Can soap ever get dirty?

Cecil replies:

You wanted the facts here, John, and that’s what you’re going to get. The active ingredient in soap is a sodium fatty acid salt, produced by the action of a hot caustic alkali solution on a natural fat or oil (most often, vegetable oil or tallow). This compound has two vital components — a water insoluble (hydrophobic) part, consisting of a fatty acid or a long-chain carbon group, and a water soluble (hydrophilic) part, generally an alkali metal. The hydrophobic part attaches to the fabric or dirt, the hydrophilic part snuggles up to the water. The result is that you force an electrically polar wedge of soap and water between the fabric and the dirt. The slime is then removed by mechanical action — “scrubbing,” to put it in layman’s terms.

By this time it should be obvious that soap always gets dirty — if it didn’t, it wouldn’t work. The whole idea of soap is that grease and dirt feel a more powerful attraction toward it than they do toward your clothes. But protein stains — blood, milk, egg, etc. — are inherently immune to soap’s charm. Insoluble in water, the complex protein molecules adhere too tightly to the fabric to let the soap get a foothold. Enzymes — compounds whose only role in life is to break down protein molecules — must be added to the detergent to make it effective.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.