Week #2: How Does Shampoo Work?

Even coffee contain lots of chemicals, shampoo as a detergent definitely needs some kinds of chemicals to clean up all the dirt and oil in your hair.

INTRODUCTORY VIDEO

Standard YouTube license: How Does Shampoo Work? uploaded by Reactions

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Dimethicone

DISCUSSION

Ingredients contained in shampoo that allows it to act like detergent mentioned are dimethicone (also called polymethylsiloxane) and polyquaternium.

Dimethicone is a polymer (can easily tell from its name - "polymethylsiloxane") made up mainly of silicon (image to your right) and is often used as conditioning agent, often for skin and hair. Its overall hazard is low, so it can be used safely¹.

Polyquaternium-7

Polyquaternium is the general name given to polycation compounds consisting of quarternary ammonium at the center of the polymer. Different polyquaternium is distinguished by numerical value, such as polyquaternium-4, polyquaternium-7, or polyquaternium-40. These numbers are assigned based on the time they were registered, so there's no relation between the numbers and their chemical structures. Each of these will have different functions, such as polyquaternium-7 creates softness to our hair. Because polyquaternium is positively charged (polycation), they neutralize negative charges produced by other chemical substances, making it easier for us to comb our hair and keep our hair aligned property with shiny effect².

Sebum and Acne⁴

Sebum is a yellow oily substance stored by the sebaceous glands under our skin to keep our skin and hair moisturized. Sebaceous glands are present everywhere in our bodies except for our hand palms and feet soles (perhaps that's why these two regions aren't oily, I never noticed this!). The most concentrated area of these glands are our back, chin, and forehead (or the T-zone)³. Besides sebum's effect towards our hair (pretty much described clearly in the video), sebum is also considered one of the factor of our acne. Excess sebum production, often affected by our hormone levels, causes thicker and stickier oil, making it more difficult to escape through our skin pores. As a result, the produced sebum pushes upwards and causes pimples (like the image illustrates)⁵.

Sodium lauryl sulfate (SLS) is present in many shampoos, soaps, toothpaste, and laundry detergents. It is a surfactant, meaning that it reduces the surface tension between two liquid phases, or between a solid and a liquid. It contains two ends, one is hydrophobic and one is hydrophilic, just like what is shown in the video, and is often used in detergent products⁸. It is considered moderate hazard, and should not be used with concentrations above 2%. There is no scientific evidence of the major issues associated with SLS like cancer, but there are studies proving it as a skin irritant⁷. So check the ingredient list, if possible, before you purchase your products. For now, I would say it is safe as it is present in many cleaning products sold and used by thousands of us, although some people may be strict and prefer SLS-free shampoo.

Finally, it is true that most shampoos are pretty similar to each other, especially shampoos produced from the same FMCGs company, such as Herbal Essences and Pantene are both produced by P&G. Their formulas are pretty much similar, so by buying Herbal Essences, you're paying more for the same formula (just better smell)⁶. So be a smart consumer ;)

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Sources 
Thumbnail is orginial work from Chemiphilic
Sebum and Acne from The Nemours Foundation/Kid Health (on image), taken from Akron's Children Hospital 
Chemical structures are from PubChem
¹ EWG's Skin Deep Cosmetics Database
² Chemists Corner
³ About Dermatology: Sebum
⁴ Akron Children's Hospital
⁵ The International Dermal Institute
⁶ It's Okay to Have Lead in Your Lipsticks [Book]
⁷ SLS Free
⁸ Surfactant [Wikipedia Page]