Unexpectedly Strong Electrostatic Screening in Nano-Ion Solutions

Electrostatic interactions in soft matter and physical chemistry are fundamentally governed by the size and charge of the interacting species. The two polyoxometalates (POMs) with the Keggin structure phosphotungstate ${\left[{\mathrm{PW}}_{12}{O}_{40}\right]}^{3-}$ $\left({\mathrm{PW}}^{3-}\right)$ and silicotungstate ${\left[{\mathrm{SiW}}_{12}{O}_{40}\right]}^{4-}$ $\left({\mathrm{SiW}}^{4-}\right)$ occupy an intermediate position between classical ions and colloidal particles. Earlier attempts to categorize POMs within this spectrum have led to inconsistent results. Thus, herein we aim to shed light on the true nature of POMs based on their electrostatic screening behavior. Colloidal probe atomic force microscopy (CP-AFM) is employed to measure the interaction forces between charged silica microspheres across various aqueous POM solutions. Initially, pure ${\mathrm{PW}}^{3-}$ and ${\mathrm{SiW}}^{4-}$ solutions are investigated over a wide range of concentrations. The CP-AFM data reveal unexpectedly strong electrostatic screening by both POMs at millimolar concentrations, which we attribute to ion aggregation. These results are more consistent with an ionic nature of the POMs than with a colloidal character. Additional investigations of ${\mathrm{SiW}}^{4-}$ in the presence of NaCl as a background electrolyte support the original hypothesis and confirm that upon addition of salt, the same effect as for the pure POM solutions at higher concentrations is observed; the electrostatic screening is more effective than expected.