Ionic specific adsorption

In some cases, e.g., the Hg/NaF q interface, Q is charge dependent but concentration independent. Then it is said that there is no specific ionic adsorption. In order to interpret the charge dependence of Q a standard explanation consists in assuming that Q is related to the existence of a solvent monolayer in contact with the wall [16]. From a theoretical point of view this monolayer is postulated as a subsystem coupled with the metal and the solution via electrostatic and non-electrostatic interactions. The specific shape of Q versus a results from the competition between these interactions and the interactions between solvent molecules in the mono-layer. This description of the electrical double layer has been revisited by... 

The potentials of zero charge considered in this chapter are those in the absence of specific adsorption of ionic as well as nonionic species. There has been no attempt to review the enormous amount of data on the effect of specific adsorption on Ea+j, except for the few cases where extrapolation back to zero specific adsorption has been used as a more accurate way to determine <7-o- However, specific adsorption is difficult to relate quantitatively to the structure of interfacial water as well as to the effect of the metal. 

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. 

Parsons distinguishes four limiting cases of adsorption at electrodes with the real situations often crossing the classification boundaries [94] (a) purely electrostatic adsorption, (b) specific ionic adsorption, (c) adsorption of uncharged species (but including dipoles) and (d) chemisorption. 

Specific ionic adsorption is that adsorption not accounted for by the... 

The Helmholtz-von Smoluchowski equation indicates that under constant composition of the electrolyte solution, the electro-osmotic flow depends on the magnitude of the zeta potential, which is determined by the different factors influencing the formation of the electric double layer, as discussed earlier. Each of these factors depends on several variables, such as pH, specific adsorption of ionic species in the compact region of the double layer, ionic strength, and temperature. 

Apart from pure ion-exchange processes, non-ionic interactions with the stationary phase are also observed with specific ionic species. The most important non-ionic interaction is adsorption. 

Most commonly, such substrate effects are expected to arise from variations in the work terms w and w, in this relation where the subscript p and s refer to precursor and successor states. For outer-sphere reactions, w and w will be determined at least in part by <() and [Eq. (a) in 12.3.7.3]. Because 4> depends upon both q" and q, substantial variations in k (up to 10- to 100-fold) may arise from this source. For solid electrodes there can be considerable uncertainties in , because the extent of specific ionic adsorption of the supporting electrolyte often is unknown. However, this difficulty can be minimized by employing uncharged or singly charged reactants. 

In the area of interfacial charging at the solid/liquid interface of metal oxide aqueous suspensions, the "surface complexation or site binding concept is commonly used [3-20]. This concept is characterised by consideration of specific ionic reactions with surface groups, rather than assuming simple binding of ions to the surface or their accumulation at the interface (adsorption). In the past decade several different models were introduced on the basis of the surface complexation model (SCM) they differ in the assumed structure of the electrical interfacial layer (EIL) and in the proposed mechanisms and stoichiometries of surface reactions leading to surface charge. 

Many recently published review articles, book chapters, and even entire books are devoted to adsorption of ionic species. Usually they cover one adsorbent (or a group of related adsorbents) or specific method(s). Some of these publications were very helpful during the preparation of this book, but current original papers were the main source of information. 

Specific ionic adsorption can alter the potential profile in the interfacial zone to an extreme degree. Figure 13.3.8 is a set of curves presented by Grahame (2) for a mercury interface with 0.3 M NaCl. Note particularly the traces for the most positive potentials. These profiles can influence electrode kinetics by mechanisms considered in Section 13.7 below. 

Ordinary chromatographic techniques depend for their effectiveness on differences (often small) in adsorption, partition, ionic charge, or size, between solute molecules of similar chemical character. A modified gel technique in which use is made of the high specificity of biochemical reactions is affinity chromatography [54,55]. 

Periodic perturbations of the potential across the Hquid/liquid boundary induce a modulation of the concentration of species located in the interfacial region. By collecting the spectroscopic signals at the same frequency as that of the potential perturbation, employing phase-sensitive detection, the interfacial sensitivity of the measurements is tremendously enhanced, as the contribution from species in the bulk of the electrolyte solutions can be effectively neglected. Based on this principle, Fermfn and co-workers introduced potential-modulated reflectance (PMR) and potential-modulated fluorescence (PMF) to study a variety of processes including ion transfer [22], electron transfer [20], and the specific adsorption of ionic species [15]. 

A specific type of the electrified interface is given by the contact of an insulating phase with electrolyte solutions. Since the charged species cannot cross the insulator, the EDL formation originates from the ionization process of surface groups (most frequently, the proton-based dissociation or association) or/and the adsorption of ionic... 

The streaming electrode measurements show a constancy of the p.z.c. value, Ea=o, for different concentrations, which is considered as the necessary condition for the absence of a specific ionic adsorption. In this case one can obtain the charge-potential relations for all concentrations by the integration of the corresponding... 

The first studies of capacitance curves for numerous PC metals were qualitatively in accordance with these expectations for carefully prepared smooth PC surfaces in the absence of the specific adsorption of ionic or neutral species, a minimum was observed in sufficiently dilute solutions, the depth of which increases for lower electrolyte concentrations. Therefore the potential of this minimum was interpreted as the p.z.c., Eo=o, of the metal-solution interface. 

Another example of specific ion adsorption was discussed in terms of the formation of interfacial ion pairs between ions in the aqueous and the organic phase. The contribution of specific ionic adsorption to the interfacial capacitance can be calculated using the Bjerrum theory of ion-pair formation. The results show that a phase boundary between two immiscible electrolyte solutions can be described as a mixed solvent region with varying penetration of ion pairs into it, depending on their ionic size. The capacitance increases with increasing ionic size in the order Ii+ < Na+ < K" " < Rb < Cs" ". Yufei et al. [22] found that significant specific ion adsorption occurs at the interface between two immiscible electrolytes... 

Siretanu, 1., Chapel, J. P., Bastos-Gonzalez, D., Drummond, C. lons-induced nanostructuration effect of specific ionic adsorption on hydrophobic polymer surfaces. J. Phys. Chem. B 117, 6814-6822 (2013)... 

Excess Charge Associated with the Specific Adsorption of Ionic Porphyrins... 

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