Colloidal silica is silica particles that are suspended in a liquid. The liquid is denser than water and has been stabilized electrostatically to allow the particles to stay suspended in the solution. The silica particles are also very small and do not have a large density. The range has to be approximately from thirty to a hundred nanometers. If the particles are too large, they will settle out of the solution. If the silica particles are too small, they are difficult to stabilize in a solution.
Preparation of colloidal silica is a multi-step process. Beginning with an alkali-silicate solution, the solution is neutralized which will cause silica nuclei to form. These particles are extremely small, usually just a few nanometers in diameter. The greatest control over the solution happens in this period.
In order to change the particle size and solution, there are two points of control. The pH and the sodium content of the solution will determine the final mixture. If the pH is reduced to below seven, making it acidic, or salt is added, the silica units will begin to fuse together, forming larger particles. These particles are called silica gels and are not stable in a solution as they can dissociate again if the solution is altered.
If the pH remains basic or neutral, then the silica particles will stay separated and actually begin to grow. After the size of the particles is maximized, the suspension is stabilized by adjusting the pH which will stop the silica particles from changing and clumping together. The solution can be adjusted by evaporated, depending on the density needed. The particle size will determine how dense a solution can become before it is unstable and the silica particles begin to precipitate from the solution.
Uses of colloidal silica vary. In paper-making plants, colloidal silica is used to drain the liquid from the paper. By allowing the water to leave the paper rapidly, the starch in the paper will remain, increasing the strength of the paper. This solution has many industrial applications as a moisture absorbent in many plants and factories. In some cases, it has been used to increase the friction of a surface. After a floor has been waxed, the surface is slippery and can be dangerous. By coating a waxed floor with colloidal silica, the friction is increased which will prevent accidents. Colloidal silica has many uses and functions and is an important product.