Specific adsorption, ITIES

The specific adsorption, which in the case of ITIES is usually the adsorption of ionic pairs [8], contributes to the Galvani potential, as well as changes the zero charge of this interface. 

Alternatively, it has been found that the Galvani potential of zero charge, in the absence of specific adsorption, equals zero. This means that there is no specific orientation of the molecules of both solvents, and the dipolar part of the Galvani potential, Eq. (12), is zero [8,22,41]. The observed discrepancies between the results of various measurements in different ITIES systems have been mainly caused by the specific adsorption [8]. Recently, the analysis of thermodynamic and free charge potentials at ITIES was performed by Volkov [42]. 

Although the correlation between ket and the driving force determined by Eq. (14) has been confirmed by various experimental approaches, the effect of the Galvani potential difference remains to be fully understood. The elegant theoretical description by Schmickler seems to be in conflict with a great deal of experimental results. Even clearer evidence of the k t dependence on A 0 has been presented by Fermin et al. for photo-induced electron-transfer processes involving water-soluble porphyrins [50,83]. As discussed in the next section, the rationalization of the potential dependence of ket iti these systems is complicated by perturbations of the interfacial potential associated with the specific adsorption of the ionic dye. 

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

Elegant studies of electrocapillarity of a nonpolarized ITIES by Gavach et al. [48] showed that the tetraethyl-, tetrapropyl- and tetrabutylammonium ions are not adsorbed within the compact layer and suggested that the interface is made of two space charge layers, described by the Gouy-Chapman theory, on either side of a central compact layer [49-51]. In a nonpolarized ITIES, the potential drop across the interface cannot be altered independently of the chemical potential of a salt of ionic constituents in either of the phases. The degree of specific adsorption cannot therefore be quantitatively estimated at a nonpolarized interface [28]. 

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... 

FIGURE 1.5 Schematic representation of the potential profiles across the ITIES in the absence (dashed line) and presence (solid line) of specific adsorption of anionic species from the aqueous phase to the interface. (Su, B., N. Eugster, and H. H. Girault, 2005, J Electroanal Chem, Vol. 577, p. 187. Used with permission.)... 

The main discrepancy between the investigations described above concerns the presence or absence of specific adsorption at the ITIES. Both Kakiuchi and Senda and Samec et... 

As mentioned by Kharkats and Ulstrup,simple dielectric considerations show that excess surface charges are expected on the side of the ITIES with a low dielectric constant, i.e., the organic phase accompanied by a surface charge depletion on the aqueous side. This very simple argument leads to the conclusion that hydrophobic ions in the organic phase are likely to be specifically adsorbed. The experimental results of Schiffrin et which show that the interfacial capacitance depends on the nature of the aqueous counterion demonstrate that this specific adsorption occurs via the formation of interfacial ion pairs. 

In particular, the coupling between the ion transfer and ion adsorption process has serious consequences for the evaluation of the differential capacity or the kinetic parameters from the impedance data [55]. This is the case, e.g., of the interface between two immiscible electrolyte solutions each containing a transferable ion, which adsorbs specifically on both sides of the interface. In general, the separation of the real and the imaginary terms in the complex impedance of such an ITIES is not straightforward, and the interpretation of the impedance in terms of the Randles-type equivalent circuit is not appropriate [54]. More transparent expressions are obtained when the effect of either the potential difference or the ion concentration on the specific ion adsorption is negli-... 

These results have been initially considered as evidence for specific ion adsorption at ITIES [71,72]. Its origin was ascribed to extensive ion pair formation between ions in the aqueous phase and ions in the organic phase [71] [cf. Eq. (20)], or to a penetration into the interfacial region [72]. The former model, which has been considered in this context earlier [60], allows one to interpret the enhanced capacity in terms of Eq. (22). Pereira et al. (74) presented more experimental data demonstrating the effect of electrolytes and proposed a simple model, which is based on the lattice-gas model of the liquid liquid interface [23]. Theoretical calculations showed that ion pairing can lead to an increase in the stored... 

The determination of isotherm data gives a first impression of the adsorbabil-ity of a compound onto a specific adsorbent. However, since it is an equilibrium method, kinetic aspects of the adsorption are not accounted for. These influences can only be determined by column tests, where a specific amount of adsorbent is contacted with a steady influent concentration of the substance under investigation. In pilot- and full-scale applications, the impact of kinetics become more important and are often limiting for the removal efficiency of a given adsorber. Therefore, column tests have to be performed in order to predict the removal potential by adsorption. 

Exanqiles of the adsoiption/desorption isotiierms are shown in Figure 3. In this representation is plotted as a function of the partial P/Po- Clearly diere are pronounced changes in QJ as toluene condenses in the porous HSQ film. Specifically there are two sets of adsorption/desorption isotherms corresponding to the isothermal P (squares) and T (circles) variations respectively. Itie open symbols denote the adsorption branch of the isotherm while the closed symbols indicate the desorption branch. Recall that the non-isothermal T variations have been transformed into their equivalent 20 °C isotherms through Eq. (2). This allows us to directly compare the true isothermal P/Po variations to their T variation counterpart. 

In a few reports, the interactions of DNA molecules with small organic molecules at the ITIES were investigated by voltammetry to observe the adsorption and desorption of their complexes. Specifically, the intercalations of DNA with A-methylphenantroline and acridine-calix[4]arene were probed voltammetrically by monitoring the transfer of the cationic intercalators. Electrostatic interactions between DNA and dimethyldioc-tadecylammonium at the DCE/water interface gave absorption waves although DNA extraction by the reverse micelles of this surfactant had been reported. ... 

There are some anomalous cases in which a specific atom shows both high electrophilicity and nucleophilicity due to the limitation of various basis set-dependent charge calculation procedures, and hence it is more appropriate to rationahze this concept of relative electrophilicity/nucleophUicity. Relative nucleophilicity is the nucleophilicity of a site relative to its own electrophilicity, and vice versa for relative electrophilicity. The idea of relative nucleophilicity/electrophilic-ity was first proposed by Roy et al. [81] to predict intramolecular reactivity sequences of carbonyl compounds. We have used a similar ratio for the first time to find the best di-octahedral smectite for nitrogen heterocyclics adsorption in terms of intermolecular interaction [82] and as well for the adsorption property of para and meta substituted nitrobenzene [83]. 

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