Updated: Sep 9, 2021
Picture this, you have just washed your hair with shampoo, rinsed it off and applied hair conditioner, then your hair just magically becomes smooth and shiny. Have you ever wondered how the hair conditioner actually works? Is it truly a magical elixir as you only leave it on for a few minutes and immediately upon washing it off, your hair becomes silky smooth?
 Photo by Kyle Smith on Unsplash
Of course, it isn’t magic and sorcery! Before we dive into the science behind the hair conditioner, we must first understand the structure of the hair. Hair is primarily made of dead and keratinized cells that are negatively charged.
The outermost layer of hair shaft is the cuticle, and it looks like a flaky layer when observed under a microscope. These cuticle flakes are normally pressed down firmly and neatly aligned by hydrogen bond.  When light reflects off the surface, it gives a healthy sheen to the hair. On the other hand, when the overlapping cuticle layers are not lying flat and are being worn down, the hair looks frizzy or dull. While the scalp produces natural sebum that keep the cuticle flakes down, however, shampoo or prolonged exposure to the sun can strip away the sebum, thus will open the cuticle and cause hair damage.  Hence, to overcome that problem, mankind had developed a concoction known as the hair conditioner.
What is hair conditioner and how it works?
Hair conditioner is generally made up of several components such as humectants, oils, and cationic surfactants (the scientific term for soaps or detergents that contain positively charge on their hydrophilic end). The chemistry behind the hair conditioner is that the positively charged surfactant will bind to the negatively charged hair strand, which create a strong attraction that allow the surfactants to cover the hair shaft completely. A trace amount of acid in conditioner will make the overlayed cuticle flakes to lay tightly even after the conditioner is rinsed out.  To top it all off, the hair conditioner can smoothen hair strands and making it less static, as well as keeping the moisture in and pollutants out.  Daily wear and tear from pulling hair back in headbands or ponytails can damage the hair shaft.  The conditioner recovers the damage by refortifying the cuticle and coats the hair shaft to preventing breakage, split ends, and even hair loss. 
Type of hair conditioner and how to use it?
There are many types of hair conditioner such as deep conditioner, cleansing conditioner, leave-in conditioner and dry conditioner. They can be used according to the labelled instructions and should only apply on the hair shaft and not the scalp. As for the traditional post-shampoo hydrator, it should be applied in fluid motions after squeezing excessive water out from the hair strands. The reason being, excess water can dilute conditioner and prevent it to get in contact with the hair. The effect of conditioner is immediate but leaving it on for 1 to 2 minutes is ideal. 
Can hair conditioner have side effects?
Very few, but they do occur. Some chemicals in hair conditioner can be potential allergens for certain individuals which could cause irritation and contact dermatitis. Furthermore, for individuals who have skin-type more prone to acne, breakout can occur if they fail to rinse out conditioner completely as conditioner contains oil substance that can clog the pores. In addition, for individuals who have fine hair, using conditioner with high oil and emollient concentration can make the hair look greasy and flat. 
All in all, hair conditioner is needed for maintaining healthy and strong hair. Although hair conditioning seems like cosmetic care, it is indeed essential after every hair wash with shampoo.
How Does Hair Conditioner Work? 12 Jan. 2014, scienceline.org/2014/01/how-does-hair-conditioner-work/.
Herbody1, director. Healthy Hair Tutorial: How Does Hair Conditioner Work [Part1]. YouTube, YouTube, 28 Aug. 2014, www.youtube.com/watch?v=lm7Qvo_E96U.
“Hair Conditioner: When, How, and Why Men Should Use It.” Birchbox United States, www.birchbox.com/guide/article/mens-hair-conditioner.
Norris, Taylor. “How to Use Conditioner on Hair: Best Practices by Type and by Hair.” Healthline, Healthline Media, 24 Oct. 2019, www.healthline.com/health/how-to-use-hair-conditioner.
Fasanella, Kaleigh. “Here's Why You Should Always Use Conditioner.” Allure, www.allure.com/story/what-does-a-conditioner-do-why-should-you-use-it
Photo by Kyle Smith on Unsplash