Stability of suspensions of alumina nanoparticles in aqueous solutions of electrolytes

Studies were performed on the colloidal behavior of aqueous suspensions of nanodisperse powders of alumina prepared by low-temperature plasma synthesis. The surface charge density, electrokinetic potential, and coagulating power of 1 : 1, 2 : 1, and 1 : 2 electrolytes depend on pH. Coagulation takes place at abnormally high concentrations of electrolytes with univalent counterions and at abnormally low concentrations of electrolytes with bivalent counterions. These facts cannot be explained by the classical concepts of the DLVO theory. High stability of suspensions with respect to univalent counterions is explained by the appearance of the structure-related component of the disjoining pressure due to the ''secondary'' hydration of the surface. This hydration is associated with the formation of highly charged aquahydroxycomplexes of aluminium on the particle surface brought into contact with acid or alkali.