Silica sol is a colloidal solution consisting of a dispersive medium (water) and dispersive phase (micelles of amorphous silica). The silica micelles filled with water molecules have developed surfaces and many functional (silanol) groups which provide high reactive capability and possibility for inoculation of the particles surfaces by ion adsorption. The silica micelles are spherical particles filled with water and ions of alkali metals (Li, Na, K, NH4). This colloidal solution is specified with lactescence and whitish colour. The silica sol stabilized with sodium ions stays stable for a long time, there is only an insignificant decrease of silicon oxide concentration.
Silica sol is manufactured by two ways. The first one is ultrafiltration when sol is concentrated in the system of filters where water is removed through the filtering membranes. The second way is concentration when water is vaporized in the reactors and a permanent sol feeding occurs. The density and dimensions of the particles change depending on the production way. So during ultrafiltration the diameter of the micelles are from 5 to 15nm, they are more water-saturated and have less spherical forms. During concentration the micells are about from 15 to 30nm, they are more consistent and spherical.
The main kind of silica sol used in industry is alkaline sol stabilized by sodium hydroxide. Commercial solutions have a concentration of silicon oxide from 15% to 30%. There are also silica sols stabilized with ammonium and potassium ions. Herewith there are some marks based on silica sol and modified with aluminium, ferrous, chromium oxides and mineral acids. Silica sols can be alkaline (рН>9) or acid ( рН<5) depending on the choice of the stabilizator during the production of hydrated silica. At the pH values of from 6 to 8 the sol is not stable and form into gel in a colloidal solution of silica sol. Acid silica sols are more inclined for coagulation and gel forming as they have decreased electric layer at the micelle surface, and this makes the micelles less stable. The most stable sol is alkaline silica sol stabilized with sodium hydroxide. Hydrated silica sol has a developed surface and can adsorb different ions or complex mixtures on it.
If silica sol is alkaline and pH value becomes acid (and vice versa), the process of gel forming starts. The micelles gather to large agglomerates that cannot disperse any more. The process of sol-gel transition is not reversible.
The price for silica sol depends on requested characteristics.
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