A swellable polyelectrolyte gel-like layer on the surface of hydrous metal oxides in simple electrolyte solutions: Hematite vs. silica colloids
Abstract
To elucidate the puzzling surface structure of hydrophilic colloidal particles of metal oxides dispersed in aqueous solutions, we reanalysed seemingly anomalous coagulation kinetics experiments carried out by various authors on nonporous as well as porous near-spherical iron oxide-hematite colloids synthesized by forced hydrolysis. The swelling degree of the shell (mimicking a loose gel layer of uniform fixed charge due to the ionization of functional groups) of the core-shell- modelled crystalline colloids was found to scale with the concentration of coagulating uni-univalent electrolyte according to the simple power law with the exponent of -3/5.
In polymer physics, this scaling law was derived for swellable polyelectrolyte gels with a low degree of crosslinking, in which a delicate balance between competing thermodynamic forces is established. The same scaling law has recently been found by us to apply in colloidal systems of amorphous silica, indicating a more generalizable appearance of the swelling gel phase at the metal oxide/solution interface than previously thought.