How Soap Cleans

As we all know, soap cleans by removing dirt particles from surfaces. What may be less familiar, is the four-phase process comprising of: wetting, absorption of soap, dispersion of soil, and prevention of future soil deposits on the clean surface. Soap can complete the four cleaning stages effectively because of its particular molecular structure. Its molecules have two distinct parts: a hydrophobic part (water-insoluble) and a hydrophilic part (water-soluble). The hydrophobic part of the soap molecule attaches itself to the water molecules and the hydrophobic part attaches itself to the dirt molecules. This double action handily traps dirt in the water/soap solution, preventing dirt from re-depositing on the clean surface.

Although it seems counter-intuitive, water is not the ideal wetting agent. In fact, it tends to bead up on the surface to be cleaned, therefore minimizing contact with the dirt particles. The contact between water and soap makes the hydrophilic part of the soap molecule break the surface of the water and allows the surface to be cleaned (wetting phase). The soap can now directly contact the soiled surface, whether skin, textile fibers, or any other type of surface, forming a film between the water and the surface and between the water and the soil (absorption of soap).

At this point, the soap can effectively separate the soil from the soiled surface by dissolving the thin layer of grease or oil that was holding dirt particles on the surface (dispersion of soil) and trapping them in foam, preventing the dirt from re-attaching. (One reason why protein stains (egg, blood, milk) are so hard to remove is because proteins are insoluble in water; they adhere strongly to fibers and prevent the soap from penetrating (protein-breaking enzymes are often added to commercial detergents for this reason). Finally, rinsing the soap washes away the soil particles trapped in the soapy solution and leaves the surface clean. Who knew such a familiar daily routine could be so complex?