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Author Biography:

Kevin Dunn  is the Elliott Professor of Chemistry at Hampden-Sydney College, and is the author of Caveman Chemistry and Scientific Soapmaking. Educated at the University of Chicago and the University of Texas, he now lives in central Virginia with his wife and several cats.



A Gentle Introduction To The Science Of Saponification
By Kevin Dunn Friday, January 22, 2016
As I was shopping for Christmas presents, I saw a man on the side of the road selling bird feeders made from old hub caps. At a shop, I saw some wall hangings in which the cheerful messages were  spelled out in letters cut from old license plates. And at a gallery, there was an end table made from the rear axle of a truck. What do these things have in common? Crafters often take things apart and put the pieces together in new and interesting ways, and I trust that such activities come as no surprise to readers of Handmade Magazine.

When crafting, some things are more difficult to take apart than others. It is pretty easy to remove a hub cap from a car—you can usually just pull it off. But you need a screwdriver to remove the license plate. And it is harder still to remove the axle from a truck, to cut it to the required height, and to weld on the other bits of steel needed to turn it into a table.

Chemists also take things apart and put the pieces together in new and interesting ways, but they do so at the molecular level. And just as in crafting, some bonds are easier to break than others. Figure 1 shows a structural formula for an oil molecule, the kind of thing a chemist might draw on a napkin. The bonds are shown as straight line segments, and wherever two bonds meet, an atom is presumed. Oxygen atoms are shown explicitly by the letter “O.” Carbon atoms are so common that we don't even draw them. Wherever two bonds meet with no letter, we presume they join two carbon atoms. And each carbon atom is presumed to bristle with one, two, or three hydrogen atoms. In this introduction, I don't expect you to understand the intricate details of molecular structure, I just want you to notice that the molecule has four parts. If you step back, it looks like a central “knuckle” with three “fingers” attached.

If you were to count the atoms in this particular molecule, you would find over a hundred of them held together by bonds of different strengths. Most of these bonds can only be broken by brute force, say, by setting the oil on fire,  or by extreme ingenuity. But three of the bonds break so easily, even a crafter can do it. You don't need a blow torch or a sledge hammer, but you can't do it with a screwdriver, either. You need the chemical equivalent of a scalpel.

The carbon-carbon and carbon-hydrogen bonds are too strong to be easily broken, but the carbon-oxygen bonds are susceptible to a little persuasion. They can be broken by strong bases like sodium or potassium hydroxide. The three hydroxide parts glom onto the central “knuckle,” and each of the the three sodium parts attach to a “finger.” This is the fundamental reaction for saponification. Each molecule of oil reacts with three molecules of sodium hydroxide, and the result is three soap “fingers” and a glycerin “knuckle.”

Each soap molecule, in turn, has two parts: the sodium part, and the rest of the “finger.” The sodium-oxygen bond is pretty weak, and so the sodium atom pops on and off easily, like a hub cap. In dry soap, sodium sticks to the “finger,” but when you add water, it pops off and is replaced by water molecules, also loosely bound to the “head” of the soap molecule, the end with the oxygen atoms. From time to time, a water molecule gets so close that one of its hydrogen atoms sticks to the head. In essence, the sodium atom pops off and is replaced by a hydrogen atom. This goes on all the time, day in, day out. Sodium popping off, hydrogen popping on and off of the hydrophilic head of the molecule. When sodium is attached, we call each “finger” a soap molecule. When hydrogen is attached, we call it a fatty acid.

You will hear people say that oils “contain” fatty acids, and if you look at the structural formula, you can see the fingers that would be fatty acids if only they were cut off. You would also say that a truck has axles. But a truck is not just a pile of axles, and an oil molecule is not just a mixture of fatty acids. And just as it is not easy to put an axle-turned-end-table back onto a truck, it is no mean feat to re-build an oil molecule from fatty acids and glycerin. You will never see a bar of soap turn spontaneously back into oil.

But you can see soap turn into fatty acid and back into soap. Sodium and hydrogen popping on and off, like hub caps. All you need is a soap solution, some vinegar, and some household ammonia. Make a 1% solution of soap in water, and use a medicine dropper to add vinegar to it. As you do, you will notice two things happening simultaneously. The solution will turn cloudy, and it will stop making suds when you shake it. The vinegar adds hydrogen atoms to the soap, turning it into fatty acid. Fatty acid is less soluble than soap, so the solution becomes cloudy and less sudsy. To reverse the process, just add ammonia, which is a base. It removes hydrogen atoms from fatty acid, turning it back into soap. I do this for myself at home—I just laugh and laugh. I hope you will, too.


 
 
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