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How does hydration influence the binding of alkali-metal ions to sulfonate resins?

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chemical structure of cation exchanger — 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 sulfonate resins, the binding strength of alkali-metal ions is inversely related to their degree of hydration.1

Because the sulfonate ion (–SO $_{3}^{-}$ ) possesses a weak electrostatic field, it is unable to strip the primary hydration shell—the water molecules directly coordinated to the ion—from the mobile cations. Consequently, these ions retain their hydration shells while interacting with the resin.1

The influence of hydration on binding follows these principles:

Hydration Preference: More strongly hydrated ions, such as lithium, prefer to remain in the surrounding solution where there is more water available.1
Binding Strength: As a result, ions that are more strongly hydrated are held more weakly by the resin.1
Selectivity Sequence: This leads to a binding order where lithium (the most strongly hydrated) is held the most weakly, followed by sodium, potassium, and rubidium, with cesium (the least strongly hydrated) forming the strongest bond with the resin.1