Are Keggin’s POMs Charged Nanocolloids or Multicharged Anions?

Owing to their multiple charges and their nanometric size, polyoxometalates (POMs) are at the frontier between ions and charged colloids. We investigated here the effect of POM–POM electrostatics repulsions on their self-diffusion in water by varying POM and supporting salt concentrations. The self-diffusion coefficients of two Keggin’s POMs [silicotungstate (SiW12O404–) and phosphotungstate (PW12O403–)] were determined by dynamic light scattering (DLS) and 1H/31P DOSY NMR, whereas POM–POM electrostatic repulsions were investigated by the determination of the static structure factors using small-angle X-ray scattering (SAXS). The self-diffusion coefficients for the two POMs and for different POM/background salt concentrations were collected in a master curve by comparing the averaged POM–POM distance in solution to the Debye length. As for classical charged colloids, we show that the POM’s counterions should not be considered in the calculation of the ionic strength that governs POM–POM electrostatic repu...