Why does the order of adsorption of hydrated alkali metals on an ion exchange resin decrease down the group?

I recently did a question on this topic and they say adsorption order is $$\ce{ Li+ > Na+ > K+ > Rb+}$$

I answered it the other way around. The reason that made me choose my answer was that $\ce{Li+}$ has a small atomic radius and hence it should have high hydration enthalpy. It should be forming $\ce{[Li(H2O)6]}$ complex which is large enough to show low adsorptivity.

So same trend should be followed as we go down the group. But, it is not the correct answer. So, please point out my mistake.

  • 1
    $\begingroup$ Consider charge concentration as well. $\endgroup$ – Güray Hatipoğlu Apr 19 '18 at 18:37
  • $\begingroup$ Mhmm.. I get it.That seems to be a reason .My post has been dead since I posted it and haven't got any other answers.. Thanks. $\endgroup$ – Nihal N.L Apr 20 '18 at 1:28

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