EMULSION OF SOAP

Soap is an excellent cleanser because of its ability to act as an emulsifying agent. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed.

How Soap Cleans

Soap is formed by molecules with a head which interacts with water (hydrophilic) and a long chain which repels water (hydrophobic).

When soap is added to the water, the hydrophilic heads of its molecules interacts with water molecules via ion-dipole interactions and hydrogen bonding, while the long hydrophobic chains join the oil particles and remain inwards (escaping from the water). In that way, they form circular groups named micelles, with the oily material absorbed inside and trapped. In these micelles, the carboxylate groups form a negatively-charged spherical surface, with the hydrocarbon chains inside the sphere. Because they are negatively charged, soap micelles repel each other and remain dispersed in water.

Although soaps are excellent cleansers, they do have disadvantages. As salts of weak acids, they are converted by mineral acids into free fatty acids:

CH₃(CH₂)₁₆CO₂^-Na⁺ + HCl → CH₃(CH₂)₁₆CO₂H + Na⁺ + Cl^-

These fatty acids are less soluble than the sodium or potassium salts and form a precipitate or soap scum. Because of this, soaps are ineffective in acidic water. Also, soaps form insoluble salts in hard water, such as water containing magnesium, calcium, or iron.

2 CH₃(CH₂)₁₆CO₂^-Na⁺ + Mg²⁺ → [CH₃(CH₂)₁₆CO₂^-]₂Mg²⁺ + 2 Na⁺