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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.

All That Glycerol
By Kevin Dunn Monday, September 18, 2017
Glycerin has been a part of soapmaking from the beginning, but it was not recognized as a substance separate from soap until the early nineteenth century. Michel Eugene Chevreul discovered that soap is not simply a mixture of alkali and fat, but rather a collection of compounds that result from the reaction of alkali with fat. He found that when fats and oils are turned to soap, they produce a sweet tasting byproduct, which he named glycerin. 

Like many natural products, glycerin has been called by more than one name. The chemical name, glycerol, reacts the fact that it belongs to the general category of alcohols, along with ethanol (grain alcohol), methanol (wood alcohol), and isopropanol (rubbing alcohol). Glycerin is the name used in commerce to describe purified products containing at least 95% glycerol. Glycerine is simply an alternate spelling.

All alcohols contain carbon, hydrogen, and oxygen. Methanol, CH3OH, is the simplest, containing only one carbon atom. Ethanol, C2H5OH, contains two carbon atoms, and isopropanol, C3H7OH, has three. The OH group can reasonably be viewed as two thirds of a water molecule (H2O, or HOH), and this is responsible for the physical and chemical properties of the alcohols. In particular, the OH group is strongly attracted to water, and these simple alcohols are all soluble in water. 

Glycerol, C3H5(OH)3, is unusual in having three OH groups, one for each carbon atom. Consequently, it is soluble in water in all proportions, from 0% to 100%. Also unusual, compared to its aforementioned kin, it has a relatively high boiling point. Methanol, ethanol, and isopropanol boil at 65°C (149°F), 78°C (172°F), and 83°C (181°F), all less than the boiling point of water, and they all evaporate more easily than water. Glycerol boils at a whopping 290°C (554°F), far above the boiling point of water.

The combination of high affinity for water and high boiling point makes glycerol hygroscopic (yes, spelled with a g). If you spill some glycerol, not only does it not evaporate (as the simple alcohols would), it actually absorbs moisture from the air. The puddle gets bigger, not smaller. Glycerol behaves similarly on skin—it does not evaporate and it retains moisture. We say it is a humectant. This makes it a desirable component for soaps and cosmetics.

Retaining moisture might seem like a great property, but it does have a down side. The same chemistry is at work in the soap dish as on the skin. Soap that contains glycerol absorbs moisture from humid air. At low humidity, the soap may dry out between uses, but at high humidity it may retain a permanent lm of water (glycerin sweat) or even dissolve in the water it has absorbed from the air. Commodity soaps seldom have this problem, because glycerin is removed during the manufacturing process. Many translucent and transparent soaps, however, contain added glycerin. These are known to consumers as “glycerin soaps.” The transparency prized in these soaps goes hand in hand with their tendency to sweat. 

Unlike commodity soaps, cold process and hot process handcrafted soap retains all of the glycerol from the saponification reaction. How much glycerol? The calculation is not too hard. The picture above shows the saponification reaction. In words, one molecule of fat or oil reacts with three molecules of sodium hydroxide to produce three molecules of soap and one molecule of glycerol. So as not to miss the forest for the trees, carbon and hydrogen atoms are shown as squiggly lines, and only the OH groups are shown explicitly. Three NaOH molecules each contain an OH group. The glycerol molecule contains three OH groups. Thus, the reaction begins and ends with three OH groups. Following them is enough to make the calculation.

The molecular weight of NaOH is 40 grams/mole and there are three of them, for a total of 120 grams/mole. Glycerol has a molecular weight of 92 grams/mole. Thus, any saponification reaction that consumes 120 grams of NaOH will necessarily produce 92 grams of glycerol. For each gram of NaOH, there will be (92/120) or 0.77 grams of glycerol. This ratio remains the same, whether you are working in grams, ounces, or pounds.