Difference Between Colloidal Silica and Reactive Silica

The key difference between colloidal silica and reactive silica is that colloidal silica is the polymeric form of silicon, whereas reactive silica is the non-polymeric form of silicon.

Silica or silicon dioxide is a crystalline compound that is common in most rocks, mineral, and sand. This substance forms when silicon and oxygen react with each other and another metal or mineral. Typically, silica in water supply exists in two forms: reactive silica and colloidal silica.

1. Overview and Key Difference
2. What is Colloidal Silica
3. What is Reactive Silica
4. Side by Side Comparison – Colloidal Silica vs Reactive Silica in Tabular Form
5. Summary

What is Colloidal Silica?

Colloidal silica is a suspension of silica particles in the liquid phase. The silica structure in this suspension can be described as amorphous, nonporous, and typically spherical silica particles. When silica particles are in the water, the surface of colloidal silica in contact with water is covered by siloxane bonds and silanol groups. Therefore, we can describe colloidal silica as a hydrophilic substance which is capable of forming hydrogen bonds.

Difference Between Colloidal Silica and Reactive Silica

Figure 01: Silica Particle Surface in Water

We can prepare colloidal silica via a multi-step process in which an alkali silicate solution is partially neutralized, leading to the formation of silica nuclei. Generally, silica particle subunits are in the size range of 1 to 5 nm. Combination of these subunits depends on the conditions of polymerization that is present in the colloidal suspension. However, the initial acidification of water-glass solution (sodium silicate solution) yields silicon hydroxide, Si(OH)4.

If we add a salt to the colloidal silica suspension (or if the pH is reduced below 7), the silica particles in the suspension tend to fuse with each other, forming chains. This product is called silica gel. However, if we keep the pH slightly on the alkaline side (above pH=7), then the silica particles remain separated and tend to grow gradually. We can call this type of silica as precipitated silica or silica sols.

There are many different applications of colloidal silica, including papermaking where it is used as a drainage aid, in nanomedicine production, in producing high-temperature binders, investment casting, carbonless paper, in catalysis, as a moisture absorbent, etc.

What is Reactive Silica?

Reactive silica is any monomeric silica, including the ionized forms and potentially any dissolved dimer of silicon. In other words, reactive silica is the non-polymeric form of silica. There are different methods we can use to remove reactive silica from a water supply. Such methods include lime softening, ion exchange, reverse osmosis, ultrafiltration, and electrocoagulation.

Key Difference - Colloidal Silica vs Reactive Silica

Figure 02: A Water Supply Tank

However, the best method to remove reactive silica from water is reverse osmosis because this method includes oxidation of iron, sulfur, and manganese, removal of chlorine, and chloramine, etc. If the water supply contains only colloidal silica, the best way to remove it is ultrafiltration.

What is the Difference Between Colloidal Silica and Reactive Silica?

Colloidal and reactive silica are two forms of silicon present in the water supply. The key difference between colloidal silica and reactive silica is that colloidal silica is the polymeric form of silicon, whereas reactive silica is the non-polymeric form of silicon. In other words, colloidal silica is highly non-reactive, while reactive silica is highly reactive and tend to undergo polymerization and other chemical reactions.

Below infographic presents the summary of the difference between colloidal silica and reactive silica in tabular form.