Nh2 rpo3h

The selectivity of these resins depends more on the complex that is formed rather than on the size or charge of the ions. Generally they are effective in polar and nonpolar solvents. However, the capacity for various ions is pH sensitive so that adsorption and elution can be accomplished by pH changes in the solution.

These chelating resins have found most of their use in metal ion recovery processes in the chemical and waste recovery industries. They may find use in fermentation applications where the cultured organism requires the use of metal ion cofactors. Specific ion exchange resins have also been used in laboratory applications that may find eventual use in biotechnology product recovery applications.[53]

A review of selective ion exchange resins has been compiled by Warshawsky.[54' A diaminotetratacetic polymer developed by Mitsubishi'55' was developed for the purification of amino acid feed solutions. The conversions of chloromethylpolystyrene into thiolated derivatives for peptide synthesis have been described by Warshawsky and coworkers.[56]

3.3 Porosity and Surface Area

The porosity of an ion exchange resin determines the size of the molecules or ions that may enter an ion exchange particle and determines their rate of diffusion and exchange. Porosity is inversely related to the cross-linking of the resin. However, for gel-type or microporous resins, the ion exchange particle has no appreciable porosity until it is swollen in a solvating medium such as water.

The pore size for microporous resins is determined by the distances between polymer chains or cross-linking subunits. If it is assumed that the cross-linking is uniform throughout each ion exchange particle, the average pore diameter of these resins can be approximated from the water contained in the fully swollen resin. The moisture content of cation resins as a function of the degree of cross-linking is shown in Figure 11 and, of anion resins, in Fig. 12. The calculated average pore size for sulfonic cation resins ranges from 16 to 20 A as the concentration of the resin cross-linking agent (divinylbenzene) decreases from 20 to 2%. The calculated average pore size of the anion resins ranges from 18 to 14 A as the cross-linking is decreased from 12 to 2%. Even at low cross-linking and full hydration, microporous resin have average pore diameters of less than 20 A. The dependence of pore size on the percent of the cross-linking is shown in Table 10 for swollen microporous resins of styrene-divinylbenzene. '57'

Sulfonic acid and carboxylic acid resins have also been equilibrated with a series of quaternary ammonium ions of different molecular weights to measure the average pore size when these resins have increasing degrees of cross-linking. These results are shown in Figs. 13 and 14.

Figure 15 shows the change in the ionic diffusion coefficients of tetra-alkylammonium ions in strong acid cation resins as a function of mean effective pore diameter of the resins.[58' As the pore diameter is increased, the penetrability of the resins with respect to the large ions also increased.

If an inert diluent (porogen) is incorporated into the monomer mixture before the copolymer is formed, it is possible to form a structure containing varying degrees of true porosity or void volume.'59"60' Variations in the amount of divinylbenzene cross-linking and diluent allow for a range of particle strengths and porosity to be made. Subsequent reactions with the appropriate chemicals result in the introduction of the same functional groups as discussed above. These are called macroporous or macroreticular resins.

Percent Divinyuehzene

Figure 11. Moisture content of strong acid cation resins as a function of divinylbenzene content.

Figure 11. Moisture content of strong acid cation resins as a function of divinylbenzene content.

Percent Divinylunzew

Figure 12. Moisture content of strong acid anion resins as a function of divinylbenzene content.

Table 10. Average Swollen Diameter of Cross-linked Polystyrene Beads in Tetrahydrofuran1571


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