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Lowering the surface tension of a solvent is a prerequisite for wetting, spreading, foaming and emulsification. Not all molecules that lower surface tension will provide all these functions, but none of these desirable properties can be provided by a molecule that fails to lower surface tension.
Water has a high surface tension (72 dynes/cm). It can be lowered into the range of 32–35 dynes/cm with traditional water-soluble fatty surfactants. Consequently, properly selected fatty surfactants can wet, foam, emulsify or facilitate spreading in aqueous solutions.
Oil differs from water in many respects, the most important of which is surface tension. Oil has a surface tension of 30–35 dynes/cm, meaning that oil-soluble fatty surfactants do not provide the desired surface tension reduction for oils. The classes of compounds that can provide surface tension reduction below 30–35 dynes/cm are based upon silicone and its fluoro compounds.
Properly chosen silicone surfactants can reduce the surface tension of oils to 20–25 dynes/cm. Properly chosen fluoro surfactants can reduce the surface tension of oils and silicones to below 20 dynes/cm. Properly chosen refers to molecules that: are amphilphilic, are soluble in the solvent chosen (i.e. clear), and can orient at the solvent surface correctly to lower the surface tension.
Formulators should carefully consider the reduction of surface tension and its role in formulation when choosing ingredients for either the oil or water phase.