Silica, or silicon dioxide (SiO2), occurs in nature as sand, quartz, flint or diatomite. It can be a colorless crystal or a white powder that is odorless and tasteless, and insoluble in water. It has many industrial uses, including the manufacture of glass, ceramic and cement and is used as a reinforcing agent in rubber and plastic. Fumed silica is a synthetic, amorphous silicon dioxide produced by the combustion of silicon tetrachloride in hydrogen furnaces (see Fig. 1).
The white powder has a very fine particle size. The manufacturing procedure was developed by Degussa Corp., Piscataway, NJ, more than 50 years ago.
Degussa markets a line of fumed silicas called Aerosil (INCI name: Silica silylate), which vary by the specific surface area. Also, the surface can be modified by chemical reaction with organosilicon compounds such as dimethyl silyl groups. Aerosil R products, such as Aerosil R 972, indicate a hydrophobic compound (R denotes repellent). Aerosil products without the R, such as Aerosil 300, are hydrophilic.
In personal care products, Aerosil works by:
• Coating solid particles with a silica layer, thus reducing the adhesive force between the particles;
• Forming a three-dimensional gel in liquids by hydrogen bonding. This gel is readily degraded by shearing (thixotropic effect);
• Adsorbing substances on its surface.
A Wide Range of Use
These properties allow it to be used as adsorbers, anti-caking agents, defoaming and drying agents, fillers, suspension stabilizers, gel formers, viscosity adjusters and as an agent to increase water resistance.
In skin care products, it can be used to gel cosmetic oils that exhibit pseudoplastic flow behavior. Hydro-philic Aerosils give higher viscosities when used at 2-10%. When making an emulsion, hydrophobic Aerosils are useful to increase the viscosity of a water-in-oil system. No heating is necessary, as Aerosil can be used at room temperature. When used at over 3%, Aerosil will overcome the greasy feel to leave a dry skin feel. It is also said to increase the water resistance of creams and lotions.
In sunscreens, it is claimed that Aerosil will increase the product’s resistance to water. The following formula is a good example of its wash-off resistance.
|PEG-7 Hydrogenated castor oil||6.0|
|PEG-45/dodecyl glyco polymer||2.0|
|Aerosil R 972 V||2.0|
Procedure: Melt first five items at 72-75°C. Add Aerosil while mixing and homogenize 5-10 minutes. Add UV filters and dissolve while mixing. In separate vessel, dissolve magnesium sulfate in water and glycerin. Heat to 72-75°C. Slowly add water phase to oil phase while homogenizing. Homogenize for 10 minutes. Cool to 40°C and add perfume and preservative. Continue cooling to 30-32°C with stirring.
Formulating with ‘Dry Water’
A novel use for hydrophobic fumed silica is to create “dry water.” If fine water droplets are produced during a mixing process with Aerosil, they will be coated by the Aerosil and prevent the water from coalescing again. A powdered substance (dry water) is the result. When this mixture is rubbed on the skin, the water will be released. Thus active liquid ingredients, such as vitamins or plant extracts, can be used as powdered products. The formula below, adapted from Degussa, contains more than 80% water with very low amounts of oils and emulsifiers.
Aqua Foundation Makeup
|Iron oxide and microcrystalline cellulose and cellulose gum||5.75|
|Liquipar Oil (paraben mixture)||q.s.|
|Sodium carboxymethyl starch||0.75|
Procedure: Add pigments to water and pentylene glycol while stirring. Homogenize briefly. Add Aerosil and homogenize 10 minutes. When completely dispersed, add next three ingredients and stir 2-3 minutes. Add starch and mix for 15 minutes.