You know shampoo cleans your hair, but do you know how it works? Here is a look at shampoo chemistry, including how shampoos works and why it's better to use shampoo than soap on your hair.
What Shampoo Does
Unless you've been rolling around in mud, you probably don't have hair that is truly dirty. However, it may feel greasy and look dull. Your skin produces sebum, a greasy substance, to coat and protect hair and the hair follicle.
Sebum coats the cuticle or outer keratin coat of each hair strand, giving it a healthy shine. However, sebum also makes your hair look dirty. An accumulation of it causes hair strands to stick together, making your locks look dull and greasy. Dust, pollen, and other particles are attracted to the sebum and stick to it. Sebum is hydrophobic. It waterproofs your skin and hair. You can rinse away salt and skin flakes, but oils and sebum are untouched by water, no matter how much you use.
How Shampoo Works
Shampoo contains detergent, much like you would find in dishwashing or laundry detergent or bath gel. Detergents work as surfactants. They lower the surface tension of water, making it less likely to stick to itself and able to bind with oils and soiling particles. Part of a detergent molecule is hydrophobic. This hydrocarbon portion of the molecule binds to the sebum coating hair, as well as to any oily styling products. Detergent molecules also have a hydrophilic portion, so when you rinse your hair, the detergent is swept away by the water, carrying sebum away with it.
Other Ingredients in Shampoo
Conditioning Agents
Detergents strip away the sebum from your hair, leaving the cuticle exposed and susceptible to damage. If you use soap or dishwashing detergent on your hair, it will get clean, but it may look limp, lacking body and shine. Shampoo contains ingredients that replace the protective coating on hair. Silicones detangle hair, smooth the hair cuticle, and add shine. Fatty alcohols help prevent static and fly-away or frizzy hair.
Shampoo typically is more acidic than soap, so it may contain ingredients to bring down the product of the pH. If the pH of shampoo is too high, the sulfide bridges in keratin can break, weakening or damaging your hair.
Protectants
Many shampoos contain additional ingredients intended to protect hair. The most common additive is sunscreen. Other chemicals protect against heat damage from hair dryers or styling aids, chemical damage from swimming pools, or build-up from styling products.
Cosmetic Ingredients
Shampoos contain aesthetic ingredients that don't affect how well the shampoo cleans your hair, but may make shampooing more pleasant or affect the color or fragrance of your hair. These additives include pearlising ingredients, which add sparkle to the product and may leave a faint glimmer on hair, perfume to scent the shampoo and hair, and colorants. Most colorants wash out with shampoo, although some subtly tint or brighten hair.
Functional Ingredients
Some ingredients are added to shampoo to keep it uniformly mixed, thicken it so that it is easier to apply, prevent the growth of bacteria and mold, and preserve it to extend its shelf life.
A Word About Lather
Although many shampoos contain agents to produce a lather, the bubbles don't aid the cleaning or conditioning power of the shampoo. Lathering soaps and shampoos were created because consumers enjoyed them, not because they improved the product. Similarly, getting hair "squeaky clean" actually isn't desirable. If your hair is clean enough to squeak, it has been stripped of its natural protective oils.
Source: About.com
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