Ion-pair adsorption

In addition to chromatography based on adsorption, ion pair chromatography (IP-HPLC) and capillary electrophoresis (CE) or capillary zone electrophoresis (CZE) are new methods that became popular and are sufficiently accurate for these types of investigations. Other methods involving electrochemical responses include differential pulse polarography, adsorptive and derived voltammetry, and more recently, electrochemical sensors. 

Immobilization method Covalent binding Adsorption Ion pair formation Entrapment or ship-in-a-bottle ... 

FIG. 9 Schematic illustration of adsorption of poly(styrenesulfonate) on an oppositely charged surface. For an amphiphile surface in pure water or in simple electrolyte solutions, dissociation of charged groups leads to buildup of a classical double layer, (a) In the initial stage of adsorption, the polymer forms stoichiometric ion pairs and the layer becomes electroneutral, (b) At higher polyion concentrations, a process of restructuring of the adsorbed polymer builds a new double layer by additional binding of the polymer. 

Direct measurement of adsorptive stripping voltaimnetric peaks using HMDE 0.60 V and accumulation potential of -0.40V Dilution in phosphate buffer and water, analyzed in Vis region Ion pair formation with octadecyltrimethylammonium bromide at pH 5.6, extraction of ion pair into n-butanol Sample solution mixed with 1 M HCl, ethanol and purification on Sephadex DEAE 25 gel, gel beads are filtered off, packed into 1 nun cell and absorbance measured... 

The interaction between the adsorbed molecules and a chemical species present in the opposite side of the interface is clearly seen in the effect of the counterion species on the HTMA adsorption. Electrocapillary curves in Fig. 6 show that the interfacial tension at a given potential in the presence of the HTMA ion adsorption depends on the anionic species in the aqueous side of the interface and decreases in the order, F, CP, and Br [40]. By changing the counterions from F to CP or Br, the adsorption free energy of HTMA increase by 1.2 or 4.6 kJmoP. This greater effect of Br ions is in harmony with the results obtained at the air-water interface [43]. We note that this effect of the counterion species from the opposite side of the interface does not necessarily mean the interfacial ion-pair formation, which seems to suppose the presence of salt formation at the boundary layer [44-46]. A thermodynamic criterion of the interfacial ion-pair formation has been discussed in detail [40]. 

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... 

Taking into account that both the voltammetric maximum and the depression in drop time-potential curves were affected by the ion pair formation equilibrium of Na DPA in LM, it is concluded that Na which has been transferred from NB to W may be adsorbed at the interface from the side of W inducing the adsorption of DPA as a counterion from the NB side. At the interface, the adsorbed Na may exist as an ion pair, which is denoted as Na DPAj, hereafter. The possibility of the interfacial ion pair formation between a hydrophobic cation (or anion) in an organic phase and a hydrophilic anion (or cation) in an aqueous phase has been proposed by Girault and Schiffrin [32], and Kakiuchi et al. [33]. 

The above-mentioned results demonstrate that the voltammetric maximum due to the adsorption of the transferring ion at one of two LM/W interfaces is requisite for the oscillation of AFwi-w2 or /wi-w2- However, the oscillation was not always realized with systems which gave maximum currents. The recovery of the concentration of the objective ion in the interfacial region of one phase from which the objective ion transfers to another is necessary in order to get the oscillation. If the ion pair which has been accumulated at the interface is desorbed to the phase to which the ion transfers, the oscillation cannot be realized. Taking into account this circumstance, the fact that the oscillation was not observed when in Eq. (11) was replaced by Br can be explained as follows. 

Whereas in many instances potentiometric non-aqueous titrations of acids can show anomalies24 depending on the type of solvents and/or electrodes (owing to preferential adsorption of ions, ion pairs or complexes on the highly polar surface of the indicator electrode, or even adherence of precipitates on the latter), conductometric non-aqueous titrations, in contrast, although often accompanied by precipitate formation30, are not hindered by such phenomena sometimes, just as in aqueous titrations, the conductometric end-point can even be based on precipitate formation34. 

In addition, (-potential measurements on PCP showed no significant effect of ionic strength on adsorption characteristics [123]. This finding suggests that no ion pair is formed, but also indicates that screening of the membrane surface by ions has little effect on the energetics of adsorption, i.e. PCP is buried at some depth below the membrane surface. 

Although adsorption of many types of species could be considered, this discussion will focus on surface hydrolysis reactions, that is, adsorption of H+ and OH . Virtually all surface hydrolysis experiments are carried out in the presence of a "background electrolyte," many of which appear to exhibit weak specific chemical interactions (e.g., ion-pair formation) with the surface (10-12). While consideration of these interactions is essential to a complete understanding of the interfacial chemistry, the topic is a subject in itself, and will not be considered in detail here. Treatment of these interactions is readily incorporated within the framework that is presented here. 

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