Understanding the stability behavior of colloidal silica in different alkali environments

This paper investigates the effect of three different alkalis, namely ammonia (NH₄OH), ethylenediamine (EDA), and tetrabutylammonium hydroxide (TBAOH) on the stability of colloidal silica, with pH controlled within the range of 8–11. As a result, NH₄OH greatly promotes the stability of silica sol at first due to the strong solvation ability of NH₄⁺ and then the stability begins to decrease because the electric double layer of silica is compressed. The addition of EDA into silica system leads to a reduction in the stability evidently followed by a slight increase, which can be explained by the cationic bridging effect of the ethylenediammonium cations. Meanwhile, the effect of TBAOH on silica dispersion is analogous to EDA, mainly caused by the hydrophobic and steric effects. The abovementioned relevant stability mechanisms are involved in non-DLVO theory.