Soap, Water and Oil

An emulsion is a type of dispersion in which two normally immiscible substances are stabilised by another substance, called an emulsifier. For example, olive oil and water will not dissolve in one another, as their intermolecular forces differ (like dissolves like). While we can agitate to form a suspension, it is temporary and the oil and water will eventually separate into distinct layers.
Soap (once dissociated in water) consists of a non-polar hydrocarbon tail and a polar head. The non-polar hydrocarbon tail interacts with non-polar substances through dispersion forces, whilst the polar head interacts with polar substances (normally water) which forms dipole-dipole interactions and hydrogen bonds with water.
The presence of soap in a mixture of oil/grease and water will create an emulsion upon agitation. The hydrocarbon tails of soap dissolves in the oil and promotes droplet formation. These droplets repel each other, as they have the same net negative charge. These individual droplets can be dissolved in water as they form ion-dipole interactions.

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There can be oil-in-water emulsions or water-in-oil emulsions, where soap is the emulsifier.

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