity, mineral partition coefficient, and chemical and sorptive properties), and vadose zone characteristics (permeability, variability, porosity, moisture content, chemical characteristics, and extent of contamination)... 

The amount of adsorbed hydrogen decreases in the presence of halide ions [395, 396]. This is due to a decrease in the M-H adsorption energy induced by ion-specific adsorption with partial charge transfer. The decrease in M-H bond strength results in an increase of overpotential. The effect is lower for Cl and higher for I -. However two joint effects are operative one due to electronic modifications, and the other one of an electrostatic nature related to a change in the local electric potential... 

Which ions are specifically adsorbed It depends, of course, on the metal, but detailed and accurate data are available only for mercury. As a rule, ions that are not hydrated tend to be specifically adsorbed. This includes most of the anions, but not F. Also, some highly symmetrical anions such as CIO, BF, and PF, are not specifi-cally adsorbed on mercury. Most cations are not specifically adsorbed on mercury. Cesium, which was found to be specifically adsorbed to some extent, is an exception. Also, large organic cations of the tetraalkyl ammonium type are found to be specifically adsorbed on mercury. For some ions, specific adsorption may be observed only at high concentrations, and it must always be remembered that such adsorption has been... 

Specifically adsorbed ions " are those which directly contact the electrode surface. As indicated in Figure 1, specifically adsorbed ions are considered to be desolvated, and they displace solvent molecules adjacent to the electrode surface. Iodide, which is weakly solvated by water, is a good example of an ion which tends to specifically adsorb on electrode surfaces. The nature of specific adsorption is a function of both electrostatic and chemical interactions between the electrode and the ion. Specific adsorption can significantly alter the interfacial potential profiles as well as the kinetics of interfacial reactions. The thin solution layer closest to the electrode surface which contains specifically adsorbed ions as well as solvent molecules is often called the inner layer or the Helmholtz layer. The inner Helmholtz plane (IHP) is considered to pass through the centers of specifically adsorbed ions (see Figure 1). 

Based on the above discussion, there are a number of factors that affect the double-layer behavior and the corresponding EDL capacitance, such as the concentration and size of ions, the ion-specific adsorption, the ion-solvent interaction, and the solvent in the electrolytes. The thickness of the EDL is typically on the order of several angstroms in aqueous solution. Since the distance separating the charges in an EDL is extremely small, the specific capacitance (capacitance normalized by the effective surface area) of EDL can reach a very high value in the aqueous electrolyte. In contrast, the specific capacitance of a typical parallel-plate capacitor is quite small... 

With the possible exception of fluoride ion, specific adsorption occurs as the electrode becomes positively charged and the anion penetrates the inner layer displacing the solvent. If specific adsorption is weak, the positive electrode charge is only partially shielded by the adsorbed inner layer charge, and consequently a diffuse layer of solvated anions is established Fig. 5.8(b). However,... 

In Eq. 11.7 the first term accounts for the compact layer thickness contribution to the potential jump, the second one for the effective dipolar layer contribution, and the third one for solvation and ions specific adsorption effects (not detailed here) (the other parameters are defined in ). X is an intermediate variable function of a and, and is the solution of the transcendental equation... 

Fig. 11. Effects of non-electrostatic electrode adsorption. The top differential capacitance curve illustrates effects from ion specific adsorption. In this case, neutral monomers adsorb weakly (a = 0.5), while anions (simple) and cations (charged monomers) display a strong...

Finally, we investigate the temperature and dielectric response dependence for an "ion-specific adsorption" electrode, with = 0.5 = 1. The DFT predictions are provided in Figure 12. Here we quite clearly see how the differential capacitance wUl drop upon a... 

Fig. 12. Temperature response, for the "ion-specific adsorption" system in Figure 11. Cases of a constant as well as temperature-increasing low frequency dielectric response are given.

For typical aqueous colloidal dispersions, the particles may carry some charges most likely due to the preferential (or differential) dissolution of particle surface ions, direct ionization of particle surface groups, substitution of particle surface ions, specific adsorption of ions, and particle surface charges originating from specific crystal structures. 

Regarding the theoretical PB-GC prediction, it is obvious that this approach cannot explain the experimental mobility unless a mechanism of ion-specific adsorption is assumed. Ottewill and Shaw argued that La + ions could be electrostatically bound to carboxylic groups. However, these groups are not present on the surface of these latexes. Accordingly, this theory is not able to reproduce the previously cited reversal and the mobility values are very different to the experimental and the HNC/MSA data (especially for the more charged system). 

Let us consider the simplest surface that shows ion-specific adsorption, namely the water-air interface. In a by now classical series of papers, Jungwirth and co-workers have shown that iodide ions do adsorb at the air-water interface, in strong contrast with the traditional view. Those simulations were performed with polarisable force fields, while the non-polarisable force fields employed at that time did not show adsorption of iodide. It was concluded that the polarisability plays a dominant role in the adsorption mechanism. Let us reconsider that problem using our novel thermodynamically optimised force fields discussed in the earlier section. We show results for the potential of mean force of a single ion at an air-water interface, calculated using umbrella sampling and the WHAM method. ... 

If the electrolyte contains an ion specifically adsorbed, the intersection point of the ao = f (pH) curves shifts and no longer represents the PZC determined with indifferent , or non-specifically adsorbed ions. Specific adsorption of cations shifts the intersection towards lower pH anions shift it towards higher pH. For example,... 

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