How to Make Colloidal Silica

Opals are natural formations of colloidal silica, which gives them their unique color and sheen.

Colloidal silica consists of silica spheres only a few hundred nanometers in diameter. Since the size of these spheres is comparable to the wavelength of most visible light, they interact strongly with light when bound in a regular series of bands and gaps known as a periodic array. In nature, the gemstone opal is an example of such an array. Most chemistry labs can easily synthesize colloidal silica.

Things You'll Need

• Graduated pipette
• Ammonia
• Distilled water
• Ethanol
• 100-milliliter plastic bottle with cap
• Magnetic stirring rod
• Magnetic stirrer
• Tetraethyl orthosilicate (TEOS)
• Syringe

Instructions

1. Use the graduated pipette to measure 6 milliliters of ammonia, 7 milliliters of distilled water and 35 milliliters of ethanol into a clean plastic bottle.

2. Place a magnetic stirring rod into the solution, and place the bottle atop a magnetic stirrer. Turn on the stirrer. Adjust the stirring speed until the solution is being stirred vigorously without splashing.

3. Fill a syringe with 2 milliliters of TEOS. Inject the TEOS into the bottle as quickly as possible (i.e., within 10 seconds), without stopping the magnetic stirrer.

4. Cap the bottle, and allow the magnetic stirrer to continue stirring the solution for two to three hours.

Tips & Warnings

• Use fresh chemicals for this reaction. The efficacy of the ethanol in particular will suffer if the bottle has been open for more than three months.

• The amount of TEOS you add will influence the size of the colloidal silica spheres. Lower concentrations will result in smaller spheres, while higher concentrations will result in larger spheres.

• Ammonia is a corrosive chemical that can damage eyes and lungs and irritate skin. Ethanol can cause severe eye irritation and moderate skin irritation. TEOS and colloidal silica are both mild eye and skin irritants.

• Always use chemical-resistant gloves and goggles when handling hazardous chemicals, and work in a well-ventilated area.

References

• UCLA Chemistry; Synthesis and Characterization of Crystals of Silica Colloids; Sarah H. Tolbert; 2003
• Allied High Tech: Colloidal Silica -- Material Safety Data Sheet
• E-Science Labs: Ammonia -- Material Safety Data Sheet
• Burdick & Jackson: TEOS -- Material Safety Data Sheet
• North American Fire Arts Association: Ethanol -- Material Safety Data Sheet