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How does cross-linking affect the structure of styrene-divinylbenzene ion-exchange resins?
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ion-exchange reaction
Figure 1: Chemical structure of cation exchanger. The exchangeable ions are marked +. The whole structure is permeated by solvent molecules, usually water (not shown).
In styrene-divinylbenzene ion-exchange resins, divinylbenzene acts as a cross-linking agent that joins polystyrene chains together to form a three-dimensional network.1
The degree of cross-linking, which is determined by the ratio of divinylbenzene to styrene, directly dictates the physical nature of this network:
- Structural Density: The network can be made either tight or loose depending on the proportion of divinylbenzene used.1
- Permeability and Swelling: While the resin beads appear smooth and impermeable, they are actually permeable to water and small ions. The openness of this structure—and consequently the rate at which ions can diffuse in and out—depends on the degree of swelling and the water content of the resin.12
- Size Exclusion: The cross-linked structure creates a network that can physically limit the entry of ions. While ions of moderate size may experience slow exchange due to the difficulty of entering the network, very large ions are unable to enter the resin structure at all.3
The standard commercial proportion for this cross-linking is typically 8 percent.1