Aggregation of Heteropolyanions in Aqueous Solutions Exhibiting Short-Range Attractions and Long-Range Repulsions

Charged colloids and proteins in aqueous solutions interact via short-range attractions and long-range repulsions (SALR) and exhibit complex structural phases. These include homogeneously dispersed monomers, percolated monomers, clusters, and percolated clusters. We report the structural architectures of simple charged systems in the form of spherical, Keggin-type heteropolyanions (HPAs) by small-angle X-ray scattering (SAXS) and molecular dynamics (MD) simulations. Structure factors obtained from the SAXS measurements show that the HPAs interact via SALR. Concentration and temperature dependences of the structure factors for HPAs with -3e (e is the charge of an electron) charge are consistent with a mixture of nonassociated monomers and associated randomly percolated monomers, whereas those for HPAs with -4e and -5e charges exhibit only nonassociated monomers in aqueous solutions. Our experiments show that the increase in magnitude of the charge of the HPAs increases their repulsive interactions and inhibits their aggregation in aqueous solutions. MD simulations were done to reveal the atomistic scale origins of SALR between HPAs. The short-range attractions result from water or proton-mediated hydrogen bonds between neighboring HPAs, whereas the long-range repulsions are due to the distributions of ions surrounding the HPAs.