Dynamic ion exchange model

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... 

There are three popular hypotheses. Two models propose extreme situations and each encompasses a substantial amount of chromatographic data. These two proposals are the ion-pair model and the dynamic ion-exchange model. The third view, which is broader in scope than the previous two concepts, accommodates both the extreme views without combining the two models. This proposal is the ion-interaction model. 

Model makers named the technique solvent generated ion exchange [7] and hydrophobic chromatography with dynamically coated stationary phase [8], thereby emphasizing a dynamic ion exchange model. 

The mechanism of IPC was hotly debated for years and continues as a matter of debate. The retention model of Bidlingmeyer [15,16] is more comprehensive than the ion-pair and dynamic ion exchange models. Lipophilic IPRs, due to their adsorbo-philic nature, dynamically adsorb onto the alkyl-bonded apolar surface of the stationary phase, forming a primary charged ion layer and, together with counter ions... 

If one assumes the dynamic ion-exchange model, then equation (3.26) is obtained, which is identical in form to equation (3.24), demonstrating that retention data alone cannot distinguish these two models of retention and that more detailed studies are necessary (Horvath et al., 1977b). 

To obtain a simple interpretation of the experimental findings in IIC, theoretical chromatographers first adopted a stoichiometric strategy that pioneered this separation mode. Unfortunately, the reaction schemes of stoichiometric models in both the mobile phase (ion pair model) and stationary phase (dynamic ion exchange model) lack a firm foundation in physical chemistry because they are not able to account for the stationary-phase modification that results from the addition of the HR to the eluent, and they fail to properly describe experimental results, as pointed out by Bidlingmeyer et al. " Key insights on these retention models were also provided by... 

The effect of sodium carbonate as an inorganic additive is mechanistically not completely clear. According to the dynamic ion-exchange model, it is to be assumed that carbonate ions are found in a competing equilibrium with solute ions for the exchange groups that are adsorbed at the surface of the stationary phase. This is a plausible explanation for the strong effect of carbonate on the retention of divalent species. 

The neutral species are partitioned between the liquid stationary and mobile phases (KD is the relevant distribution constant). Separation is based upon the relative values of the distribution coefficient of the different neutral species. This model most closely explains the experimental results obtained with non-bonded reversed-phase columns (e.g. n-pentanol coated onto silica gel), in which the stationary phase behaves as a bulk liquid. The ion-pair model is, however, unable to explain ion-pair interactions with chemically bonded reversed-phase columns, and the working of these clumns is more appropriately explained by a dynamic ion-exchange model. 

Fig. 1 Schematic of (A) the ion-pair model (B) the dynamic ion-exchange model and (C) the ion-interaction model for the retention of anionic solutes in the presence of a lipophilic cationic HR. The solute and the HR are labeled on the diagram. The large hatched box represents the lipophilic stationary phase, the black circle with the negative charge represents the counter-anion of the HR, whilst the white circle with the positive charge represents the counter cation of the solute.

On the other hand, the results of several systematic studies of the parameters which govern retention in this type of chromatography were consistent with the predictions of the model for dynamic ion-exchange. Knox and Laird (53) examined the effect of hetaeron concentration on the retention of sulfonic acids and related dyestuffs on a short alkyl silica (SAS) stationary phase. The hetaeron used was cetrimide (cetyltrimeth-... 

A particularly compelling argument for dynamic ion-exchange has put forward the observation that retention of anionic and cationic sample components increases and decreases with increasing concentration of a cationic hetaeron, respectively. Whereas anionic hetaerons are expected to promote the elution of anionic eluites and to enhance the retention of cationic eluites, the quantitative data presented in this regard (226) are not wholly consistent with the model since the hetaeron concentration at which the effect is half-maximal is different for anionic and cationic eluites. If the observed phenomena were due to the presence of bound hetaeron in both cases, the two effects would have identical dependence on the hetaeron concentration in the mobile phase. 

Many subsequent stoichiometric mixed mode models are based on various combinations of these ion-pair and dynamic ion exchange extreme mechanisms. The effect of the IPR counter ion [13] and the reduction of available hydrophobic surfaces... 

According to the ion-exchange model [8-10], the lipophilic ion is adsorbed at the surface of the stationary phase forming LH. This renders the non-polar resin a dynamic ion exchanger. The solute ion E may subsequently interact with LH ... 

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