Colloidal Stability in the Presence of Salt

Colloidal silica dispersions are sensitive to the presence of salts. Addition of a salt to a colloidal silica dispersion leads to a shrinkage of the thickness of the diffuse part of the electrical double layer which is surrounding each particle in the dispersion [56]. However, silane-modified silica dispersions are expected to be much more resistant towards addition to salt as such dispersions are not primarily stabihzed by electrostatic forces. In order to evaluate the salt stability of silane-modified silica dispersions, various amounts of sodium chloride were added to two different types of unmodified and silane-modified silica dispersions. After addition of sodium chloride to the different silica dispersions, the gel time was measured. The definition of gel time in this context is the time it takes to obtain a rigid gel, i.e., the 

Colloidal silica, size (nm), type Charge [meq/g] pH Silane per nm surface area SiOj-content [wt%] 

Added NaCl cone. (wt% in dispersion) Silica dispersion number (1-3 30 wt% SiO 4-5 15 wt% SiO ), particle diameter and degree of silane modification  

4 Stability against Gelling/Coagulation upon Freezing and Thawing 

Unmodified colloidal silica dispersions stabilized with alkali ions or aluminate-modified colloidal silica dispersions are not stable towards gelling or coagulation upon freezing and subsequent thawing [58]. The particles in such dispersions are irreversibly aggregated and cannot be brought back to a stable colloidal state. Samples of unmodified sihca dispersions and silane-modified sihca dispersions (two different degrees of silane modification) were put in a freezer at -20°C for 24 h [27]. Then they were thawed at room temperature for 16 h and were finally visually inspected to see 

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