How Soap Does its Thing

Water molecules are attracted to each other by means of the very subtle, but certainly existest, Van der Waals force. In a normal body of water, water molecules are equally pulling on each other, except at the surface of the water. There, the water molecules pull together creating a dome shape. This "surface tension" is what causes water bead and to almost encapsulate itself. This surface tension is great if you are an insect floating on water (the surface of water is actually more resilient because of this tension) or if you are drying off anything (the water will collect into beads). This surface tension is a problem if you really want something to get wet -- clothing, stains, things that need cleaning -- because the water will attract more to itself than to other people. This is where soap comes in.

Soap works because its a surface-acting agent, or surfactant. Surfactants break up the surface tension on water. With the surface tension broken up, water will "melt" into any substance and becomes more "porous", if you will, to things it contacts. Surfactants work because they have molecules with hydrophillic (attracted to H20 molecules) and hydrophobic (repelled by water) ends. The hydrophillic end binds to the water while the other hydrophobic or lipophillic (attracted to lipids, namely the debris of stains or dirt). Therefore, the surfactants kind of become miniature links between H20 and debris, thus allowing water to bring the dirt with it as it is washed away.