How Does A Shampoo Work?

Tanmay Ketkar

HAIR. We spend a considerable amount of money on haircare. Nowadays, haircare products are often likened to the plethora of potions in Harry Potter, primarily due to the names and number of ingredients that are added. The cosmetic market is highly competitive and haircare, even more so! Huge, expensive advertising campaigns and celebrity endorsements characterize this sector. 

Variety of shampoos catering to a specific customer base are available nowadays 

To get things into perspective, let us look at the size of the ever-growing Indian haircare market. According to a 2018 report by Nielsen, the Indian haircare industry is valued at around INR 22,500 crore (US$ 3.3bn). Haircare associated businesses have seen an annual growth of just under 10% over the past few years. Maintaining and hair-grooming has become a daily routine for most of us, and the market is flooded with a range of products promising healthy, beautiful, and flowing hair. So, how do some of our favorite shampoos work?

Origins of shampoo

Etymologically, the word shampoo originates from the Hindi word champ?, meaning to press and massage hair and skin. Traditionally, soaps were used to clean both hair and skin, but they suffer from poor lathering capabilities, hair damage, and are no longer recommended. Shampoos as we know them today, containing synthetic surfactants, were first introduced less than a century ago. 

What is hair made of?

Our hair is made of a protein called keratin, and has three cross-sectional layers- the cuticle, cortex, and the medulla. The cuticle is the outermost layer, consisting of scaly projections that protect the hair surface whilst also providing shine. The layer beneath is the cortex, which contains the pigments which give the hair its characteristic color. The innermost layer is the medulla. 

Hair is covered in an oily coating called sebum, which is secreted by glands situated near the hair follicle. This coating absorbs and traps any dirt from the environment, and even our scalp flakes, and hence our hair needs to be frequently washed. Shampoos help clean our hair, as the oil being hydrophobic in nature, cannot be washed off by water. 

What are surfactants and how do they act?

Oily, or hydrophobic substances, can be made more water soluble by reducing surface tension between oil and water. That is how cleansing agents like soaps and detergents work. The most important molecule in a detergent is a surfactant. Surfactants are derived from fatty acids. Fatty acids occur naturally and are found in various plant and animal sources. The materials used most often to make the surfactants used in shampoos are extracted from coconut oil and soybean oil. 

A surfactant molecule consists of two parts- a hydrophilic (water-loving) polar head group and a hydrophobic (water-hating) non-polar tail group. The hydrophobic part is also called lipophilic (oil-loving). The polar group attracts other polar molecules, such as water, and repels non-polar molecules such as oils. On the contrary, the non-polar tail is attracted to oils and is repelled by polar molecules. The non-polar lipophilic site collects deposited oily dirt and the hydrophilic polar end binds to water. This creates an emulsion of the two. In this way, the surfactant reduces the interfacial surface tension, facilitating the washing mechanism, and removes the sebum from the hair shaft.

Surfactant molecules at an oil-water interface

There are various types of synthetic surfactants- like anionic, cationic, non-ionic, and amphoteric. Commonly used are sodium lauryl sulphate (SLS), sodium lauryl ether sulphate (SLES), and ammonium lauryl sulphate. 

Structure of sodium lauryl ether sulphate. Wikimedia Commons

The mechanism of working of each type depends on the presence (or absence) of charged polar group, and the nature of charge (positive or negative). Natural surfactants like saponins were used earlier, although their extent of action is now largely inadequate, in comparison. 

Why shampoo, and not soap?

Normally, hair has an acidic pH of about 4-5, at which the non-covalent interactions binding and maintaining the hair strand are strong. Soaps, having an alkaline pH of about 8.5, can disrupt some of these interactions by causing breakdown of the disulphide bonds. This leads to hair damage. Highly alkaline substances can have a detrimental effect on hair, by destroying these bonds. Prolonged use of soap on hair can decrease the number of non-covalent interactions, thereby increasing the amount of weakening and damage. As a result, most shampoos now employ anionic surfactants, which are not only minimally disruptive, but also offer efficient cleansing, lathering, and foaming capabilities.

What are the ingredients in a shampoo formulation and why are they added?

Water: Yes, the most abundant ingredient in shampoos is in fact, water! It makes up around 75-80% of the total formulation. Specially treated water, called deionized water, is used in shampoos. This water is made free from any dissolved ions or particles. 

Surfactants: The next most abundant ingredient and the driving force of the shampoo is the surfactant, which as we saw earlier, brings about the cleansing action. The dual hydrophilic-lipophilic nature of the surfactant allows it to bind to water and oily dirt particles from separate ends. When surfactant molecules line up in this manner, it forms a structure called micelle which gives the shampoo its cleansing ability.

Foaming agents: Also derived from fatty acids, they are added to enhance the foaming characteristics of the formulation like amount of foam, lathering, size of bubbles. Commonly used foaming agents are lauramide DEA and cocodiethanolamide (cocamide DEA).

Thickeners: They are added to increase the viscosity of the formulation and make the shampoo thicker. Examples include methylcellulose and even sodium chloride (common salt). 

Sequestering agents: Ethylenediaminetetraacetic acid (EDTA) is used as sequestering agent to chelate and remove calcium and magnesium ions from the formulation. This prevents itching and dulling of hair due to hard water. 

pH adjusters: The pH of a shampoo formulation needs to be approximately 5.5, which is pH of the scalp. Use of detergents causes alkalization of the hair due to the negative charge, and acids like citric acid or glycolic acid are added to balance and maintain the pH.

Preservatives: They are mainly added to prevent bacterial and microbial contamination and associated decomposition of the formulation. Sodium benzoate, disodium EDTA, 1,3-dimethylol-5,5-dimethyl (DMDM) hydantoin, methylisothiazolinone, etc. are some commonly used preservatives. 

Other additives: Perfumes, plant extracts, fragrance oils, colouring agents, etc. are added to give the formulation a distinct, pleasant aroma, and an aesthetically pleasing colour. 

Outlook and recent advances: The FMCG sector being very dynamic, manufacturers are developing newer technology depending on consumer demand. Nowadays, 2-in-1 shampoos providing both cleansing and conditioning, are rapidly becoming the norm. 

Latest advances include better conditioning products to prevent hair damage. According to various studies published in the Journal of Cosmetic Dermatology, use of silicones like dimethicone lead to improved conditioning and hair colour retention. Use of skin care ingredients in hair care products is another major development. For instance, collagen and peptides lead to improved moisturization and help strengthen the hair shaft. Recent research also shows that amino acids like lysine, tyrosine, and histidine, when added to shampoos, help repair hair damage by restoring the amino acid balance. Companies are heavily investing in R&D of polymers and silicones in shampoos and other cosmetics.