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ChemMatters April 1983 Page 8
© Copyright 1983, American Chemical Society
pH & Hair Shampoo
by Kathleen J. Dombrink and David O. Tanis
The sebaceous glands in the scalp secrete a continuous flow of sebum, a greasy, semiliquid substance composed primarily of glycerides (50%). Waxes, fatty acids, and squalene, an intermediate in the biosynthesis of cholesterol, are the other major constituents of sebum. The sebum serves several functions. First, it coats the cuticle, or outer layer of the hair, preventing loss of water from the inside of the shaft. This water inside the shaft keeps the hair soft and flexible. Second, the sebum coating makes hair look smooth and shiny. It also prevents the growth of bacteria, such as streptococci, and spread of fungus infections, such as ringworm, on the scalp.
However, because sebum is sticky, it attracts dirt, eventually making hair look dull and dirty. Shampoos are supposed to remove the dirt and excess sebum, leaving a light coating of sebum on the hair and the scalp. How well a shampoo cleans depends on the detergent used in the shampoo.
A detergent molecule contains two major sections: a polar and a nonpolar portion. The nonpolar portion is soluble in grease and oil, which are also nonpolar, while the polar end is soluble in water, a polar solvent. A simplified representation of a detergent molecule is as follows: The detergent coats the grease, allowing the grease to be dispersed in water. The grease can therefore be rinsed away with additional water.
Detergent molecules produce alkaline solutions (solutions with a pH greater than 7) when reacted with water. However, this does not necessarily mean that a very alkaline pH is desirable in a shampoo. The effects of an alkaline pH on the nature of hair must also be considered.
Each strand of hair consists of many long, parallel chains of amino acids. These chains, or polymers, of amino acids are made of proteins. There are three ways in which these long chains can connect to each other: 1) hydrogen bonding; 2) salt bridges between acid and base groups; and 3) disulfide bonds.
Water will reversibly break the hydrogen bonds. This makes wet hair easy to shape and set. When the water evaporates, hydrogen bonds form at new positions, holding the hair in this set. In strongly acidic solutions where the pH is 1.0 to 2.0 both hydrogen bonds and salt bridges are broken. The disulfide bonds, however, can still hold the protein chains together in the strand of hair. At a slightly alkaline pH of 8.5, some disulfide bonds are broken. This causes the cuticle or outer surface of the hair strands to become ruffled. This roughness catches light unevenly and makes the hair look dull. Repeated washing with slightly alkaline shampoo will damage the hair by breaking more and more of these disulfide bonds. This is one cause of the dreaded "split ends."
At a pH of 12, which is a strongly alkaline solution, all three types of bonds are broken, and the hair dissolves! Commercially available depilatories, such as Neet® and Nair®, contain strong alkaline ingredients.
Hair is normally acidic. Human skin has a pH between 3.0 and 5.0, and hair has its maximum strength in a pH range of approximately 4.0 to 5.0. Shampooing often leaves hair alkaline. To restore the hair's naturally acidic pH, acid rinses can be used. The old-fashioned acidic rinses such as lemon juice, which contains citric acid, and vinegar, which contains acetic acid, are generally sufficient to counteract the temporary effects of alkaline shampoos.
Commercial treatment such as bleaching, dying, or a permanent, as well as blow drying and the use of electric curlers, can also leave the hair without its protective sebum. Conditioning rinses put back this oily coating, often penetrating the hair shaft itself. Because conditioners cling best to the damaged portion of the hair where the cuticle has become ruffled, thorough rinsing usually removes the conditioner from healthy sections of the hair, and the conditioner works only where it is needed. Too much or too heavy a conditioner will not damage hair, but it can make the hair stickier. The dirt attracted to this sticky hair often may make more shampooing necessary.
Shampoos that are "acid balanced" and promise to maintain the normally acid pH of hair usually contain an acid such as citric acid to counteract the alkalinity of detergents. Conditioning shampoos contain an oil such as lanolin. Since shampoos are designed to remove oil from the hair, the effectiveness of such conditioning shampoos is debatable.
REFERENCES Corcoran, F.; Akona, K. “The pH of Hair Shampoos: A Topical High School Experiment.” Journal of Chemical Education, 54:553-54, 1977. Rinzler, C. A. “The New and Improved Chemistry of Cosmetics.” Science 82, 3:54-61, April 1982.