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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.

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    $\begingroup$ Consider charge concentration as well. $\endgroup$
    – Güray Hatipoğlu
    Commented Apr 19, 2018 at 18:37

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I think the order should be the other way around. The size of the hydrated Li ion will be the greatest, followed by Na, K and Rb. Refer

This will make the charge to size ratio smallest for the hydrated Li ion, giving it the lowest adsorptivity. That is confirmed by the relative selectivity coefficients of these ions with certain ion exchange resins. Refer

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