Metal cations are one of the most important ions in regulating solution behaviors of colloid particles. In aqueous media, the binding affinities of metal cations widely-accepted in literature follow trends: Li+&lt;Na+&lt;K+&lt;Rb+&lt;Cs+ for monovalent cations and Mg2+&lt;Ca2+&lt;Sr2+&lt;Ba2+ for divalent cations, which are regulated by their radius of hydrated ions. Herein we report that the widely-accepted trends of binding affinities for metal cations can be reversed in a nanoconfined cavity with high charge density and suitable size. We use a lacunary Keggin-type polyoxometalate K7[α-PW11O39] as the model and study its ion-pair formation with metal cations by using isothermal titration calorimetry (ITC) and nuclear magnetic resonance (NMR) techniques. The results show that the hydration shells of metal cations will be destroyed significantly during their binding processes to the cavity, which leads to consequence that their binding affinities are more dependent on radius of bare ions rather than hydrated ions. As a result, the binding affinities of metal cations follow reversed trends: Li+&gt;Na+&gt;K+ for monovalent cations and Mg2+&gt;Ca2+&gt;Sr2+&gt;Ba2+ for divalent cations in the cavity.

Reverse binding affinities of metal cations in nanoconfined cavity

2019 Spring National Meeting

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