Selectivity
of ion exchange resins
The selectivity coefficient of resins is defined in the ion exchange reactions page. The affinity of the resin for an ion is governed by the size of its hydrated form as well as its charge. The larger the ion, the more the resin bead must expand to accommodate it in the structure. The expansion is opposed by the restraining cross-links, so that large ions require a greater force to penetrate the resin than small ones. Ions with a higher charge have a higher affinity for the functional group of a given ion exchange resin.
Although the actual ionic diameter of each ion increases as we increase atomic number, the number of water molecules that are attracted as the hydration sphere goes down as atomic number increases. This means that charge density increases with atomic number.
| Ion | Atomic number | Hydrated diameter (Å) | Naked diameter (Å) |
|---|---|---|---|
| Alkaline (monovalent) | |||
| Li+ | 3 | 3.82 | 0.60 |
| Na+ | 11 | 3.60 | 0.95 |
| K+ | 19 | 3.31 | 1.33 |
| Rb+ | 37 | 3.28 | 1.48 |
| Cs+ | 55 | 3.27 | 1.69 |
| Alkaline earths (divalent) | |||
| Be++ | 4 | 4.59 | 0.31 |
| Mg++ | 12 | 4.26 | 0.65 |
| Ca++ | 20 | 4.11 | 0.99 |
| Sr++ | 38 | 4.10 | 1.13 |
| Ba++ | 56 | 4.05 | 1.35 |
A SAC resin has thus, among alkaline metal ions, the highest selectivity for caesium and the lowest for lithium.
For carboxylic and chelating resins, the above effect is not applicable, because divalent ions are complexed and no longer hydrated. On the other hand, the complex increases dramatically the resin selectivity. See the page about resin structure.
The pore size of a gel type resin is about 10 to 20 Å, which means the pores can accommodate easily inorganic ions. Resins with a higher degree of cross-linking have less elastic and slightly smaller pores. Macroporous resins have — in addition to the small gel pores — much larger pores (200 to 1000 Å or more), which can accommodate large organic ions.
You can see on separate pages selectivity tables for 
