And here again is one of those terms which pepper the inside of most crafts, making the newcomer feel adrift in a sea of syllables. So, indeed, what on earth is it? At it simplest, saponification is the process by which lye turns oils into soap.
Let’s back up a bit. The reaction of an acid with a base is one of the first and simplest reactions demonstrated in elementary chemistry, and plays a role in everything from grade school science fair volcano eruptions (baking soda + vinegar) to the rising action of quick breads (baking soda again, with buttermilk and/or cream of tartar). Most of these reactions produce fizz and heat, but one thing every single one of them produces is salt. No, not (usually) NaCl/table salt, but still, salt.
Let’s back up a bit more. An acid/base reaction which does produce table salt is this:
NaOH + HCl –> NaCl + H2O
Sodium hydroxide (lye, caustic soda) added to hydrochloric acid produces table salt and water. When you look at it that way, a salt can be handwavingly described as the front half of a base and the back half of an acid joining up, and leaving the remainder of their original compounds to fend for themselves and find new compounds to be part of. In the above reaction, it’s pretty simple, but some reactions of this category are a bit more complex.
What does all this have with soap and saponification? Well, saponification is, at its heart, an acid-base reaction. Wait, what? Acid? Yes, acid, though it’s true they’re not what we traditionally think of as acids. I’ve talked before about triglycerides and fatty acids (and indeed touched on several of these points at that time), and it’s those which provide the other half of this reaction. A triglyceride molecule is a big, unwieldy thing; it has one “head” – the “glyceride” part – and three long “tails” – the fatty acid chains. The first step in the saponification reaction is to knock the head off from the tails, leaving the head and the tails both free to react with something new. This step requires the presence of water, which is part of why we dissolve our lye crystals in water when making soap (the other reason is that without dissolving in something very like water, NaOH will not separate (“disassociate”) into Na+ and OH- ions and therefore will not be able to enter into chemical reactions). After this, the sodium from the sodium hydroxide binds to the fatty acids, creating soap, and the former glyceride “head” binds to the oxygen and hydrogen (the “hydroxide” portion) from the sodium hydroxide, creating glycerol, which we usually refer to as glycerin.
Image credit: V8rik at en.wikipedia.
Here’s a picture to help that make a bit more sense. The “R”s in this picture represent long fatty acid chains, shortened this way so we can concentrate on the bits of the molecules which are participating in the chemical reaction. On the left is a triglyceride molecule. The arrow represents the reaction with water and sodium hydroxide/lye, and on the right are three soap molecules and one glycerol. The reaction is the same with potassium hydroxide, by the way, for making liquid soaps.
OK, so, that’s a whole big pile of chemistry talk. I’ll be honest, I don’t really spend a whole lot of time thinking about the nuts and bolts of the chemistry when I’m actually making soap, but there are a few things which I try to keep in mind. For one thing, the early part of this reaction requires heat, but a later part generates it – which is part of what creates the “sweet spot” of heat when soaping. Too cold and the reaction will take a very long time to complete – several days or even a week; too hot and it will create a runaway chain reaction and you’ll end up with a soap volcano. It explains the function water plays in the process, and why we don’t want to discount our water too steeply when soaping. It explains where the glycerin that is part of the reason handmade soap is so awesome has come from.
Anyway, why is saponification such a significant reaction? Well, you know the old saying about how oil and water don’t mix? Which, true, they don’t. There are two general categories of substances, and by and large, things will only mix with other things from the same category as themselves. So you can mix olive oil and canola oil, and you can mix water and beer, but if for some insane reason you wanted to mix olive oil and beer, you’d be out of luck. Which in that case is probably a good thing, because, eww. But saponification produces a molecule which is one kind of substance – the Na and O end of the thing – at one end, and the other kind – the fatty acid tail – at the other. So it is able to stick to oily, yucky, dirty stuff with the tail, and then the other end is able to hook up with water and wash the whole mess away, soap and dirt alike. Neat, huh? (Chemists call this kind of molecule “amphiphilic”, but we don’t need to worry about that.)
So, suds up – soap is awesome!