As far back as most can remember, our hair has been washed with shampoo first and conditioner second, but it seems many of us don’t really know why, or even understand the differences between the two.
The washing process of hair can be understood when looking at the composition of hair (figure 1A). Within hair follicles are the sebaceous glands which produce a yellow oily substance called sebum. Sebum is made up of many different chemicals (figure 1B) and is important as it acts as a barrier, protecting the skin from bacterial and fungal infections, as well as keeping skin and hair moisturised.1 Sebum is continuously excreted from these glands, and over a short period of time many will notice their hair looking oily as the sebum builds up on the scalp.2 Due to the sebum’s waxy texture, particulates from the air can also adhere to the scalp and hairs surface, and at this point many feel it’s time to wash their hair.
We use shampoo to remove the excess sebum from our scalps as water alone cannot. The composition of sebum, as seen in figure 1B, is predominantly made up of oily, fatty molecules which cannot dissolve in water. Similar to when cleaning dishes (https://glamsci.blog/2019/05/08/why-we-need-soap-to-clean-dirty-dishes/), some form of surfactant must be added to allow water to dissolve and remove the oily components. The primary surfactants present in most shampoos used to be lauryl sulfates (figure 2), compounds chosen for their ability to clean as well as foaming capabilities.2 Whilst many shampoos still contain lauryl sulfates, more recently laureth sulfates (figure 2) have been preferred by manufacturers as they show a decrease in irritation when in contact with the eyes. Shampoos are designed to not only clean the hair, but to also leave behind a pleasant, fresh smell to the hair once washed off.
Shampooing the hair will inevitably remove not only excess oil, but also important moisture from the hair and can leave it feeling dry. Conditioners are often used after the hair has been shampooed to lower friction between hair fibres2 which makes brushing the hair easier, as well as adding moisture back into the hair.
Conditioners contain cationic surfactants (figure 3), molecules which possess a positive charge. Strands of hair are negatively charged due to the deprotonation of amino acids that make up outer layer of the hair, and so when you put conditioner on your hair, the cationic surfactant molecules will preferentially deposit on the surface of the hair.4 Once the positively charged surfactant molecules bind to the negatively charged hair, the overall coupling results in neutralisation and therefore reduces the overall charge of a person’s hair, reducing static. Conditioners also contain polymers which help to detangle hair, as well as silicones which are heat resistant and work to lubricate hair strands, adding gloss and shine.
- R. Y. Lochhead, Pract. Mod. Hair Sci., 2012, 75–116.
- M. . Paridah, A. Moradbak, A. . Mohamed, F. abdulwahab taiwo Owolabi, M. Asniza and S. H. . Abdul Khalid, Intech, 2016, i, 13.
- Indian J. Dermatol., 2015, 60(3), 248-254