Chemistry has everything to do with whether your hair is straight or curly; blond, red, or brown; easily shaped into your favorite style; or a difficult-to-manage unruly mop. In fact, a chemist probably can provide as much—if not more—insight as your hair stylist on what makes the difference between a bad hair day and a good hair day.
Given the time and money most of us lavish upon our crowning glory, it's hard to believe that hair is merely dead skin. As skin cells in a cell layer known as the hair matrix die, they produce a tough protein called keratin. The pressure of new cells packs the dead cells and keratin together and pushes them to the surface of the scalp, where they emerge as strands of hair.
The average human head has between 100,000 and 150,000 strands of hair. From a chemist's viewpoint, that hair is virtually the same as bird feathers, horse hooves, cat claws, sheep's wool, and even snake scales—all of which are made primarily of keratin. Like all proteins, keratin is constructed from building blocks called amino acids. Much of keratin's strength is attributed to its high content of the amino acid cystine, which has particularly strong chemical bonds between sulfur atoms.
The Physical and Chemical Structure of Hair
The physical structure of a human hair can be seen through a microscope. It consists of three layers of dead skin cells: the cuticle, a smooth, outer covering fashioned from cells that overlap like fish scales; the cortex, a thick, middle layer made of spindle—shaped cells filled with keratin and hair color; and the medulla, a relatively narrow central column containing cube—shaped cells and pockets of air.
Hair's chemical structure cannot be seen under a microscope. Like all matter, hair is made up of tiny units called atoms. The outer part of an atom consists of even smaller particles called electrons that whirl around the atom's center region. When two or more atoms join to form molecules—combinations of atoms—they do so by exchanging or sharing electrons.
At the molecular level, hair is composed mostly of chains of keratin molecules. These chains are cross-linked by chemical bonds in much the same way that the sides of a ladder are held together by rungs. The bonds include very strong links between sulfur atoms found in cystine and much weaker electrical attractions between hydrogen atoms in other parts of the keratin molecules.
In straight hair, bonds form between atoms located at approximately the same sites on neighboring keratin chains. This creates relatively level “rungs” that allow the sides of the “ladder” to remain straight. In wavy or curly hair, molecules from different sites along adjacent keratin chains are attracted to each other, forming arching bonds that cause the sides of the keratin ladder to bend or loop around.
As for color, the shade of your hair is determined by natural pigments called melanins that are produced by skin cells. Surprising as it may seem, just one melanin—a black pigment called eumelanin—is responsible for most hair shades, from blond to black. The formula is simple: the greater the eumelanin level, the darker the hair. However, eumelanin cannot take credit for red hair, which owes its unique hue to a rarer, reddish type of melanin called pheomelanin. When skin cells grow old and stop producing eumelanin or pheomelanin, a person's hair turns gray or white.
Our genes determine the color and texture of our hair, but there are ways of getting around genetics. As the one-third of American women who color their hair and the half who perm or relax their locks can attest, hair chemistry can be easily changed. Perms, relaxers, dyes, shampoos, conditioners, and even curling irons and blow dryers all work their magic by tinkering with hair's normal chemical structure.
In fact, every time you wash your hair or go swimming, you generate a small chemical reaction. Hydrogen-rich water molecules snap the weak hydrogen bonds between your hair's keratin chains, prompting even the curliest of locks to straighten temporarily when wet. As the hair dries, the hydrogen bonds are reestablished, and waves or curls return.
The opposite occurs with straight hair that is dampened and then wound around pins or rollers so that different parts of the keratin chains are adjacent to one another. As the hair dries, it forms new hydrogen bonds and is temporarily transformed from straight to curly or wavy. Blow dryers, electric rollers, and curling irons act along the same principle, but use heat energy to reinforce the new bonding pattern.
But as people whose hair style depends on a curling iron or a blow dryer know, their efforts can be undone in minutes. All it takes is a little water in the form of a sprinkling of rain or a moist sea breeze to send your hair's hydrogen bonds scrambling back into their normal alignments.
For more lasting changes to your hair style—and your hair chemistry—you must turn to permanent waving (perming) or straightening. These processes not only alter hair's hydrogen bonds but also split apart stronger chemical bonds that are unaffected by water. Although waving and straightening (also called relaxing) produce opposite results, the two procedures employ the same chemical tricks.
Whether hair is permed or relaxed, the first step, called softening, involves reduction. Reduction is a chemical reaction that adds hydrogen to a chemical compound. In the perming or straightening process, reducing agents, sometimes together with heat, break the hair's water-stable bonds—primarily the sulfur-to-sulfur links in cystine.
In the next step of the process, called rearrangement, the keratin chains are molded into the desired configuration. This is accomplished by winding the softened hair on rods to produce curls or holding it flat to straighten it.
The final step, hardening, makes the rearrangement permanent by rebuilding the sulfur-to-sulfur bonds and other water-stable molecular links that hold hair strands together. This is accomplished through oxidation—a chemical reaction that reverses reduction by adding oxygen to a compound. For perms and relaxers, the chemicals that initiate oxidation are either sodium bromate or hydrogen peroxide solutions.
Although perming and straightening are called “permanent,” perms and relaxants do not change the hair matrix. Thus, their effects eventually disappear as hair grows out.
When properly formulated and used according to directions, products that perm or relax hair should not change its color or significantly weaken it. But both hair and scalp can be damaged if such products are carelessly manufactured or applied. In 1995, for example, the U.S. Food and Drug Administration (FDA) warned against the use of two hair relaxers sold under the name of Rio Hair Naturalizer System. The FDA took that action after more than 1,800 consumers complained that the products had irritated their scalp, caused their hair to break off, or turned it green. FDA investigators found that the products contained too much acid and were improperly labeled.
Another process that must be carefully controlled to avoid unpleasant results is bleaching. Hair is usually bleached with hydrogen peroxide, a chemical that lightens hair by oxidizing its melanin pigment.
The greater the concentration of hydrogen peroxide and the longer it is left on, the lighter the hair becomes until it approaches the harsh yellow-white of the stereotypical “bleached blonde.” The bleaching process is halted by rinsing the hair with very hot water or a mild acid solution, such as pyruvic acid.
The results achieved by bleaching are also determined by how much ammonium hydroxide, a chemical that activates oxidation, is added to the peroxide solution. Too much of this ammonia compound can produce unattractive reddish tints.
Hair tinting, which is capable of producing a rainbow of natural-looking hues from ginger gold to chestnut brown, also relies on oxidation. Colorants are a complex brew of chemicals. Oxidation is provided by a developer, usually hydrogen peroxide or another oxidizing agent. The developer acts on an intermediate. Intermediates are synthetic compounds, often derived from coal tar, that develop into dyes when they are oxidized.
Other components of hair color include modifiers, compounds that enhance or stabilize certain shades of color; antioxidants, which prevent the intermediates from oxidizing before they are applied to hair; and alkalizers, chemicals that improve the absorption of dye by softening the hair.
Like other hair treatments, coloring does not alter the cells that produce hair or its pigment. Thus, people who tint their hair must touch up their natural roots periodically if they want their growing tresses to retain an even color between complete dye treatments.
In recent years, many women have experimented with semipermanent and temporary dyes. Semipermanent colors use lower concentrations of oxidizing agents and wash out in four to six shampoos. Temporary colors, or rinses, are acid dyes that coat only the surface of the hair and readily wash out in the next shampoo. Because they simply add a tint to the surface of hair, rinses can be used only to produce a darker hair color.
Shampoos and Conditioners
Perming, relaxing, and coloring are the harshest things you can do to your hair from a chemist's viewpoint. But shampoos and conditioners also take advantage of chemistry to get their work done.
The job of shampoo is to clean the hair, so it's not surprising that the main ingredient in nearly all shampoos is detergent. Detergents create an anionic, or negatively charged, solution when combined with water. Such a formula is good for cleaning hair by attracting positively charged ions, such as oil. But it also tends to ruffle hair's outer cuticle, explaining why just-washed hair is often flyaway, unmanageable, and rather dull.
To counteract these undesirable effects and smooth down the hair cuticle, people often follow a shampoo with a rinse of a cationic, or positively charged, solution. In years past, people used vinegar, lemon juice, or beer for this purpose. Such rinses made hair glossier and easier to comb, but they could leave hair smelling like a tossed salad or a brewery. Modern conditioners achieve the same results and also have a pleasant fragrance more appropriate for hair.
One of the most important classes of conditioning agents is the quaternary ammonium compounds. These cationic substances help counteract static electricity and flyaway hair by binding to the anionic strands of shampooed hair. Because hair that has been permed, relaxed, or colored is considerably more anionic than normal hair, manufacturers design special conditioners packed with cationic ingredients for hair that has undergone a strong chemical treatment. Some formulas can even temporarily repair or strengthen chemically processed hair through the use of cationic proteins that cling to the hair's weakened keratin chains through hydrogen bonds.
Protecting Your Hair
Of course, it would be best to avoid damaging your hair in the first place. Experts have a few tips to offer on how to keep your hair from looking like the result of a failed chemistry experiment. First, they caution, do everything in moderation. Don't test the limits of your hair's ability to form and reform chemical bonds by subjecting it to repeated perming, coloring, or straightening over a short time.
Second, don't begin any treatment until you have thought it over and know for sure what look you want to achieve. If you impulsively decide to put a poodle perm in your stick-straight hair or bleach your raven locks to platinum blond, it will be weeks or months before you can change your look again without seriously damaging your hair.
Third, follow all instructions on hair-care products to the letter. And before starting any time-sensitive chemical process, such as coloring or a perm, do a dry run—go through the whole process in your mind and set out all the items you will need.
Lastly, if you end up creating a hair disaster, see a hair-care professional. Don't try to fix the problem yourself with home remedies or other chemical products. A professional may be able to salvage your hair, provide you with products to repair the damage, or at least cut your hair so that it looks reasonably attractive while you wait for your mangled tresses to grow out.
And here's one final tip: Consider being content with the hair you were born with, be it kinky or straight, dishwater blond or carrot red. You'll save a lot of energy—both chemical and personal—if you develop a sense of style that emphasizes your hair's natural beauty rather than an artificial look that requires you to constantly beat your hair into submission.