Soap

Because soap is a salt, it partially separates into its component ions in water. The active ion of the soap molecule is the RCOO^-. The two ends of this ion behave in different fashions. The carboxylate end (-COO^-) is hydrophilic (water-loving), and is said to be the "head" of the ion. The hydrocarbon portion is lipophilic (oil-loving) and is called the "tail" of the molecule. This unusual molecular structure is responsible for the unique surface and solubility characteristics of soaps and other surfactants (agents affecting the surface of a material).

In a mixture of soap and water, soap molecules are uniformly dispersed. This system is not a true solution, however, because the hydrocarbon portions of the soap's ions are attracted to each other and form spherical aggregates known as micelles. The molecules tails that are incompatible with water are in the interior of these micelles, while the hydrophilic heads remain on the outside to interact with water. When oil is added to this system, it is taken into these micelles as tiny particles. Then it can be rinsed away.