Counter-ion exchange

The protonation of the triplet jtjt state of 3-bromonitrobenzene is shown to be responsible for the acid-catalysed promotion of halogen exchange which follows a S y23Ar mechanism26 (equation 23). Cationic micellar effects on the nucleophilic aromatic substitution of nitroaryl ethers by bromide and hydroxide ions have also been studied27. The quantum efficiency is dependent on the chain length of the micelle. The involvement of counter ion exchanges at the surface of ionic micelles is proposed to influence the composition of the Stem-layer. 

Finally, clays such as the smectites almost invariably have a net negative structural charge because of isomorphous substitution of cations of lower charge than would be present in a balanced structure. In kaolinite, the amphoteric nature of the hydrated aluminum and silica surface contributes more to surface charge than does substitution. As a result of either substitution or surface dissociation, a region of counter ions (exchangeable and... 

Edges and plates require different techniques to determine a°. For the former, the titration technique as used for oxides is appropriate, whereas on the plates only counter ion exchange can be realized, for which the maximum number of exchangeable groups can be established, the cation exchange capacity (c.e.c.). The problem is that one cannot easily measure either the plates only or the edges only, but only their summed responses. This is of course also the case with other techniques like conductometry emd electrophoresis. 

A decrease of C Cl and the formation of C Pi result because of a counter ion exchange reaction... 

This interfacial convection is possible during a notable time only if some process of desorption occurs. Now, the desorption can be spontaneous with a short chain surfactant, which is easily desorbable. In this case, the kinetics of transfer, adsorption and desorption have to be considered simultaneous in order to account for some instability, that is the same molecule will be adsorbed and desorbed. When the surfactant is not or only slightly desorbable - that is if the chains are long - the desorption cannot be spontaneous but it can happen that it is assisted by an interfacial reaction which transforms a very adsorbable species into an easily desorbable one as by our counter ion exchange reaction. 

Batey has reported Pd-catalyzed cross-coupling reactions of tetraalkylammo-nium aryl trifluoroborate salts bearing functional groups such as acetyl, nitro and chloro with functionalized aryl bromides under mild conditions 35]. These tetra-alkylammonium organofluoroborates are prepared from the respective boronic acids using counter ion exchange protocol. They are air and moisture stable and are soluble in various organic solvents (Scheme 3.21). 

Ion-exchange methods are based essentially on a reversible exchange of ions between an external liquid phase and an ionic solid phase. The solid phase consists of a polymeric matrix, insoluble, but permeable, which contains fixed charge groups and mobile counter ions of opposite charge. These counter ions can be exchanged for other ions in the external liquid phase. Enrichment of one or several of the components is obtained if selective exchange forces are operative. The method is limited to substances at least partially in ionized form. 

The use of totally permeated flow markers in aqueous GPC offers similar advantages along with many of the same shortcomings that one finds in nonaqueous GPC. One problem commonly found in aqueous GPC is that salt peaks due to the on-column ion exchange of counter ions of a polyelectrolyte with dissimilar ions in the GPC mobile phase will occur at or near the total permeation volume of the column. These salt peaks will often obscure the flow marker used in the analysis. Short of preconditioning the sample to exchange... 

Interestingly, the specific environment of the ionic solvent system appears to activate the chiral Ni-catalyst beyond a simple anion-exchange reaction. This becomes obvious from the fact that even the addition of a 100-fold excess of Fi[(CF3S02)2N] or Na[BF4] in pure, compressed CO2 produced an at best moderate activation of Wilke s complex in comparison to the reaction in ionic liquids with the corresponding counter-ion (e.g., 24.4 % styrene conversion with 100-fold excess of Fi[(CF3S02)2N], in comparison to 69.9 % conversion in [EMIM][(CF3S02)2N] under otherwise identical conditions). 

The major chemical problem met in ion-exchange practice is the fouling or poisoning of the anion resins by organic matter. The various counter measures deployed include pre-flocculation, oxidation of the organic material, the use of specially developed resins, and treatment of the fouled resins by brine and/or hypochlorite. 

Increasing the number of ion-exchange sites by the use of counter-current (counterflow) regeneration... 

A design feature of an ion exchange plant whereby service flow and regenerant flow are in the same direction. The opposite of counter-current (counterflow). 

Exchange of counter-ions (and solvent) between the polymer and the solution in order to keep the electroneutrality in the film. In a compacted or stressed film, these kinetics are under conformational relaxation control while the structure relaxes. After the initial relaxation, the polymer swells, and conformational changes continue under counter-ion diffusion control in the gel film from the solution. 

There has been much interest in modifying the ion-exchange properties of conducting polymers with substituents or polymeric counter-ions. Cationic substituents, such as ammonium234 and pyridinium235 groups, increase the polymer s anion-exchange capacity and increase anion trans-... 

Continuous ion-exchange separation technology, which adopts counter-current processing in a rotating turntable, is currently making an impact and 70% of the world s lysine manufacture units use this technology. 

In coulometry these exchange membranes are often used to prevent the electrolyte around the counter electrode from entering the titration compartment (see coulometry, Section 3.5). However, with membrane electrodes the ion-exchange activity is confined to the membrane surfaces in direct contact with the solutions on both sides, whilst the internal region must remain impermeable to the solution and its ions, which excludes a diffusion potential nevertheless, the material must facilitate some ionic charge transport internally in order to permit measurement of the total potential across the membrane. The specific way in which all these requirements are fulfilled in practice depends on the type of membrane electrode under consideration. 

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