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Surfactants

Surfactants, i.e. Surface Active Agents, are molecules which are added to a liquid in tiny amounts and change the properties of the liquid at a suface or interface, i.e. changes the surface tension of the liquid. In everyday life, this effect is seen with washing up liquid.

The general structure of a surfactant consists of a hydrophilic and hydrophobic part.

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Surfactants tend to exist at the boundary between two different liquids - in silicone chemistry for example between a silicone fluid and water. Used as an emulsifier, it is added to these two immiscible liquids in order to make an emulsion and not just two layers of liquid. The emulsifier uses its molecular structure where the hydrophilic part in highly-soluble water and the hydrophobic part is highly-soluble in the silicone fluid.

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Another example for an emulsion is milk, where casein works as emulsifiers to stabilise the equilibrium between the oil and water phase. The surfactant film can stabilise an emulsion for months and in some cases even for years. However, over time the fine droplets in the emulsion will coalesce and separate into two layers. This also happens very quickly when the emulsion is kept below 0°C and the water freezes. Basically there are two different kinds of an emulsion : water in oil (W/O) and oils in water (O/W).

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Depending on the kind of silicone fluid (chain length, functionality etc.) and the ratio fluid/water as well as the kind of emulsifier, hundreds of silicone emulsions are available tailored to the requirements of the customs. Additional softening coemulsifiers are also in use for specific applications.