Washing Away

Patrick Brown/Year 12 Chemistry 2010

s a consumer we expect the shampoo we buy to have specific properties; those of which clean our hair, make it

smell pleasurable, protect our hair and be cost effective. Chemically, this is achieved through the action of surfactants on your hair follicles.

AShampoos are essentially a form of detergent which aims to clean your hair instead of washing your dishes or cleaning your clothes. Detergents are very versatile chemicals. As the compounds which are combined with the detergents change, so does the purpose of the detergent. These primary compounds are called surfactants.

“Surfactant: Wetting agent: a chemical agent capable of reducing the surface tension of a

liquid in which it is dissolved”wordnetweb.princeton.edu/perl/webwn

Water and oil are immiscible substances. This is due to their difference in polarity. Water is polar because there is unequal sharing of electrons between the hydrogen and the electronegative oxygen. This results in a slight positive charge on the hydrogens and a slight negative charge on the oxygen in the bent-

shaped molecule. This makes water a "polar" solvent. Therefore, only hydrophilic (water-loving) substances are appreciably miscible in water. Oils, on the other hand, are made of hydrocarbon chains. The elements and structure of oils make them non-polar, or hydrophobic. Only polar solutes dissolve in polar solvents and vice versa, this is why water and oil don’t mix.

The reduction of surface tension achieved by the surfactant makes the water ‘wetter’ and hence easier to separate in terms of dirt and grime. Surfactants have dual affinity: they are both hydrophobic and hydrophilic. A surfactant molecule consists of a hydrophobic tail group, which links to greasy soil, and a hydrophilic and polar head group, which renders the molecule water-soluble; this arrangement helps to disperse and rinse away greasy soil. A water molecule, (being polar because of the unequal sharing of its electrons in its "bent" structure) binds to the polar head of the surfactant. The non-polar hydrocarbon tail of the surfactant binds to the non-polar hydrocarbon chains found in dirt and grime hence lowering the surface tension. Chemically, this is why the tail is hydrophobic.

Surfactants are classified according to the nature of the hydrophilic head. There are three main classes: Anionic e.g. Sodium Lauryl Sulfate, Cationic e.g. Trimethylhexa-

decylammonium Chloride and Nonionic e.g. -Pentaerythrityl Palmitate.

Primarily, anionic surfactants are used in shampoo’s due to their affinity for negatively charged oil and grease particles. Today, the most common anionic surfactants used are alkyl sulphates e.g. Ammonium Lauryl Sulfate, (CH3(CH2)10CH2OSO3NH4) and alkyl ether sulfates e.g. CH3(CH2)10CH2(OCH2CH2)nOSO3NaC11+nH23+4nNaO4+nS

These detergents work best in soft water that contains little calcium carbonate and magnesium, as these elements bind to the detergent and make an insoluble scum.

In the process of washing, (i.e. a reaction with H20) the lauryl sulfate anions and the ammonium cations separate. They continue to align themselves into what is known as a micelle, in which the ions form a sphere, with the polar heads (the sulfate) on the surface of the sphere and the non-polar hydrophobic tails pointing inwards towards the center. The water molecules around the micelle arrange themselves around the polar heads, but this disrupts their hydrogen bonding with the water surrounding them. The overall effect of having these micelles in an aqueous environment is that the water is given a greater ability to penetrate things like cloth fibres or hair, and also becomes more readily available to dissolve anything coming off the substrate.

The Lauryl Sulfate micelle

Cocamide DEA (Coconut fatty acid monoethanolamide) is used as a foaming agent, to make the lather. The other surfactants will generate a certain amount of suds, but this foaming agent is added to get the amount just right. Besides its foam stabilizing effects, it is also a viscosity booster i.e. it makes the shampoo clearer and seem more ‘pure’ which is an effective marketing strategy.

The detergent cocamidopropyl betaine is added for several of its special properties. It is milder on the skin than the benzine sulfonates (C18H29NaO3S), so adding it to the mix reduces the amount of the harsher detergents needed. Cocamidopropyl betaine is thicker than the other ingredients, so it can be added to make the mix have the right viscosity. It has anti-static properties, so the hair doesn't generate an electric charge and jump to the plastic combs and brushes used when drying the hair. It is a humectant, attracting moisture from the air, thus keeping hair from drying out. Lastly, cocamidopropyl betaine has antibiotic properties that can prevent spoiling of the shampoo (Science Toys, 2003). The surfactant ammonium xylenesulfonate is also added to bind with other preservatives like methylchloroisothiazolinone (C4H4ClNOS) which are combined with the shampoo for marketing purposes to produce aroma. Such chemical is called a hydrotope which is a compound which makes it easier for H20 to dissolve other molecules.

Ammonium xylenesulfonate

C8H13NO3S

In summary, a plain and simple formula for a shampoo is a combination of lauryl sulfate, lauryl ether sulfate, cocamidopropyl betaine and coconut fatty acid monoethanolamide. A desirable pH of 5–6 is achievable with the addition of citric acid. An optimum viscosity is usually obtainable by incorporating sodium chloride into the formula. A formulator will typically add preservatives, perfumes and dyes to complete the formula. To maintain all properties and marketing claims, some modifications with other additives may be necessary.

Only a few of the many surfactants and chemicals added to shampoo are in fact used for cleaning the hair. The power behind this cleansing action ultimately comes down to a molecule which overcomes the contrasting polarities of oil and water (Megan Junqwi, 2009). The hydrophobic tail of the surfactant molecules in Ammonium Lauryl Sulfate, (CH3(CH2)10CH2OSO3NH4) for example, will bind

to the grease and oil hydrocarbons. When H20 is applied, it binds to the anionic hydrophilic head and then the surfactant molecule is washed away.

However, because of the bi-polar properties of surfactants, the hydrophobic tail also binds with sebum on the hair follicle. The role of sebum is to protect the hair shaft and without it the hair follicle is unprotected from dirt hydrocarbons. Sebum is made up of 25% wax, 41% triglycerides (RCO2CH2CH(O2CR')CH2O2CR'' + 3 H2O), 16% free fatty acids (e.g. RCO2H, R'CO2H, R''CO2H) and 12% squalane (C30H62). A ‘good’ shampoo should be able to hydrolyse the already present triglycerides into free fatty acids and glycerol. The hydrolysis makes it easier for the surfactants to act upon the sebum and hence replace it. Thus, modern shampoos contain chemicals which replace the sebum; like dimethicone (C8H24O2Si3). The flexible chain structure of dimethicone lends itself to repairing sebum and providing hair strands with elasticity and shine. Such flexible chains become loosely entangled when molecular weight is high. Practically, these loose chains of dimethicone rap around the hair follicle.

Anionic surfactants are the most widely used surfactants in shampoos because of their ability to wash away grease and oil whilst maintaining sebum and follicle health however, harsh anionic surfactants such as Ammonium Lauryl Sulfate damage the scalp. People with sensitive skin may experience scalp and eye irritation. This is due to the strong hydrophilic nature of the anionic head. The head [in ammonium lauryl sulfate] has a strong affinity for H20 and other water based proteins. Therefore, it causes skin and eye irritation because it causes a ‘dry effect’.

Dimethicone – Sebum replacement and shaft protection.

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Essentially, if left on for too long, or used in high dosages, the surfactant will dry out the scalp and cause irritation. This is why world standard health and safety regulations recommend using shampoos with lower levels of Ammonium Lauryl Sulfate.

Leading on from safety regulations, they also state that the shampoo must be biodegradable. Because shampoos are predominantly a detergent during the production of detergents measures are made to ensure the biodegradation of the surfactants are safe to the environment.

“Biodegradability is the capability of material to decompose over time as a result of

biological activity, especially to be broken down by microorganisms.”

http://www.dolceta.eu/malta/Mod5/spip.php?rubrique73 (2007)

In detergents, biodegradability is the susceptibility of a detergent to be effected by the common processes involving bacterial action by which organic matter in waste water is normally decomposed. A pure chemical is either biodegradable or not. Also, the rate of breakdown varies among biodegradable pure chemicals. In mixtures of pure chemicals, the rate of degradation and completeness are both affected. Detergents can vary between primary and ultimate aerobic biodegradation.

Primary degradation involves microorganisms changing the structure of a surfactant in the detergent, thereby resulting in the loss of its surface-active [verb: Surfactant] properties due to the degradation of the parent substance and consequential loss of the surface-active property.

Ultimate aerobic biodegradation is achieved when the surfactant is totally used by

microorganisms in the presence of oxygen, resulting

in its breakdown to carbon

dioxide, water and mineral salts of any other element present.

Detergents which meet the criteria for ultimate aerobic biodegradation may be placed on the market without further limitations relating to biodegradability.

In conclusion, in order for a shampoo to be released into the marketplace, it must meet the needs of the consumer. These include a pleasant aroma, hair protection, have a nice feel and lather and be cost effective. In order for this to be achieved certain chemicals like Ammonium Lauryl Sulfate, Dimethicone and Methylchloroisothiazolinone need to be combined in the compound. It is important to remember that all brands of shampoo have different quantities and types of these chemicals.

Shampoos such as Garnier Fructis Fortifying Daily Care, Suave Daily Clarifying Shampoo, Nizoral and Johnson’s Baby Shampoo/Original are all low cost, quality shampoos voted by consumers on ‘Consumersearch.com’. A customer can expect to pay around $5-7 for one of these shampoos depending on the volume of each. A consumer can expect to pay more than $20 for salon and luxury shampoos such as Nexxus, Giovanni and Ojon which contain premium surfactants.

Manufacturing companies must remember that the modern consumer is discerning with their choice and any product with dangerous chemicals or ones which don’t biodegrade will not be bought. Thanks to the internet the consumer’s knowledge is growing rapidly and so is the retail industry therefore, in today’s society one must always be informed to make the best possible decision.