Hair Care Physicochemistry
All mammals have hair, but for humans, it plays a role in shaping opinions on their appearance and self-confidence. This causes individuals to want to change the appearance of their hair in many different ways. From shampoos to conditioners, dyes, mousses, lotions, and heat-styling tools, humans constantly test different methods to achieve their ideal hair appearance (Fernandes et al., 2023). But, from a scientific standpoint, what matters when ensuring that hair is in its glossiest and softest form?
To understand how to maintain hair’s healthiest form, it is vital to understand hair itself and the many different layers that make up each strand. Hair strands are mostly composed of protein, taking up 65-95% of the total hair weight. The main protein in hair is keratin, a protein mostly made of tyrosine, glycine, and cysteine amino acids. The cysteine amino acids contain a considerable amount of sulfur, meaning there are disulfide bonds between keratin chains, creating “cystine units.” These units are the chain of amino acids that make up the inside of every strand of hair on someone’s head. Specifically, they are inside the hair’s medulla (Fernandes et al., 2023).
The medulla of a hair strand is the innermost part. It is loosely packed, disordered, and made of many air spaces. The medulla is more dominant in thick hair, but is not very heavy, no matter the hair's consistency, due to the air spaces. Surrounding the medulla is the cortex. The cortex makes up most of the hair. It consists of elongated cortical cells with alpha-helical keratin filaments and sulfur-rich proteins such as keratin. Unlike the light medulla, the cortex takes up 90% of the weight of the hair. The cortex is a vital component of the hair to ensure the hair stays strong. The helical keratin structures provide the cortex with its elasticity, which allows it to store moisture and pigments. The outermost part of the hair is the cuticle. The cuticle of hair is made of six - 10 overlapping flat cells that point toward the end of the hair, and it looks similar to animal scales or roof shingles. The cuticle also has sublayers of its own. Those sublayers are called the epicuticle, A-layer, exocuticle, endocuticle, and the cell membrane complex (Fernandes et al., 2023).
Even with all the layers that make up hair, it is still very prone to damage. Damage to hair causes the cuticle to stay intact for a shorter amount of time, sometimes even removing the cuticle entirely. Due to the effect hair damage has on the cuticle, it causes the proteins in the cortex to degrade as well. Hair can be damaged by two categories of factors: mechanical and chemical. Bleaching, perming, relaxing, and straightening are all different chemical processes that are known to cause significant damage to hair fibers. Mechanical damage can result from poor brushing or combing, or from blow-drying at extremely high temperatures. With further damage, hair may lose its strength and elasticity and, eventually, its complete structural integrity, ultimately resulting in hair breakage (Fernandes et al., 2023).
So, the question is, what actions could one take to prevent their hair’s structural integrity from completely falling apart? Simply using shampoo and conditioner without understanding how they work is not very effective.
First and foremost, shampoo is a necessary step to ensure that hair is in its healthiest condition. Using shampoo ensures that the scalp is clean by removing dust, residue from styling products, and excessive sebum. Sebum is a fatty lubricant matter secreted by the sebaceous glands of the skin (“Sebum Definition,” n.d.). The scalp creates sebum as a way to protect the skin near the hair follicles from infection, but when sebum is combined with sweat, dead skin cells, and other debris, it can cause buildup that makes hair appear greasy and can cause flaking (Sherry Christiansen, 2024). Shampoo can remove all of the unwanted substances by solubilizing them with the surfactants in the product. This allows the soluble products to be washed away easily. This prevents hair from having excess oils and fatty products, which is extremely helpful when ensuring hair’s health. However, while it leaves the hair clean, it can also leave the hair looking rough and dry.
The way to help with the dry look left after cleaning with shampoo is by applying conditioner to the hair. Conditioner repairs hair, making it look smoother. Since effectively using shampoo to clean the scalp requires applying the physicochemical process of anionic surfactants and scrubbing abrasion, it removes some necessary lipids and keratin. These can be brought back to the hair with the help of conditioner (Fernandes et al., 2023).
Conditioners can cover and fill the surface of the hair shaft, fixing the damage caused by external factors, and even shampoo. The goal is that individuals apply conditioner to their hair when damage has only reached the cuticle and can, therefore, prevent the damage from increasing to the hair’s cortex. Conditioners usually contain silicone polymers, cationic polymers, bridging agents, viscosity builders, pH adjusters, and other components for general physical appeal (Zhang et al., 2015). These different common ingredients in conditioner can help your hair, but when too much is applied, these components hurt the hair more than they help it.
Silicone smooths, detangles, conditions, and adds volume to the hair. The issue with silicone is that over time, silicone can weigh down the hair and create a layer that does not just protect the hair from further damage, but actually guards the hair from further moisture (Campbell, 2022).
Cationic polymers are always beneficial in the conditioning process. Naturally, hair’s pH is acidic, at a level of 4.5 and 5. When hair follicles are damaged, that can cause the pH level to decrease. Cationic polymers are used to add to that pH level, making it more basic, because cationic polymers have a positive charge. In the end, cationic polymers are, in fact, the part of the conditioner that does the conditioning. The cationic polymers are one of the main products that bond to the hair shaft to help improve the state of the hair cuticle (WS, 2019).
Hair care comes in many forms, but when purchasing, it is important to understand the effect different products have on your hair. Doing so is the most effective way to ensure hair is in its healthiest and most attractive state. It is important for individuals to understand how to take care of their hair in the most efficient way. This is extremely important for helping consumers purchase the products that are best for their hair, ultimately allowing people to have an easier time maintaining healthy hair.
References
Campbell, S. (2022, August 25). Is there silicone in your shampoo and conditioner? Why you should avoid it. KIND2. https://kind2.me/blogs/kind2/is-there-silicone-in-your-shampoo
Fernandes, C., Medronho, B., Alves, L., & Rasteiro, M. G. (2023). On Hair Care Physicochemistry: From Structure and Degradation to Novel Biobased Conditioning Agents. Polymers, 15(3), 608. https://doi.org/10.3390/polym15030608
Layer of healthy and damaged hair structure. (n.d.). dreamstime. Retrieved September 25, 2025, from https://www.dreamstime.com/layer-image207689306
Sebum definition. (n.d.). In Merriam-Webster Dictionary. https://www.merriam-webster.com/dictionary/sebum
Sherry Christiansen. (2024, December 8). How Sebum on the Scalp Can Benefit or Damage Hair. Verywell Health. https://www.verywellhealth.com/sebum-buildup-on-scalp
WS. (2019, December 21). Cationic Compounds in Cosmetics. Blogspot.com. https://science-yhairblog.blogspot.com/2011/07/cationic-compounds-in-cosmetics.html
Zhang, Y., Alsop, R. J., Soomro, A., Yang, F.-C., & Rheinstädter, M. C. (2015). Effect of shampoo, conditioner and permanent waving on the molecular structure of human hair. PeerJ, 3, e1296. https://doi.org/10.7717/peerj.1296