A surfactant is a molecule with two portions that, if they were pure, would be insoluble in one another. Generally, these two portions are an-oil soluble group and a water-soluble group. Such molecules are called amphilic.
These molecules travel to an interface where they lower surface tension. The surface tension of fatty surfactants are around 32 dynes/cm2. At a point called the critical micelle concentration (CMC), micelles or aggregates of molecules form.
Silicone surfactants are also amphilic materials but the group other than the silicone-soluble group can be oil- or water-soluble. The silicone surfactant, if composed of silicone- and water-soluble groups, will lower the surface tension of water to around 20 dynes/cm2.
If the silicone surfactant has oil- and silicone-soluble groups, the surface tension of the oil will be lowered to 20 dynes/cm2.
The lowering of a surface tension is a necessary step toward wetting, detergency, spreading and emulsification. Most formulators are quite comfortable with water-based systems incorporating surfactants.
The same properties can be observed in oil-based systems and it is perfectly legitimate to explore what the CMC of stearyl dimethicone is in isopropyl myristate, or to use cetyl dimethicone to improve wettability of a pigment in oil.
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