Exchange of cation

Figure C2.12.2. Fonnation of Br0nsted acid sites in zeolites. Aqueous exchange of cation M witli an ammonium salt yields tlie ammonium fonn of tlie zeolite. Upon tliennal decomposition ammonia is released and tire proton remains as charge-balancing species. Direct ion-exchange of M witli acidic solutions is feasible for high-silica zeolites.

A. lCBr titration of Enniatin B in methanol. Since exchange of cation between solution and carrier is relatively rapid only one signal is seen per chemically distinct carbonyl. The titration shows the magnitude of the chemical shift observed. Since Enniatin B may be considered a cyclic analogue of Gramicidin A, these chemical shifts indicate the magnitudes of chemical shifts that can be expected in the Gramicidin A channel (see Fig. 6 and 13) for direct interaction of carbonyl with cation. 

The exchange of cations for protons occurs through the transmembrane domain, whereas regulation (fine-tuning) of the exchanger is largely exerted by the C-terminal domain. NHEs are believed to form homodimers in the membrane. 

Figure 35. Schematic representation of the reversible variation of volume associated with the electrochemical switching of polypyrrole. Changes in free volume are mainly due to two effects electrostatic repulsions between fixed positive charges and exchange of cations, anions, and solvent molecules between the polymer and the solution. (Reprinted from T. F. Otero, H.-J. Grande, and J. Rodriguez, J. Phys. Chem. 101, 3688, 1997, Figs. 1, 3,6, 7, 13. Copyright 1997. Reprinted with the permission of the American Chemical Society.)...

If two ionic compounds have the same structure type, but in such a way that the cationic positions of one compound are taken by the anions of the other and vice versa ( exchange of cations and anions ), then they sometimes are called antitypes . For example in Li20 the Li+ ions occupy the same positions as the F ions in CaF2, while the O2- ions take the same positions as the Ca2+ ions Li2Q crystallizes in the anti-CaF2 type . 

Many mineralogic reactions involve exchange of cations or anions. Hence, geochemists commonly need to determine equilibrium lines in terms of activity ratios. Consider, for example, the reaction at 25 °C between the clay kaolinite [Al2Si205(0H)4] and the mica muscovite. The RXN commands... 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

Zeolite, zeolite group A collective term for a family of aluminosilicate minerals characterized by framework structures that allow easy access and exchange of cations and small molecules (see chapter 2), The name derives from the Greek terms zein, meaning to boil, and lithos, meaning stone. The term is also applied to synthetic materials of comparable composition, crystal structure, and physical properties (see chapter 2). 

FIGURE 7.4 Characteristic ion-exchange isotherms of zeolites (a) Regular system with selectivity for cation A, (b) incomplete ion exchange of cation A, (c) irregular system with selectivity reversal, (d) regular system with selectivity for cation B, and (e) irregular system with selectivity reversal. 

Zinc(II) and chromium(IV) cations are smaller (0.88 and 0.69 A respectively) than oxygen(II) anions (1.24 A), so one way to view this reaction is through cation migration. The zinc cations need to extract themselves from their oxide octahedral hole and move to the chromium compound to find an available tetrahedral hole. Simultaneously, chromium cations are moving in the direction of the zinc compound. After a few exchanges of cations, the two crystallites look as shown in Figure 5.2. 

A double displacement reaction involves the exchange of cations between two ionic compounds, usually in aqueous (water) solution. A double displacement reaction is also known as a double replacement reaction. 

Exchange of cations at localized sites driven by the presence of structural charges. MD and MC simulations give clear evidence that cations in the interlayer spacing are localized within a particular energy well caused by structural charges.24,25... 

The conditions of Donnan equilibrium derived in Equation 34.6 can be understood in a slightly different way. For the coupled transport described previously, the heterogeneous exchange of cation A in the aqueous phase and cation B in the membrane phase is represented by the following equation ... 

The variation of the far-infrared and Raman spectra of zeolites, induced by the exchange of cations, clearly indicates that the cationic degrees of freedom manifest themselves in that frequency region where large-amplitude deforma-... 

As the name implies, demineralization occurs through ideally simultaneous exchange of cations and anions across a uniform mixture of strong acid cation and strong base anion (Type 1) resins in the hydrogen and hydroxide forms respectively, all contained in a single vessel ... 

The zeolites are aluminosilicate Framework minerals of general formula [Al,.Si,0,f.. 2j -zH 0.- They are characterized by open structures that permit exchange of cations and water molecules (Fig. 16.2). In the synthetic zeolites the aperture and channel sizes may sometimes be controlled by a sort of template synthesis—the zeolite is synthesized around a particular organoammonium cation. This yields channels of the desired size. The zeolite framework thus behaves in some ways like a clathrate cage about a guest molecule (Chapter 8). The synthesis of zeolites also involves several other factors such as the Al/Si ratio, the pH. the temperature and pressure, and the presence or absence of seed crystals -- ... 

The ratio of the permeabilities of two cations in a cation exchange membrane is equal to the product of the ion exchange equilibrium constant and their mobility ratio (1). Therefore it is important to characterize the equilibrium ion exchange selectivity of ion exchange polymers in order to understand their dynamic properties when used in membrane form. Nafion (E.I. du Pont de Nemours and Co.) perfluorinated sulfonate membranes have found wide use in a variety of applications, many of which involve exchange of cations across membranes that separate solutions of different ionic composition. The inherent cationic selectivity of the polymer is an important consideration for such applications. Results of ion exchange selectivity studies of Nafion polymers are reviewed in this chapter, and are compared to those of other sulfonate ion exchange polymers. 

Verburg K., and P. Baveye. 1994. Hysteresis in the binary exchange of cations on 2 1 clay minerals A critical review. Clays Clay Miner. 42 207-220. 

In the same vein ESR technique has been used to try to characterize solid-solid reaction between a given salt or oxide and a zeolite matrix particularly by Kucherov, Slinkin and others (19, 20, 57) with elements as Cu, Cr, Fe, V on HZSM-5 (19) and on H-Ga ZSM-5 (20) and Fe and Mn on ZSM-5 (57). Usually the interpretation consists in assigning the interaction to cationic exchange of H" " or Na" by the added elements. This holds probably true but as mentionned above for B2O3 impregnated on HZSM-5, part of the element may be isomorphously substituted as T element in the framework. Exchange of cations as Cr " and their characterization was already studied on Y-type zeolite in the early 70 ies (60,61). 

Zeolite-hosted dyes can be prepared by ion-exchange of cationic dye molecules or by incorporation during hydrothermal crystallization [100]. Optical absorption bands of molecular sieve-accomodated dyes can be... 

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