How Things Work: Shampoo

Millions of years ago, man was covered in hair.
With so much hair to go around, it wasn?t given much attention. Cave-dwellers were covered in a matted, oily hairstylist?s nightmare. Over the years, evolution has caused our hair to recede all the way to the tops of our heads.
Today, many cherish their hair as if every day would be its last, and an entire industry of hair care products has sprung up to cater to this craze. There are waxes, gels, mousses, sprays, colorings, washes, and everything in-between that can give hair any look imaginable. The most important of these, however, is also the simplest: shampoo.
We know of shampoos today as a liquid product meant for cleaning hair. According to the FDA, shampoo is actually a cosmetic, unless it is intended to treat a medical condition such as dandruff or lice (in which case you have bigger problems than the government?s definition of shampoo). As simple as shampoo may seem, it isn?t very easy to concoct. Imagine that you are the lead scientist for Head & Shoulders. The CEO and marketing managers call you into the conference room. ?It?s time we get down to brass tacks,? they say. ?We need you to design us a totally new shampoo. That damn Herbal Essences and its clever, sexy marketing is ruining us! We want a shampoo that will make our customers scream out in ecstasy too!? Such a task is easier said than done. Putting aside marketing, what do customers look for in a shampoo?
First and foremost, it must clean one?s hair and remove oil. At the same time it must not be so harsh that it removes the hair?s natural oils. It must be non-toxic if ingested in small quantities. Its formulation can?t sting the eyes too much. It must have proper viscosity: not too runny, but not too thick that it can?t be spread easily. It must lather the hair enough to satisfy buyer image. And it must look and smell appealing.
Finally, this completed product must last on store shelves for months.
How is all this done?
Certainly not with one single wonder chemical! There is no deity that rains down shampoo mixes like manna from Heaven. Let?s take a look at some of the main ingredients of shampoo.
Cleaning one?s hair is made possible by a class of chemicals called surfactants. Surfactants act on boundary surfaces. One end of each surfactant molecule is water-attractive, or hydrophilic, while the opposite side is water-repulsive or hydrophobic. These ends naturally align, and the hydrophobic ends attach to oily deposits known as sebum on each strand of hair. When enough of these have attached, the sebum acquires a negative charge and repels itself from the similarly charged hair. Rinse your hair, and down the drain it goes!
Lather is actually unnecessary. The large amount of lather we see on shampoo advertisements is just a marketing ploy to glamorize shampoo. Lather is the result of surfactants wastefully interacting with air, which actually reduces the cleaning ability of your shampoo. Take that, sexy Herbal Essences lady!
Many other chemicals exist to fulfill secondary purposes, with names that would make all but the most diehard chemists cringe. Emulsifiers are added to boost viscosity. Preservatives are added so bacteria don?t multiply inside the shampoo bottle.
Often some conditioners are added, even though most people think of conditioner as a separate product. Conditioners use positively charged molecules, which attach to the hair, canceling the hair?s natural negative charge and eliminating static. They don?t wash away when rinsed, either, leaving a protective layer on the cuticle.
Fragrances are frequently added as well, even if the shampoo is unscented. It turns out that this mass of harsh chemicals doesn?t smell like wild roses on a dewy morning. Fragrance cancels out the otherwise objectionable smell.
Never thought shampoo was so complicated, did you? Whether your hair is straight, curly, wavy, or afrotastic, and whether it?s black, brown, blond, or red, shampoo knows no bounds. Except for a small minority of computer science majors, we all enjoy a freshly shampooed head of hair. Just don?t lather too much.