Model ion exchange

Several theoretical models, such as the ion-pair model [342,360,361,363,380], the dyneuaic ion-exchange model [342,362,363,375] and the electrostatic model [342,369,381-386] have been proposed to describe retention in reversed-phase IPC. The electrostatic model is the most versatile and enjoys the most support but is mathematically complex euid not very intuitive. The ion-pair model emd dynamic ion-exchange model are easier to manipulate and more instructive but are restricted to a narrow range of experimental conditions for trtilch they might reasonably be applied. The ion-pair model assumes that an ion pair is formed in the mobile phase prior to the sorption of the ion-pair complex into the stationary phase. The solute capacity factor is governed by the equilibrium constants for ion-pair formation in the mobile phase, extraction of the ion-pair complex into the stationary phase, and the dissociation of th p ion-pair complex in the... 

Sellergren, B. and Shea, K. J., Chiral ion-exchange chromatography Correlation between solute retention and a theoretical ion-exchange model using imprinted polymers, /. Chromatogr. A, 654, 17, 1993. 

Clay ion-exchange model May be useful for predicting adsorption of heavy metals. Aqueous-phase-... 

The clay ion-exchange model assumes that the interactions of the various cations in any one clay type can be generalized and that the amount of exchange will be determined by the empirically determined cation-exchange capacity (CEC) of the clays in the injection zone. The aqueous-phase activity coefficients of the cations can be determined from a distribution-of-species code. The clay-phase activity coefficients are derived by assuming that the clay phase behaves as a regular solution and by applying conventional solution theory to the experimental equilibrium data in the literature.1 2 3... 

The ion exchange model is most commonly applied in geochemistry to describe the interaction of major cationic species with clay minerals, or the clay mineral fraction of a sediment it has also been applied to zeolites and other minerals, and to ions besides the major cations (e.g., Viani and Bruton, 1992). As the name suggests, the model treats not the sorption and desorption of a species on the surface and in the interlayers of the clay, but the replacement of one ion there by another. 

To allow for its numerical solution, we formalize our discussion of the ion exchange model by including in the basis a species Ap (e.g., >X Na+) representing the exchanging site (Eqn. 9.21). This species has a molal concentration mp, and the... 

No value for dmq/dmp is needed to evaluate the and Freundlich models. For the Langmuir model and the ion exchange model under the Gaines-Thomas and Gapon conventions,... 

Fig. 2 Reactions of benzoic anhydride in CTABr , 0.01 M NaOH , 0.02 M HC02Na. The lines are calculated using the ion-exchange model. (Reprinted with permission of the American Chemical Society)...

Some examples of micellar rate enhancements of bimolecular reactions of electrophiles are shown in Table 5. Generally the surfactant was SDS with added electrophile, e.g. H30+ or a metal ion, but sulfonic acids were also used so that HaO+ was the counterion and there was no interionic competition. The maximum rate enhancements, knl, depend upon the specific conditions of the experiment, and, as predicted by the pseudophase ion-exchange model, generally decrease with increasing concentration of the electrophilic ion. In some cases the reactions were too fast for measurement... 

The acid hydrolysis of micellized alkyl sulfates (Kurz, 1962 Motsavage and Kostenbauder, 1963) has recently been very carefully reinvestigated (Garnett et al., 1983). For relatively dilute micellized alkyl sulfate, salt inhibition follows the predictions of the pseudophase ion-exchange model, with the expected salt order. But this order is not followed with more concentrated alkyl sulfate, and these results are a very interesting deviation from the widely observed pattern of micellar salt effects. 

The ion-exchange model has also been successfully applied to reactions of hydrophilic anions in microemulsions or alcohol-swollen micelles (Mackay, 1982 Bunton and de Buzzaccarini, 1982 Athanassakis et al., 1982). 

A very careful analysis of the pseudophase ion-exchange model has been given by Romsted who reviewed the evidence up to 1982 and considered the limitations of the treatment (Romsted, 1984). 

An alternative approach to this problem is to assume that deprotonation of a weak acid in the micellar pseudophase will be related to the concentration of bound OH-, which should follow the ion-exchange model (Section 5). As a result much of the work has been based on the use of very weak acids which are deprotonated only at high pH. 

CPE XI returned to Cairo, Egypt in 1997, and papers and posters were presented on adsorption, analytical methods, chemical/biological/treatment, groundwater studies, ion exchange, modeling, risk assessment, waste minimization and treatment, and for the first time, ISO 14001, which focuses on environmental management and quality systems. 

The data in fig. 5 were simulated using the multi-site ion exchange model of Barrer and Klinowski (92). The model essentially consists in assigning intrinsic selectivity coefficients to the... 

A pseudophase ion exchange model has been applied to reactions in micellar systems with varying success (1-7). According to this model, the distribution of nucleophile is considered to depend on the ion-exchange equilibrium between the nucleophile and the surfactant counterion at the micelle surface. This leads to a dependence on the ion-exchange constant (K g) as well as on the degree of dissociation (a) of the surfactant counterion. The ion exchange (IE) model has recently been extended to oil in water microemulsions (8). 

This reaction has been used as a test of the ion-exchange model in micelles ( ) and the value of K g for cyanide differs from that of fluoride or hydroxide by about an order of magnitude. 

Figure 3. Ion-exchange model plot (vide text). The data are for...

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

These authors showed that near surface decreases in the Ca/Ti ratio determined by XPS are accompanied by the formation of an amorphous Ti-rich layer up to 10 nm thick, as observed by TEM (Pham et al. 1989 Turner et al. 1989). The authors proposed a base catalysed hydrolysis and ion exchange model to account for their observations, whereby surface Ca2+ is released to solution via exchange with H+ and Ti-O-Ti surface species are converted to Ti-OH species via reaction with OH and H20. The overall reaction can be written as follows (Pham et al. 1989) ... 

Mehran and Tanji (1974) Proposed irreversible first-order kinetics for nitrification, denitrification, mineralization, immobilization, and plant uptake and reversible first-order kinetics for NH4 ion exchange. Model verified with published incubation data. 

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