Questions & Answers
Basic know-how about silica
1. What exactly is silica?
Silica is another name for silicon oxides, for example SiO2. It can be found in nature in crystalline form, e.g. as quartz (sand), and is the most abundant component of the earth's crust. Amorphous silica, on the other hand, is industrially manufactured in a variety of forms.
2. What exactly does "colloidal" mean, anyway?
A colloid, or sol, is a stable dispersion of particles. In other words, the particles are small enough that gravity doesn't cause them to settle, but large enough not to pass through a membrane, allowing other molecules and ions to pass freely. Particle size ranges from 1-100 nm, usually in the region of 5-100 nm.
3. How does colloidal silica differ from fumed, fused, or precipitated silica?
Colloidal silica varies from other types of silica in several significant ways. The most noticeable difference is that it's in liquid form, as opposed to powder. In addition, it has the widest ranging surface area, and its aggregate size can be as small as the actual size of the primary particle.
4. Whats the difference between sodium silicate (water glass) and colloidal silica?
Colloidal silica, CS, consists of dense, amorphous particles of SiO2.
The building blocks of these particles are, in fact, randomly-distributed [SiO4]4- tetrahedral. This random distribution is actually what makes amorphous silica different from, say, crystalline silica, which is ordered on a molecular level.
In addition, sodium silicates are alkaline solutions. The pH of sodium silicates typically ranges from 12-13, compared to 9-11 for colloidal silica. This means that they mainly consist of silicate monomers, whereas colloidal silica is polymeric.
Like the sols, sodium silicate solutions are usually sold at SiO2 concentrations of approximately 30% by weight. However, the weight ratio of SiO2/Na2O is much lower - less than 3.4 - whereas for colloidal silica, it's generally greater than 50.
Finally, the viscosity of sodium silicates is much higher. Think of it as a kind of syrup, whereas colloidal silica resembles water.
5. How exactly do you produce your silica sol?
We use several different methods to produce the aqueous colloidal silica that meets our consistency standards. The general principle is to cation exchange a sodium silicate to the desired pH, where the polymerization takes place. Then the sol is stabilized and concentrated to the desired content. For sols with larger particles, we start by building on smaller particles.
The Diagram on this web page offers a deeper look into the process.
6. Can you explain surface modification- anionic/cationic/surface charge?
In brief, the surface of CS consists mainly of hydroxyl groups, or -Si-O-H. However, other groups have also been identified. These include silandiol, -Si-(OH)2, silanetriol, -Si(OH)3, surface siloxanes, -Si-O-Si-O-, and surface-bound water.
The surface of CS is anionic at an alkaline pH level. lt's stabilized with cations such as sodium or ammonium. In the case of cationic sols, polyaluminum chloride is the stabilizing agent.
The surface can also contain Al-O-. By subjecting CS to sodium aluminate, isomorphous substitution takes place, leading to the formation of -Al-OH -groups. This results in a highly negatively-charged surface, down to pH 3. For this reason, aluminate-modified CS has an increased stability range with regard to pH.
Properties of colloidal silica
7. What about temperature change? Does it affect colloidal silica?
Sols should be stored at 5-35 oC (40-95 oF). If the sol is subjected to freezing conditions, it will irreversibly lose its stability. Highly elevated temperatures may accelerate the growth of micro-organisms, as well as decrease the long-term stability of the silica sol.
8. How will a change in pH affect the stability of colloidal silica?
The stability of colloidal silica can be defined in several different ways. The most important considerations, however, are particle growth and aggregation. Then come pH, ionic strength, silica concentration and storage temperature.
Particle growth occurs primarily in silica sols that feature particle diameters smaller than 10 nm - due to so-called Ostwald ripening. This is characterized by a decreased specific surface area.
In terms of aggregation, stability depends on the surface charge and surface area. In equal conditions, large particles are more stable than smaller ones. For very large particles (with diameters greater than 100 nm), settling can be a problem at low viscosities. Aggregation can, in fact, be monitored by estimating viscosity: the more gel, the higher the viscosity.
The illustration on this web page illustrates the effects of pH in the colloidal silica water system.
9. Can colloidal silica be mixed with other chemicals?
Positive counter ions balancing the negative surface charge are diffusely oriented around the particle. The negative potential is therefore declining by distance from the particle, and the repelling forces between particles extend for some distance out from the particle surface. As salt is added, the counter ions move much closer to the particle surface, which reduces the distance through which the repelling forces act. This causes a reduction in sol stability by increasing the probability of inter-particle collision.
As a rule, polyvalent cations are more effective in shrinking the diffuse layer. This makes them more effective gelling agents for colloidal silica.
Organic solvents
Alcohol, acetone and other polarized solvents can be mixed with colloidal silica. However, there is a limit to how much solvent can be mixed into the silica sol - gelling may occur. Compatibility is enhanced by increasing the dielectric constant of the solvent, decreasing the pH of the colloidal silica, decreasing the silica concentration, and to some extent increasing the particle size.
Emulsion resin and water-soluble resin
When they have the same pH and surface charge as the resin, sols are compatible with both emulsion resin and water-soluble resin. However, in the case of emulsion resin, the emulsifier should be carefully selected, as it may cause gelling or separation.
Generally speaking, the anionic grades are compatible with anionic and non-ionic surfactants, whereas the cationic grades are compatible with cationic and non-ionic surfactants.
In both cases, the colloidal silica may become incompatible with the surface-active agent, depending on the surfactant's composition and impurities. This needs to be taken into account when selecting surface-active agents.
10. What are the differences between the grades of colloidal silica?
Particle size and pH are what differ most between the grades of colloidal silica.
Particle size can also be expressed in terms of specific surface area, i.e. the higher the specific surface area, the smaller the particle size. The particle size also affects the maximum possible SiO2 content. That is, small particles can only be stable in more dilute silica sols.
The pure silica sols are anionic and are typically sodium- or ammonium-stabilized to a pH of 9-11. Through modification using sodium aluminate, however, the sols are stable down to a pH of 3-4.
Cationic silica sols are stable at pH 4-5, and deionized sols are stable at a low pH, typically 2-3.
Practical advice about handling silica
11. Does shelf life influence the properties of colloidal silica?
Stability is what generally determines the shelf life of a sol. Checking the stability simply involves measuring the viscosity and specific surface area.
12. How does colloidal silica react to steel, plastic and other materials?
Colloidal silica does not react with stainless steel or plastic material. We recommend that you don't use mild steel, because the iron will discolor the product.
13. What types of pumps can I use with colloidal silica?
You can choose from a variety of pumps:
Piston membrane pumps, preferably with a pressure equalizer
Membrane pumps
Hose pumps
Excenter screw pumps
Centrifugal pumps
It is recommended that all pumps are cleaned with water, and dried before and after use.
14. What about cleaning colloidal silica?
The simplest way to clean pipes, valves, pumps or spills is to rinse them thoroughly with water right after use - before they've dried out. However, for tanks and containers with large volumes of deposits, you should first empty the tank and flush it with water. Next, inspect the walls and bottom of the tank for any leftover solids. These will have to be removed with a high-pressure hose.
A second alternative, if NaOH is compatible with the tank and its contents, is to use 4-5% caustic soda (NaOH). Mix with an agitator, or circulate the tank. This type of cleaning usually takes between 2-5 hours. For better results, heat the caustic soda to 50-60 oC.
If it isn't possible to use caustic soda due to restrictions and/or danger, follow the manual method to clean the tank/container.
CAUTION: Caustic soda is extremely powerful; always protect yourself by wearing a full mask, helmet, rubber boots and goggles.
Safety and the environment
15. Is colloidal silica dangerous for the environment?
Since colloidal silica products essentially consist of only amorphous silica and water, they rank as one of the most environmentally-friendly industrial chemical products and usually do not present a problem. However, check with state and other relevant authorities.
16. Does it pose any particular health hazards?
All our silica sols contain solely amorphous silica which, unlike crystalline silica, has generated no confirmed cases of silicosis to date.
Before handling the material, however, read the corresponding Material Safety Data Sheet (MSDS) for health, safety and environmental information for each product.
Since the sodium- and ammonium-stabilized sols are slightly alkaline, always wear safety goggles and appropriate PPE when handling them. If you get sol in your eyes, rinse immediately with water. If problems persist, seek medical attention
Application areas
17. Can colloidal silica be modified?
1. Our sols can be stabilized with sodium, potassium or ammonium ions and organic modified.
2. To obtain a high negative surface charge even at a low pH, silica sols can be surface modified. In the surface modification, trivalent aluminum atoms are substituted for part of the tetravalent silicon atoms in the surface of the particles. This creates a fixed negative charge that's independent of pH. For these grades of our sols, lowering the pH reduces the amount of charge attributable to reaction between hydroxyl ions and surface silanol groups, but not that arising from the alumina substitution. Therefore the stability of the alumina-modified sols will increase continuously with decreasing pH.
3. Cationic sols consist of colloidal silica in which each particle is coated with a layer of Al2O3. This converts the surface charge from negative to positive. The stabilizing counter ion is chloride.
4. Some special grades of deionised colloidal silica are also available. The pH is approximately 2-3. The silica sols contain neither anions nor cations as stabilizing ions.