In the modern sense, shampoo isn’t soap because soap is made from organic sources like beef tallow, palm oil, or—in the movies or your local coven—human fat. Mix up those acidic fats with an alkaline solution like sodium hydroxide and you’ll make a salt. (The process is called “saponifcation,” while the salt itself is a “surfactant,” or “surface active agent; in this case, soap.)

Shampoos, on the other hand, are derived from petrochemicals. They’re synthetic, possibly developed in World War I when the Germans faced a shortage of organic fats from which to make soap. They’re also surfactants, just ones made from stuff that bubbled up from inside the earth instead of growing on top of it.

Earlier shampoos were actually just special soaps, usually with additives like attractive-smelling herbs. In fact, the name shampoo originally referred not to the soap itself, but to the process of washing the hair with a massage.

To understand the distinction between today’s soaps and shampoos, it’s easiest to explain why we don’t wash our hair with soap in the first place.

Both soap and shampoo are detergents, decreasing the surface tension of water to allow it to mix in with the natural oil sebum produced by the follicles’ sebaceous glands, as well as the dirt, bits of scalp, and left-over hair care products that are gunking up your mane. Try washing your hair some time without a surfactant—the water will just bead up and bounce off. You need something to make the water “wetter,” for which soap, like any detergent worth its salt, will work just fine.

But soap has one nasty habit in our hard, mineral-laden water: It reacts with metal ions to form “scum,” another salt—typically calcium or magnesium salt—which stays in your hair (or on the side of your bathtub) no matter how much you rinse. Shampoo, on the other hand, doesn’t react with the hard water, allowing it to act as a detergent without leaving your hair scummy.

Unfortunately, the synthetic detergents in shampoo—ammonium lauryl sulfate, ammonium laureth sulfate, sodium lauryl sulfate, etc.—don’t foam up very well, so manufacturers are forced to add lather builders—cocamide mea, cocamidopropyl betaine, etc.—to give you that sudsy, this-is-really-working feeling. And of course the detergents/surfactants in shampoo tend to be more harsh than those in most soaps, stripping away more of your hair’s natural sebum that is optimal. Hence conditioner, developed to replace your natural oils with synthetic substitutes.

So if you’re freaked out about continually stripping the natural oils from your hair only to respackle with clean but fake ones, well, maybe don’t wash so much. We advocate washing as frequently as you feel necessary, predicated by the amount of shmutz like smoke, sweat, and barbecue sauce that finds its way into your flowing locks. Since everyone’s sensitivity to commercial hair care products differs, experiment with differing intervals between washes. (We wash our hair every day, but have thick, resilient hair.)

Just don’t assume that your more hippy-dippy shampoos from companies like our well-loved Lush aren’t using detergents like sodium lauryl sulfate—they are.

It seems like the optimal wash would be with a mild soap and rain water, collected in a mineral-free catch. Why don’t one of you Dethronerds give that a shot and report back the results.

References

• Re: What is in soap scum? [MadSci.org]
• What is the difference between soap and shampoo? [ChemistryQuestion]
• Shampoo [Everything2]
• How Your Water Affects Your Hair [Chagrin Valley Soap and Craft]
• Soap production [Science at Home]
• Shampoo [Wikipedia]
• Soap Chemistry [Cleaning 101]

Image courtesy Auburn University Special Collection and Archives.