synthetic macromolecular (polymeric) organic ion exchangers. According to the general classification of ion exchangers, ion-exchange resins are divided into cation-exchange resins (polyacids), anion-exchange resins (polybases), and amphoteric, or bipolar, resins (polyampholites). Cation-exchange resins may be of the strong-acid or weak-acid types; anion-exchange resins may be of the strong-base or weak-base types. If the electric charge carriers in the molecular network of an ion-exchange resin are fixed ions (functional, or ionogenic, groups) of a single type, such as sulfonic acid groups, the ion-exchange resin is called monofunctional. However, if the resin contains ionogenic groups of various types, it is called polyfunctional.
According to structural criteria, a distinction is made between microporous, or gel-like, resins and macroporous resins. The particles of gel-like resins are homogeneous. Ion exchange in a gel-like resin-solution system is possible as a result of diffusion of ions undergoing exchange through the molecular network of the swelled ion exchanger. Macroporous resins are heterogeneous; their particles have a spongelike structure—that is, they are penetrated by a system of open pores with the average diameter (200–300 to 1,000–1,200 angstroms) greatly exceeding the sizes of the solvent molecules and the ions being exchanged. The electrolyte solution freely penetrates the particles of such ion-exchange resins through the pores, which greatly facilitates ion exchange, particularly in nonaqueous systems.
Ion-exchange resins may be regarded as insoluble polyelectrolytes.
|Table 1. Properties of some Soviet industrial ion-exchange resins|
|Expressed in terms of the number of milliequivalents of ions absorbed by 1 g of dry resin upon contact with standard solution of sodium hydroxide (for cation-exchange resins)|
|Static exchange capacity1(meq/g)||Specific volume2(ml/g)||Maximum use temperature (°C)||Basic raw material|
|Strong-acid cation-exchange resins|
|KU-1 ................||4.2-4.5||2.6-3.0||80||phenol, formaldehyde|
|KU-2 ................||4.8-5.2||2.5-2.9||130||styrene, divinylbenzene|
|Weak-acid cation-exchange resins|
|KB-2 ................||10-11||2.6-3.0||100||acrylic acid, divinylbenzene|
|KB-4 ................||85-10||26-30||100||methacrylic acid divinylbenzene|
|Strong-base anion-exchange resins|
|AV-16 ................||8.0-9.5||3.6-4.2||90||polyamines, epichlorohydrin, pyridine|
|AV-17 ................||3.5-4.2||2.5-3.0||50||styrene, divinylbenzene|
|Weak-base anion-exchange resins|
|AN-2F ................||85-10||25-32||50||polyamines phenol|
|AN-18 ................||3 5-5.0||20-25||60||styrene divinylbenzene|
|EDE-10P ................||8.5-9.5||2.6-3.2||45||polyamines, epichlorohydrin|
The multicharged ion, which forms the structural network of the ion-exchange resin, is virtually immobile because of its high molecular weight. This ion network, or ion grid, binds small, mobile ions of opposite sign (counterions), which are capable of equilibrium exchange for ions from the surrounding solution. The properties of some industrial brands of Soviet ion-exchange resins are listed in Table 1. The average particle size of such ion-exchange resins is 0.2–2.0 mm, and the corresponding bulk densities are 0.5–0.9 ton/m3.
Ion-exchange resins are produced by polymerization, poly-condensation, or polymer-analogous transformations (so-called chemical treatment of polymers that initially did not have ion-exchange properties). Ion-exchange resins derived from styrene-divinylbenzene copolymers have become widely used in industry. These resins include strong-acid cation exchangers, as well as strong- and weak-base anion exchangers. The basic raw materials for the industrial synthesis of weak-acid cation exchangers are acrylic and methacrylic acids and the corresponding esters. Ion-exchange resins based on phenol-aldehyde polymers and polyamines are also manufactured in large quantities. Controlled synthesis of ion-exchange resins results in materials with the desired engineering characteristics.
Ion-exchange resins are used for desalinization of water, extraction and separation of rare elements, and purification of the products of the organic and inorganic synthesis.
L. A. SHITS