How do clothes full of dirt and grass stains come out of the washing machine clean and bright? Soap, of course. What exactly does soap do?
First we need to know how soap is made. The main ingredient for making soap is surprisingly one of the macromolecules we discussed…fats. Specifically, molecules called triglycerides, which are esters of fatty acids. In the diagram of saponification shown below, these are represented by the molecule on the left, the reactant. Imagine long hydrocarbon chains, like the ones present in fatty acids, replacing the "R" symbols. These triglycerides, when heated and mixed with a strong base (in this example, Sodium Hydroxide is used), produce soap, which is represented by the molecule on the right. Remember, the "R" group represents a long hydrocarbon chain.
What we have is a long hydrocarbon chain, which is non-polar, attached to a polar head. Really, it is the best of both worlds, a molecule that repels water with a top section that can interact with water via hydrogen bonds. An almost perfect molecule that looks something like this:
The majority of food stains are oil based. The salad dressing that dripped down your shirt, the pizza cheese that fell on your jeans: both are oil based. Oil and fats are non-polar. Since "like dissolves like," these oils are attracted to, and dissolved by, the long, non-polar hydrocarbon chain of the soap. The soap and the oil huddle together away from the water in the washing machine, and the non-polar tops of the soap form a shell around them. This is called a micelle, a hydrophobic ball of soap and oils in the middle, with a hydrophilic shell around it.
The oils are trapped in the micelle and then are washed away in the rinse cycle. The oily stain, and evidence of a late night pizza stop, are washed out of your jeans forever (or at least until the next time).
If you don't believe these diagrams, check out this video.
This same principle is behind those funny poufs people use in the shower. The pouf helps to loosen and remove dirt from your skin, making it easier for soap molecules to trap it in their hydrophobic core.