Chemistry of Shampoo Cleaning Action
Shampoo’s main purpose is to clean hair. Here’s how shampoo does it.
Shampoo marketers go to great lengths to make their product seem special, but the truth is that shampoos generally contain the same ingredients and work in the same fashion. Despite marketing claims, shampoo does not nourish or revive hair, as the hair shaft is dead once it leaves its root. Shampoo does however clean hair and chemistry can explain how.
Why Water Alone Does Not Clean Hair
Joe Schwarcz describes how shampoo works in his December 9, 1998 article for the Washington Post “Secrets of Shampoo”. Around the root of a hair are sebaceous glands that secrete an oily substance called sebum. The purpose of sebum is to protect the hair from drying out. However, sebum also attracts dirt, and water alone cannot wash it away. This is because water and oil do not mix, a fact due to the polarity of the molecules.
How Shampoo Cleans
Joe Schwarcz goes on to explain how the cleaning agent in shampoo is called a surfactant. Surfactant molecules have a hydrophilic end, which is attracted to water, and a hydrophobic end, which is repelled by water but attracted to oils. The surfactant helps separate the oil molecules so they are no longer attracted to each other. The surfactant also acts as a bridge between the oil and water molecules so that the dirty sebum can be washed away with the water. Shampoo works just as soap does. Elmhurst College’s website provides a detailed explanation with pictures of the chemistry behind soap.
Shampoos Strip the Sebum Off
All shampoos strip away the sebum from the hair. However, sebum is important for protecting hair shafts. Thus, shampoos must contain an ingredient that replaces the sebum. Schwartz describes how silicones like dimethicone coat the hair and replace sebum. The hair shaft is covered with translucent cells that overlap each other like roof shingles. In damaged hair these shingles are rough. Dimethicone can smooth the shingles, changing hair’s reflective properties. This gives hair that much sought-after shine.
Ingredients in Shampoo That Do Not Clean
There are plenty of other ingredients in shampoo that do not clean. Television commercials have taught the public that foamy shampoos are better and more effective. Some shampoo companies use surfactants that are better at creating foam. However, foam is just gas encased in a liquid and has no bearing on cleanliness. Shampoos also contain thickeners to give it the expected texture, preservatives to prevent bacterial contamination, and a variety of fragrances. Proctor & Gamble offers a list of ingredients used in shampoos matched with their purpose.
List of Shampoo Ingredients
|Ingredient Type||Function Examples||Examples|
|Styling polymers||“Active ingredients” that form bonds between hair strands||
|Plasticizers & Neutralizers||Ingredients that modify polymer properties to make them more flexible and easier to wash out of hair||
|Solvents||Liquids that serve as carriers for other ingredients and then evaporate after product is applied to hair (usually water or ethanol)||
|Thickeners||Ingredients make gels easier to apply||
|Conditioning Agents||Help to improve softness and shine attributes||
|Preservatives||Prevent microbial contamination of product||
|Propellants||Pressurized gases to provide necessary force to push product out of container||
|Surfactants||Used to help dissolve oils in a water-based system. Also used in mousses to work together with propellant to create foam. Can improve spreadability of product||
|Fragrances||Impart a pleasant scent or cover up a raw material odor|
Shampoo contains many ingredients but only a few are actually used for cleaning. The power behind this cleansing action ultimately comes down to a molecule which overcomes the contrasting polarities of oil and water.