Cation exchanger cross-linking effect

Fig. 29. Effect of the extent of crosslinking of sulfonated cation exchangers (quantity of cross-linking agent) on diffusion coefficient of tetracycline in sorbent grains 1) SDMDMA, 2) SHMDMA, i) SEDMA, 4) Dowex-50W...

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

Resin Beads. The resin beads are prepared from commercially available Dowex 50W-X8, a sulfonic acid cation exchange resin. The 8% cross-linked resin was chosen because it does not shrink or expand excessively in acid-water systems and because it is readily available. Resin cross-linkage had no detectable effect on the final oxide products in the development test runs. The resin is hydraulically classified to produce a size fraction that is predominately 60-80 m in diameter. The classified resin is washed with 6 M HC1 to remove contaminants such as iron and rinsed thoroughly with water. 

FIGURE 19.3 Schematic of a cation resin bead showing the polystyrene chains with sulfonic groups, the divinylbenzene cross-links, and the active sulfonic groups where the ion exchange occurs. Because the void spaces between the polymer chains and crosslinks contain water, the effective ion-exchange surface area of the bead is not limited to its outside diameter. 

Sulfonated cation exchangers having the same 8% DVB as the cross-linking agent in the polystyrene matrix were found to be much less effective in the separation of electrolytes compared to the above anion exchangers. This fact can be related to the sufficiently larger swelling of the... 

Ion exchange resins are cross-linked polymers that form salts with ions from aqueous solutions. The ion exchange resins that are used to effect such exchanges are of two types cation exchange and anion exchange resins. By using the two types either singly or in combination, it is relatively simple to alter, remove, or recover the ionic content of dilute solutions at will. 

Pectic and Polygalacturonic Acids.—The effects of cross-linking of pectic acid on the cation-exchange properties, on the binding of endo-polygalacturonase, and on the biodegradability have been examined. ... 

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