Shampoo Chemistry

Do you use shampoo?  Right now, you’re probably thinking, “That’s a silly question, of course I use shampoo.  How else would I keep my hair clean and luscious?”  However, I bet that you don’t know too much about what is in your shampoo or why it cleans your hair.  Why would you; it’s not as if you’re lathering a diverse group of chemicals into your head, right? If you read on, you will become educated about what is in a shampoo and how it works.

Shampoo Basics

Let’s start with the basics, what does shampoo do to your hair?  Intuitively, one would say that shampoo is used to clean hair. Hair must be kept clean to prevent hair from becoming oily and greasy by a substance called sebum. Sebum keeps your hair healthy and protected, but also attracts dirt, causing hair to become greasy. Shampoo contains sulfates that, when rinsed with water, carry oily substances out of your hair. Thus, shampoo actually cleanses and dries hair. Moisturizers can be added to shampoos to allow the hair to retain water, and become moisturized. Conditioners can also be added to prevent tangling of hair. There can be other additives to shampoo, such as UV-absorbers and colorants, but ultimately, the purpose of shampoo is to successfully clean and dry hair.

Click the picture for a more detailed view from Chemistry Views.

Shampoo Components and Applications           

Shampoo has many components and each has a specific role. Major components, aside from water, are surfactants, conditioning agents, and preservatives.  Shampoo manufacturers look at aesthetic qualities, foaming agents, pH buffers, and thickeners.  The surfactants clean your hair.  These molecules are both hydrophobic and hydrophilic. The hydrophobic tail consists of hydrocarbons with a chain length of between C8 and C18.  The shorter the chain is, the greater power to remove grease it will have. However these chains are harsh on hair.  A long chain is very mild, but does a lot less to clean hair, so most manufacturers will use a chain of roughly length C12.  The hydrophilic head is made of many functional groups and the surfactant’s nature will likely be determined by this part of the molecule.  The four types of surfactants are anionic, cationic, nonionic, and amphoteric. Anionics will foam and clean your hair more than the other types of surfactants. The most common forms are sodium laureth sulfate, sodium lauryl sulfate, and ammonium lauryl ether sulfate.  The ammonium group has the properties of an increase in steric hindrance and a low ionization level, so it is most commonly used in a bottle of 2 in 1 shampoo and conditioner. This is because the cationic surfactants are used as conditioners, and can cause both the anionics and the cationics to precipitate out, but the properties of the ammonium group prevent this.  Cationics cling to the hair and do not come off easily, which leaves the hair shiny.  The most commonly used cationic surfactant is polyquarternium-10 which is a cellulosic polymer that is quarternised for ideal results.  Nonionics are very uncommon today and probably do not apply to your shampoo unless it specifically targets grease build up. This is because a nonionic surfactant is very damaging to the hair.  It strips away all of the fats and can leave the hair damaged and irritating. Amphoteric surfactants are just the opposite.  They are present in most shampoo because they are extremely mild and produce an excellent lather.  These surfactants carry both a positive and a negative charge unlike nonionics which do not carry a charge and the most commonly used one is cocamido propyl betaine.  To condition your hair, a glycerol or a silicone polymer is added.  These add shine, but accumulate on hair.  Some other aspects of conditioners are that they make hair easy to comb with cetyl alcohol and shiny with cetrimonium bromide.  If you are looking to buy an individual conditioner, polyquats or PEG fatty glycerides are used, as they work better and are easier to remove.  There are innumerable preservatives and there will be at least four in your shampoo as a wide range may be necessary to kill all potential microbes.  In an aesthetic sense, there are many fragrances for scent, opacifiers and colorants for the appearance of the shampoo, and UV-absorbers to protect the shampoo from the Sun. Foaming agents do not help to clean, but they do aid in producing a better lather.  pH buffers are used to keep the shampoo acidic.  At a pH level of five the hair is shiny and sleek, but at higher pHs of seven to nine, the hair becomes dull.  Thickeners are for convenience as they prevent the shampoo from running freely as it leaves the bottle as well as to make the shampoo viscous enough that it does not run into your eyes.


Relations to Properties of Solutions

To show the entire chemistry behind shampoo, let us assume that it contains conditioners and surfactants. The surfactants are anionic, or negatively charged, and conditioner molecules are cationic, or positively charged. To prevent the two from coming together and depositing out of solution, the conditioner molecules form a crystalline matrix around themselves. At high concentration, the surfactant molecules are stable with the conditioner matrix. Before washing the hair, the sebum gland protects the hair from drying out, but attracts dirt and is not water soluble. Once shampoo is applied, the surfactants form a colloid, or are in suspension, around the dirt and grease molecules. Then, the surfactants carry the dirt and grease molecules out of the hair. The hydrophobic end of the surfactants attaches itself with these molecules, and the hydrophilic end allows these molecules to be washed away by water. After the hair is rinsed with water, the surfactant concentration is diluted, causing the conditioner matrix to lose its stability and release conditioner molecules. Since the surface of the hair has been negatively charged by the surfactants, the cationic conditioner molecules are attracted to the hair, and provide a coating. This smooths out any roughness caused by surfactant molecules.


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