Copyright © Karl Dahlke, 2023
After a day at the beach, your body is covered in sand, and you even have sand in your hair. You take a shower, but the benefits of soap and shampoo are largely psychological. After a thorough wash, you notice that there is still sand in your hair. Each grain is a tiny rock that remains insoluble and intact, undisturbed by lather. They must be physically dislodged by flowing water, like stones tumbling downstream. Run water through your hair again and again until the sand is gone; there isn't much else you can do.
This is not a typical day in the tub. Most of the time you are trying to remove oil from your skin and hair - your own oil perhaps, or oils from the outside world. Grease from the barbecued ribs is on your hands, and your dishes, and maybe even your clothes if you are a bit sloppy; all these items must be cleaned. You can run water on your hands all day and it doesn't seem to do any good. These substances are not entrained in the stream like sand, they just stick to your skin and your clothes, and do not wash off. Something more is needed, something to help these compounds dissolve in water, for as we all know, oil and water don't mix.
In 2800 B.C., a curious fellow, somewhere in Babylon, combined water, alkali, and cassia oil (from the local cassia tree), and heated the mixture over a fire. A strange and useful substance emerged, an early form of soap. So useful was this product, that he wrote down the formula on a clay tablet. For the first time, people could wash. What a change that must have been!
Did our early Babylonian inventor wonder why a compound that is made of mostly oil should be so effective at washing oil away? It seems counterintuitive, does it not? Even moreso when you look at the underlying chemistry. A fatty acid is a fairly large molecule, and yet, all you need do is replace one hydrogen with sodium, and you get soap. It seems like a small change, 1 atom out of 38, but it makes a big difference. Let's look at the chemicals involved.
As you recall from an earlier chapter, a molecule of oil is a triglyceride, three fatty acids tied to a glycerol backbone, like three chains hanging off a stick. Under the right conditions, e.g. heat + water, the ester bonds are broken, and the three organic acids are set free. If a base is present, such as sodium hydroxide, a second reaction is possible. This is base + acid = salt + water, probably the first reaction you learned in high school chemistry. The simplest inorganic example joins sodium hydroxide with hydrochloric acid to make table salt and water. The hydrogen and sodium trade places, and the formula looks like this.
NaOH + HCl = NaCl + HOH
Of course HOH is usually written H2O, the more recognizable formula for water, though some chemists prefer HOH, since many reactions, including the ones on this page, displace one of the two hydrogens, separating water into H and OH. Here is the chemical reaction in its more traditional form.
NaOH + HCl = NaCl + H2O
Apply the same base + acid chemistry to lauric acid, a 12 carbon chain acid that is present in palm kernel oil. The result is sodium laurate, sometimes called sodium palm kernelate, as listed in the ingredients for Ivory Soap.
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| HOH | + |
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This process is called saponification, and many different soaps are possible. The variations are almost endless, using different alkaline metals and different oils. sodium + olive oil yields Castil soap, which is very mild, while calcium + olive oil makes a thicker soap that has been used as a lubricant for centuries. It probably greased chariot wheels in the Roman Empire. Even aluminum soaps, drawn from the third column of the periodic table, have industrial uses. Returning to the first column, potassium soaps are softer than sodium soaps, and are often liquid. Along with the metal ion, there are many oils to choose from. The well known brand Palmolive is so named because it uses palm oil and olive oil. When this mixture is combined with sodium, the resulting soap has a nice feel, and lathers well. So discovered B. J. Johnson, founder of the Palmolive Corporation in 1898.
In addition to the metal + oil variations, modern soap may include colors, fragrances, deodorants, moisturizers, emollients, inorganic salts, free fatty acids, glycerin, and other compounds. But this is window dressing, isn't it? It's still the sodium fatty-acid that gets you clean, just as it did 4,800 years ago in Babylon.
So - how does it work? The lauric chain, (or myristic or palmitic or stearic or whatever you are using), gathers around an oil molecule and captures it; then the sodium ion makes the entire aggregate soluble in water, whence it can be rinsed away. suddenly the action of soap is not so mysterious after all. Sodium makes almost anything soluble in water, such as sodium chloride = table salt. Start with a fatty acid, and replace the disassociated hydrogen with sodium to produce a molecule that captures other oil molecules, but still dissolves in water. How cool is that?
As you stroll down the cleaning aisle of the grocery store, a dozen national brands vie for your attention, from Caress to Zest, each with its own enticing scent. Convenience and variety notwithstanding, some people choose to make their own soap, just as some people grow vegetables in a small garden for a fresh, homemade salad. However, making soap is not a trivial task. Too much lye, and the soap is caustic, and could harm skin or eyes. too much fatty acids, and the soap leaves a greasy residue on the skin, which is not what you want from your shower. Precise stoichiometry is required. fortunately, online tutorials are available. follow the directions, and the bar will be just as you like it. As for me, I am content to buy my soap from Kroger.
I will close with a song, Singing in the Bathtub, that conveys the joy of getting clean. It was originally written in 1929 for the film The Show of Shows, and was, in part, a parody of MGM's song Singing in the Rain, that was released the same year. Since Warner Bros. had full rights to the Bathtub song, they used it often - in the very first Looney Tunes animated short in 1930, and in 20 shorts all together. Perhaps the most familiar is Tweet and Lovely, which begins with Tweety in the bath. He sings two of the four verses, whereupon his song, and his bath, are interrupted by Sylvester the cat.
I'm singing in the bathtub Happy once again Watching all my troubles Go swingin down the drain Singing through the soap suds Life is full of hope You can sing with feeling While feeling for the soap --- Tweety's song stops here, due to catus interruptus --- Oh! A ring around the bathtub It isn't so nice to see But a ring around the bathtub Is a rainbow to me Reaching for a towel Ready for a rub Everybody's happy While singing in the tub |