how does soap remove fats and oils if it’s made of fats and oils?


What’s the process that makes it good at something so far off from its composition?

In: 28

Precisely *by* being made of fats and oils.

Water is what is called a polar molecule. This basically means there’s one end that is much more negatively charged and another end that’s more positively charged. Water tends to arrange itself so that the negative end of one molecule is pointing toward the positive end of another.

Water is very good at dissolving and washing away other things that are polar. They interact well with water and water is able to carry them away. Non-polar things, like most oils and fats, don’t interact well with water. Soaps contain surfactants, these are molecules that have both a polar and non-polar part of them. They interact well with both water and oils and therefore can help water dissolve and carry away non-polar molecules.

One of the things that I remember from chemistry class…

**Like dissolves like**

Water can’t dissolve oil, but other oils can.

The fats and oils in soap have been changed in the process of making them into the soap. This change allows them to cling to both water and oils; since you rinse with water, that carries the soap-oil mix away with it, leaving your skin oil-free.

Fats already have a fat-friendly part. The soap-making adds a water-friendly part. Now you have soap, that can enclose fat in a layer of (outward) water friendly soap.

There’s a rule of thumb in chemistry that similar substances dissolve each other well (“like dissolves like”). So fats and oils can dissolve other fats and oils, but don’t dissolve well in water. But soap dissolves in water -what’s going on?

Normal fats and oils are triglycerides- three fatty acid chains (think long chains of carbons and hydrogens) attached to a glycerin backbone. When making soap, triglycerides are reacted with a strong base (eg potassium hydroxide) in a reaction called saponification, which causes the glycerin backbone to break off and the free fatty acids to form a salt with the potassium (or sodium if using NaOH) ions. Now the ionic end (the “head”) of the fatty acid salts dissolve well with water, and the tail of the fatty acid dissolves well with oils. And now we have something that can mix with both water and oils!

When you wash with soap, the tail ends of the soap molecules attract oils on your skin, and the “head” of the soap molecules attract water. Because each end wants to be with its “own kind”, a bunch of soap molecules end up arranging themselves in a “bubble” where all the tails point inward, trapping a little ball of oil, and all the heads point outward, forming the water-soluble outside surface of this little bubble, called a micelle, and then all the micelles rinse away with the water.