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 gamma-cyclodextrin (gamma-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 = H-2(2+), B3+, Si4+ or P5+. Titration experiments using complementary techniques (ITC, DOSY NMR, and electrochemistry) revealed that the affinity between gamma-CD and polyoxometalates (POMs) is directly correlated to the charge density of the Keggin anion as reflected in the values of the binding constants K-1:1. These constants increase dramatically following the order: [H2W12O40](6-) < [BW12O40](5-) < [SiW12O40](4-) < [PW12O40](3-). Additionally, cloud point experiments on a non-ionic surfactant resulted in the same series of POMs, emphasizing the general affinity of these inorganic Keggin-ions to non-ionic organic soft matter due to a general solvent effect arising from the weakening of the hydration sphere with decreasing ionic charge. Furthermore, single crystal X-ray diffraction analysis showed distinct organizations of the POMs with gamma-CD in the solid-state, where the moderate chaotrope [BW12O40](5-) interacts with the external wall of the gamma-CD, while on the other side of the series, [PW12O40](3-) penetrates deeply into the cavity of the gamma-CD through its secondary rim offering optimal contact area. Finally, our investigations revealed the unique behavior of [PW12O40](3-), which displayed not only the highest affinity of the POMs to gamma-CD (K-1:1 > 10(5) M-1), but also the ability to interact with both CD faces resulting in a wide variety of supramolecular aggregates.

Automated summary

By varying the charge density of Keggin-type polyoxotungstate anions, the affinity of polyoxometalates (POMs) to gamma-cyclodextrin (γ-CD) was found to be directly correlated, providing insights into their interactions and behavior within supramolecular aggregates.