Secrets of Shampoo
Special to The Washington Post
Wednesday, December 9, 1998; Page H01
A stroll down the shampoo aisle is always an eye opener. What are shampoos doing these days? Will they revitalize or energize our hair? Add luster or remove buildup? Nourish or rejuvenate? What's the newest "miracle" ingredient? Sperm extract or quinine? Alpha hydroxy fruit complex or beta carotene?
It's all very confusing. In fact, sometimes it's hard to tell if you're in a pharmacy or a grocery store, with shampoos loudly proclaiming that they contain mango, papaya, apple pectin, wheat germ or Swiss vanilla, whatever that may be.
This bewildering cacophony of claims and esoteric ingredients stems from the basic dilemma faced by shampoo manufacturers. How do you convert an essentially simple product that cleans hair into a magical lotion that increases sex appeal? The answer lies in a little basic science and a lot of clever marketing.
First, shampoos do not feed, resuscitate, enliven or revive hair. They can't, for the simple reason that hair is not alive. It is a shaft of dead proteins, notably including a tough fibrous material called keratin that also shows up in fingernails.
So what can shampoos do? They can clean hair, a none too difficult task. All that is required is removal of the thin layer of oily material known as sebum. Produced by sebaceous glands in the skin, it coats and protects the hair. Unfortunately, sebum also acts as a virtual magnet for dirt and residue from hair treatment products.
Suds and Surfactants
Prime ingredients in all shampoos are substances called surfactants, short for "surface-active agents." Responsible for cleaning action and lathering properties, they largely determine the hair's condition after shampooing. There are many kinds, all sharing a common feature.
Their molecules are composed of long chains of atoms in which one end is hydrophilic, attracted to water. The other end, termed hydrophobic, is repelled by water but shows great affinity for oily substances.
Cleaning action works like this: The hydrophobic end secures itself in the oily layer of sebum while the hydrophilic end remains anchored in water. As the hair is rinsed, the soiled sebum is washed away.
Removal of the sebum is facilitated by the ability of surfactant molecules to form aggregates called micelles. These are spherical groupings of 40 to 100 molecules in which all hydrophobic ends point toward the center and all hydrophilic ends stick out in the surrounding water.
Any tiny oil droplets removed from the hair by the surfactant will be attracted to the center of the micelle, keeping the oil drops from coalescing and redepositing themselves on the hair before they can be rinsed away.
Surfactants also contribute to cleaning in a completely different fashion by affecting a physical property of water known as surface tension. Water molecules are electrically "polar," with regions of positive and negative charge.
The reason is that the two hydrogen atoms are not symmetrically distributed around the oxygen atom but are on the same side, like the ears on Mickey Mouse's head. As a result, the negative area of one molecule is attracted to the positive region in an adjacent one.
This is why water forms beads on a surface or drops in the air; the surface area of the water in contact with the air is minimized because water molecules are attracted to each other much more strongly than to the air. For the same reason, plain water won't make bubbles.
To clean best, water needs a greater "wetting" ability. Surfactants accomplish this because their molecules wedge between water molecules, reducing surface tension and allowing water to flow easily into every nook and cranny on a surface.
Lathering of a shampoo also is the result of the activity of surfactants. Foam is nothing more than dispersion of a gas, in this case air, in a liquid. A close look at foam produced by a shampoo reveals that it consists of air bubbles surrounded by a thin layer of water.
To achieve this effect, the liquid's surface tension must be reduced so it no longer needs to minimize the surface area exposed to air. The water can stretch around the air bubble.
There is no clear link between a surfactant's ability to clean and to produce foam. Indeed, very effective shampoos that do not lather well can be formulated. But they do not sell well either. Somehow, in consumers' minds, sudsing and cleaning go together. So most shampoos incorporate surfactants with strong lathering properties although they may not be ideal in terms of conditioning or irritant potential.
The first commonly used surfactant was soap. Indeed, the first shampoos were just solutions of soap prepared from water, soap and soda (sodium carbonate) by British hairdressers during the heyday of the Empire on which the sun never set. They coined the word shampoo from "champo," a Hindi word meaning to massage or knead.
These products cleaned as long as water had low mineral content. But when the water contained an appreciable amount of magnesium and calcium, the soap formed an insoluble precipitate deposited on the hair, resulting in a dull appearance.
The advent of synthetic detergents, or "syndets," in the early part of this century solved this problem. Lauryl alcohol isolated from coconut fat or palm kernel oil was used to make sodium lauryl sulfate (SLS), which was a very effective cleaner, foamed well and did not form insoluble compounds with calcium or magnesium. It remains a mainstay of the industry but requires support from other surfactants.
Sodium lauryl sulfate is referred to as an "anionic" surfactant because its hydrophilic end is negatively charged. Positive charges are called "cationic." It is a rather harsh detergent, meaning that it removes sebum very effectively.
Since sebum protects the hair from drying out and conditions its surface, using SLS alone as a surfactant would lead to dry, fly-away hair. SLS also can irritate eyes and in rare cases, skin.
In addition, SLS is not very soluble in cold water and so cannot be used to make shampoos that look "clear." It dissolves readily at water temperatures normally used for shampooing so the problem of insolubility is more a cosmetic than a practical problem.
Related compounds such as ammonium lauryl sulfate or triethanolamine lauryl sulfate are much more soluble, so less can be used in a shampoo, resulting in more gentle cleaning. These typically are used in shampoos for dry or damaged hair.
Sodium laureth sulfate (SLES) is a chemical relative of SLS that was developed because of its high solubility. Therefore, it can be used to make clear shampoos. It has very good cleaning and foaming properties but is more expensive to produce than SLS.
Numerous other surfactants are available for use in shampoos. Cationics, with a positive charge on their hydrophilic end, foam and clean very well but can hurt the eyes.
They can, however, be used at low concentrations as conditioners. Their positive end is attracted to damaged parts of the hair, which tend to accumulate negative charges. Neutralization of the charges reduces static buildup.
Another class of surfactants, known as non-ionics because they don't carry full-fledged electrical charges, are good cleaners but do not foam well. Amphoterics, which can behave as cationics or anionics depending on the pH of the solution, do not foam well either but are very gentle to eyes and skin.
The classic shampoo ingredients have been subjected to various eye irritation tests over the years, and manufacturers have learned which detergents are least likely to cause trouble. Amphoterics are in this category and often are employed in "no tears" baby shampoos.
Shampoos contain far more components other than surfactants. There are thickeners (xanthan gum), preservatives (parabens), emulsifiers (glycol distearate), color additives and foam boosters (cocamide monoethanolamine).
Quaternary ammonium compounds, known as "quats," are added to shampoos to allow easy detangling when combing. These are the same compounds found in fabric softeners, and they cling to the hair's surface to produce a smooth finish.
Some shampoos also include panthenol, a molecule that can diffuse into the hair shaft and bind to proteins, strengthening their structure. Various proteins such as elastin and collagen or synthetic polymers often are added in the hope that they will bind to the hair's surface to enhance thickness.
Results are variable because most of these substances are rinsed away with the detergent. In some cases, a residue may accumulate, referred to in the trade as "buildup." This has resulted in introduction of specialty shampoos claiming to eliminate the social horror of limp, sticky and dull hair.
Actually, any shampoo without added conditioners will serve this purpose.
All shampoos, no matter how gentle, face a common problem. As the protective sebum is stripped away, the outside layer of hair, the cuticle, is exposed.
In healthy hair, the cuticle consists of translucent cells overlapping like shingles on a roof. In damaged hair, these shingles are more open and ragged. As the rough adjacent hairs rub against each other, transfer of electrons can produce a static electrical charge. The result is the dreaded affliction of fly-away hair.
Ideally, a shampoo should smooth down the cuticle and cover it with a clean coating of a sebum-like material. The smoothing effect is readily achieved by controlling the shampoo's acidity. As long as the pH is between 5 and 8, the "shingles" will close down. So all shampoos, whether they make the claim or not, are "pH balanced."
The proper pH range is maintained by addition of "buffering agents," such as citric acid. Humectants, which help to retain moisture, also are added. Examples are glycerol or propylene glycol, which is related to active ingredients in antifreeze. These form strong bonds to water and prevent it from evaporating.
Replacing soiled sebum with a clean protective coating is a more formidable task. After all, how is a shampoo to know that it is supposed to remove one oily substance and leave behind another one? Modern "two-in-one" formulations have gone a long way toward solving this problem.
Silicones such as dimethicone are relatively non-greasy materials resembling sebum. They can coat the hair, add gloss and provide a smooth surface to ease combing. They also can fill in damaged areas where the cuticle has been worn away and change hair's reflective properties, producing more shine.
Frequent use of the word "balsam" on hair-care labels signals an alternative. Balsam oil, from the tree of the same name, is chemically similar to sebum. The idea is to remove dirty sebum and replace it with clean balsam oil.
The search for novel ingredients that advertisers can hype seems never-ending. How about sperm shampoo? This pricey product touts the wonders of hyaluronic acid as the answer to bad hair days.
Why? Because this is the substance used by sperm to penetrate the egg. Therefore, in a logical non sequitur, it supposedly penetrates the hair shaft. Hyaluronic acid, which also occurs naturally as a lubricant in your joints, is a good moisturizer. The penetration stuff, however, is just hoopla.
The newest entries in the shampoo sweepstakes are products featuring alpha hydroxy acids (AHAs). These compounds, which occur naturally in fruits, do loosen adhesion between skin cells on the scalp and are helpful for people suffering from flaky, dry scalp associated with dandruff, psoriasis or eczema. They also happen to be good humectants (moisture-enhancers) and will prevent hair from drying out.
The fact that AHAs may have value in shampoos does not justify imaginative promotion of products that may contain "natural" extracts of fruits such as mango or papaya. In most cases, these are not present in amounts great enough to do much but perhaps contribute to the fragrance. Similarly, inclusion of other esoteric ingredients such as thyme, jojoba oil, vitamin E or placenta extract is far more effective for marketing than for the hair.
Shampoos have an excellent track record in terms of safety. But you certainly wouldn't get this impression by surfing the Internet. Numerous web pages are devoted to the horrors of sodium lauryl and sodium laureth sulfate, accusing these compounds of causing ailments ranging from cataracts to cancer. These sites usually are maintained by promoters of ĎalternativeĎ beauty products and are piled high with falsehoods, misinterpretations and conjectures.
Most of the negative "information" provided can be traced to studies that were done 20 years ago and found that certain shampoos were contaminated with small amounts of nitrosamines, a class of compounds known to be carcinogenic. The ethanolamine lauryl sulfates were implicated as the source of contamination.
Manufacturers addressed this issue, and today's products are routinely monitored for nitrosamines. But somewhere along the line, it seems that the distinction between ethanolamine lauryl sulfate and sodium lauryl sulfate was lost, at least among some members of the public.
This is more than you absolutely need to know about shampoos. In truth, despite the amount of interesting science behind these products, finding one you like is a matter of trial and error.
Remember that those young people tossing their luxurious golden tresses seductively in slow motion on television have just spent hours with expert hairdressers.
Forget the ads, disregard the fruit salad, save the vitamins for pills, ignore price and find a product you like. I often use diluted dishwashing detergent (containing, yes, sodium lauryl sulfate) and find it quite acceptable. However, I have noted an unusual desire to scrub pots and pans.
Joe Schwarcz is a professor of chemistry at Vanier College and McGill University, both in Montreal. He is the 1999 winner of the American Chemical Society's James T. Grady-James H. Stack Award for Interpreting Chemistry for the Public.
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