SOAP: HOW CAN SOMETHING MADE FROM GREASE GET YOU CLEAN?

At least 3,000 years ago somebody figured out that if you mix ashes and grease, you can make not just really messy gunk but something that seems almost the opposite. Mix the two under the right conditions, as the ancient Romans and Phoenicians learned to do, and you get soap. Chemically speaking, you get a molecule with two parts -- a head and a tail. The head is an alkali such as the potash that is abundant in wood ash. The tail is a long, dangling chain of atoms called a fatty acid. A typical fat molecule, such as those discussed in the New Tech piece (next door), is made of three fatty acid chains attached at one point. (See triglyceride model in photo.) In making soap, the chains disconnect and each binds to an alkali molecule. Technically, soap does not clean anything. It helps water to do the cleaning by solving water's problem with oily materials. Oil and water don't mix. Oils are water repellent or, as chemists say, hydrophobic, from the Greek for "water fearing." Soap molecules solve this problem by acting as go-betweens. The fatty-acid tail is hydrophobic and sticks to fats. The alkali head is water-loving, or hydrophilic, and faces the water. By coating greasy objects, soap molecules help water come closer to surrounding them. With a little agitation or scrubbing, the entire dirt particle becomes coated with soap molecules and lifts off into the water. Detergents work the same way. In fact, soaps are one class of detergents. Both are called surfactants, a contraction of the phrase "surface acting agents." Since ancient times, soap makers have found many things that can be added to soap to enhance its appearance, odor and cleaning ability. But most still are made from animal or vegetable fat. Instead of wood ashes, however, they use synthetically made alkalis such as lye, which is sodium hydroxide. Here's how you can make soap at home. Dissolve a 13-ounce can of flake lye in 2.5 pints of water. Lye can be dangerous to handle, so youngsters should not try this without adult supervision. Melt six pounds of fat saved from cooking or bought as lard. Cool the melted fat to about skin temperature. Stir the fat while slowly pouring in the dissolved lye. Mix well, pour into a shallow dish mold, give it a day to harden and cut it into bars. HOW SOAP WORKS Typical surfactant molecule (the main type of chemical in soap and detergent) FATTY-ACID TAIL: (hydrophobic) repels water, attracts oil and grease ALKALINE HEAD: (hydrophobic) attracts water, repels oil and grease Particle of greasy stuff on surface repels water, stays stuck on cloth or skin. Surfactants attach hydrophobic tails to greasy stuff, allowing water to come closer. With scrubbing or agitation, dirt particle is loosened and immediately coated with surfactants. This "lifts" dirt, allowing it to float away in water.

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