SHAMPOO
The average human scalp contains approximately 125,000 to 150,000 hair fibers. Hair is nonliving material composed mostly of the fibrous protein keratin. Sequential bundling of keratin fibrils leads to the formation of the main body of the hair fiber, the cortex. Surrounding the cortex are lifeless cells known as cuticle cells that allow for protection by forming a sheath around the hair. At the center of this structure is the hair shaft. Coiled proteins surrounding the shaft provide hair with the properties of elasticity and bounce and contain melanin pigment that pro- vides hair coloration. While the hair shaft extends outward from pores of the upper epidermis layer of the skin, the deeper dermis skin layer con- tains the hair bulb and root located within the hair follicles, the structures responsible for hair growth. Located adjacent to each hair follicle is a sebaceous (oil) gland, which produces and releases sebum. Sebum coats the cuticle cells of the hair and also aids in maintaining hair flexibility, shine, and gloss by keeping the cuticle sheath lying flat and preventing dehydration.
As a protein, keratin consists of amino acid chains sustained by different types of forces, including hydrogen bonds, salt bridges, disulfide linkages, and hydrophobic interactions. Of these forces, hydrogen bonds and salt bridges are the most important when considering the action of shampoo, as hydrogen bonds between protein chains are disrupted by water and salt bridges are destroyed by changes in pH. A strand of hair is most stable and strongest at a pH of 4 to 6 (slightly acidic), as the maxi- mum number of salt bridges exists at a pH of approximately 4.1. Under basic conditions, the cuticle cells tend to become unstacked, and this dis- lodging of the protective tile cells leads to reduced light reflection and the appearance of hair lacking luster. In addition, the raised cuticle cells may also lead to tangling and the escape of cortex moisture leads to dry and brittle hair. Thus, the control of pH (between pH 4 and 9) is paramount when considering the chemistry of hair care products.
Rinsing hair with water alone will cause the keratin to become more stretchable by absorbing water and softening. However, adding a synthetic cleansing agent such as those found in shampoo to the hair during water rinsing will also allow for the removal of the water-insoluble oily sebum and associated debris from the hair shaft. Detergent action associated with shampoos is the result of surfactants. Surfactant is an abbreviation for “surface-active agent.” Surfactants possess hydrophilic (water- soluble) heads and hydrophobic (water-insoluble) fatty acid tails. The water-soluble head of the surfactant may carry a negative charge (anionic surfactant), a positive charge (cationic surfactant), may lack any charge (nonionic surfactant), or the surfactant molecule may carry both a posi- tive and a negative charge (amphoteric surfactant). Most major surfac- tants used in modern shampoos include all but cationic surfactants because of their skin-irritating potential. Surfactants promote the solubi- lization of sebum-based oil and dirt in water by lying across the oil-water interface and emulsifying the oil droplets, thereby allowing the oily soil to be removed from the hair via water rinsing.
The word “shampoo” is thought to come from the Hindu word champo, which means “to massage” or “to knead.” The first successful retail shampoo was developed in 1930. Until that time, and through World War II, the cleansing agent in shampoos was soluble soap (usually sodium and/or potassium salts were preferred). Soap-based shampoos readily formed insoluble calcium and magnesium salts in the presence of hard water, thereby leaving a dulling film on hair. Consumers would often apply a rinse containing vinegar or lemon juice to remove the “scum” film.
The most common primary surfactants used in modern shampoos are the lauryl sulfates and lauryl ether sulfates. The preferred counter-ions for these anionic surfactants are sodium, ammonium, or potassium to con- fer good water solubility. Cosurfactants, which are frequently included for their ability to enhance foaming power, include chemicals such as coco monoethanolamide, lauric acid diethanolamine, coco diethanolamide, cocamidopropyl betaine, and other betaines (e.g., lauramidopropyl be- taine). The foaming ability of shampoos may be additionally boosted by adding compounds such as fatty acid alkanolamides. Shampoos designed for oily, normal, and dry hair seem to differ primarily in the concentra- tion of surfactants (e.g., oily hair: more concentrated detergent; dry hair: more dilute detergent). Amphoteric surfactants are often used to allow for skin mildness (e.g., in shampoos designed for infants) and include chemicals such as sodium alkyl amphoacetates, sodium 3-dodecylamino- propionate, sodium 3-dodecylaminopropane sulfonate, and N-alkyltaurines.
Other functional ingredients included in modern shampoos are those that confer benefits other than cleaning, such as agents to provide mois- turizing body (e.g., proteins such as keratin, collagen, silk protein, hydrolyzed soy protein, hydrolyzed wheat protein), conditioning (e.g., oleyl alcohol, glycerine, stearyl stearate, glycerides derived from natural plant and animal oils, shea butter, allantoin, Aloe vera gel, dimethicone, and cationic polymers such as polyquaternium-10 and polyquaternium-7), antistatic properties (e.g., cationic detergents such as trimethyl alkyl am- monium chloride, tricetyl methyl ammonium chloride), UV protection (e.g., benzophenone-3), or antidandruff properties (e.g., piroctone ola- mine, zinc pyrithione). In addition, modifiers are added to confer desir- able flow properties and to stabilize suspensions of insoluble particulate components (e.g., volatile silicones, carbomers [cross-linked polyacrylic acids], acrylates, cellulosics [e.g., hydroxypropylmethylcellulose], xanthan gum, glycol stearate, cetyl palmitate, glyceryl distearate, sodium sulfate).
Solvent fillers (e.g., water, propylene glycol), preservatives (e.g., form- aldehyde, DMDM hydantoin, BHT, methylisothiazolinone, methylchlor- oisothiazolinone, parabens), pH adjusters that enhance foaming action (e.g., triethanolamine, myristic acid, sodium chloride, citric acid, lactic acid), chemical sequestrants/ligands (e.g., EDTA), herbal extracts (e.g., lavender, bergamot, rosemary, peppermint, soapwort, yucca, Iceland moss, orange flower, lemongrass, grapefruit seed, cucumber, watercress, pars- ley, matricaria, mushroom, chamomile, jojoba, fennel seed, golden seal), vitamins (e.g., panthenol, ascorbic acid, retinyl palmitate, riboflavin, tocoph- eryl acetate), colors (e.g., caramel, henna, D&C colors, FD&C colors), and fragrance also can be added to enhance the composition, purity, chemical effectiveness, and aesthetic quality of the product.
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