Comparatively Speaking: Higher vs. Lower Levels of Surfactant Silicone

Surfactants are comprised of two or more groups that are not soluble in each other in pure form; they are clear and they lower surface tension when added to a formulation. (see Comparatively Speaking: Aqueous Surfactant vs. Silicone Surfactant). 

Therefore, for a fatty surfactant to be functional in oil-based systems, it must be clear and must lower surface tension. The surface tension of the fatty surfactant and most oils is around 32 dynes/cm2, and many fatty surfactants are of limited functionality in oil-based systems.

Alkyl silicone compounds can be effective in oil-based systems, as they are soluble (i.e. clear when added) and lower surface tension when added to oils. They first lower surface tension by increasing their concentration at the interface, then they form micelles. It is this property that makes them amphiphillic and allows them to function at low concentrations. Figure 1 illustrates what happens when behenyl dimethicone is added to soybean oil.

The reduction of surface tension makes the oil feel like silicone. Because of the low surface tension, the oil feels like cyclomethicone. As one increases the concentration, micelles form. If an alkyl silicone with a melting point above ambient is chosen, a gel will form upon cooling. The gel is reversible, liquefying upon heating and re-solidifying upon cooling. The gel is thixotrophic, meaning it liquefies under pressure.

The synthetic alteration of the “a” to “b” ratio in the molecule changes the gel's clarity and the occlusivity of the blend. Figure 2 shows the effect. The behenyl dimethicone added to the oil has differing amounts of silicone.

The ability to alter the aesthetics by lowering surface tension and/or form thixotropic gels using natural oils can create different skin feel and properties.

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