The stability of amorphous colloidal silica

Abstract

The stability of amorphous silica hydrosols has been studied in the presence of alkali metal cations at various pH's. Results show that silica is very stable within appropriate pH ranges, notwithstanding the presence of very high electrolyte concentrations (sometimes reaching saturation). Moreover, coagulation is often reversible. Although all the alkaline cations are able to coagulate silica between pH's about 7 and 11, they can be classified into two groups according to their behavior in more alkaline media: above pH 11, Li+ and Na+ cations bring coagulation whereas K+, Rb+, and Cs+ remain ineffective. It is suggested that the attraction forces responsible for coagulation are related to acid-base interparticle bonding of two types (salt bonding by acidic cations and silanol-dissociated silanol bonds) rather than to London-van der Waals forces.