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.