Hofmeister effect in the Keggin-type polyoxotungstate series

The chaotropic character of Keggin-type polyoxotungstate anions was evaluated with respect to their ability to bind to γ-cyclodextrin (γ-CD) by varying the global charge density of the nanometer-sized polyanion. The strengths of the host–guest association were analyzed within the series of isostructural [XW12O40]n− anions where the ionic charge varies from 6- to 3- depending on the heteroatom, respectively, X = H22+, B3+, Si4+ or P5+. Titration experiments using complementary techniques (ITC, DOSY NMR, and electrochemistry) revealed that the affinity between γ-CD and polyoxometalates (POMs) is directly correlated to the charge density of the Keggin anion as reflected in the values of the binding constants K1:1. These constants increase dramatically following the order: [H2W12O40]6− 105 M−1), but also the ability to interact with both CD faces resulting in a wide variety of supramolecular aggregates.