Chemistry class: What is a fatty acid?


Versatile fatty acids are the cornerstone of making soap and other cosmetics, and are lauded for their skin care properties, as John Woodruff, founder, Creative Cosmetic Developments, explains

While we are familiar with acid products used around our households, from our kitchens to our garages, consumers and even many in the cosmetics industry who aren't scientists are less familiar with the fatty acids that are the cornerstone of so many cosmetic ingredients and end products.

In this article, John Woodruff, formulator and founder of Creative Cosmetic Developments, discusses the different kinds of fatty acids and their cosmetic and toiletry uses.

We are familiar with mineral acids, such sulphuric acid found in car batteries and organic acids, like citric acid found in lemons and acetic acid found in vinegar, but what are the fatty acids frequently found in personal care products?

In their simplest form fatty acids consist of a methyl group (CH3) followed by a straight hydrocarbon chain (CH2) of an even number of carbon atoms, with hydrogen atoms along the length of the chain and a carboxyl group (―COOH) at the end. It is the carboxyl group that makes it an acid and the length of the chain that differentiates one acid from another.

Thus, caproic acid comprises six hydrocarbon molecules and is an oily liquid, but capric acid has ten carbon atoms and is a crystalline solid. Their IUPAC names reflect the number of carbon atoms, so caproic acid is hexanoic acid and capric is decanoic acid. However, their INCI names are generally the common name and so will be used here.

These fatty acids are saturated as they do not have double bonds and those most common in cosmetics are lauric acid (C12), myristic (C14), palmitic acid (C16) and stearic acid (C18). They have increasing molecular weights and melting points. If neutralised using an alkali, soap formed is of increasing hardness. Thus, lauric and myristic acids may be used for a shave cream and palmitic and stearic acids to form bar soaps. The alkali used also has an effect, with potassium hydroxide yielding a much softer soap than sodium hydroxide. Fatty acids are also called carboxylic acids, and their salts and esters may be called carboxylates.

For many years, the most common oil-in-water emulsifier system found in cosmetics was stearic acid neutralised with triethanolamine. If not fully neutralised the stearic acid would form a soft cream with a lustrous pearliness, exemplified by traditional vanishing cream.

Today saturated fatty acids are used as consistency modifiers to give body and substance to emulsions and as co-emulsifiers. It is possible to purchase pure versions, but often a small amount of another fatty acid doesn’t make a difference to the product and can help costs.

Fatty acids are not found in a free state in nature; commonly they exist in combination with glycerol in the form of a triglyceride. Saturated fatty acids are found in animal fats and are the result of conversion of fats into the acid plus glycerin, which is a source of energy for the animal, or human. Palmitic acid makes up as much as 30% of body fat and from 5-50% of lipids in vegetable fats, being especially abundant in palm oil.

If there is a single double bond in the carbon chain the acid is termed monosaturated and the first example is oleic acid, which has a carbon chain length of 18, but is liquid at room temperature. The presence of double bonds lowers melting points and renders the acid more reactive and less resistant to oxidation.

Unsaturated fatty acids are commonly of plant origin, and it is thought the Mediterranean diet is healthier than North European diets because of increased consumption of fruit and vegetables and the ubiquitous use of olive oil, which contains unsaturated fatty acids that can improve blood cholesterol levels. Common sources of unsaturated fatty acids are canola oil, avocados, peanuts, nuts and seeds, and these are often found in cosmetic products because of the trend for natural ingredients. Such products need to be protected against oxidation and going rancid.

Polyunsaturated fats are found in high concentrations in sunflower, corn, soybean, and flaxseed oils, walnuts and fish, and these provide polyunsaturated fatty acids such as linoleic acid and alpha-linolenic acid. Both have 18 carbon atoms, but linoleic acid has two double bonds whereas alpha-linolenic has three. linoleic acid is an omega-6 fatty acid and alpha-linolenic acid is an omega-3 fatty acid. They cannot be formed within the human body and must be introduced via our diet; thus, they are termed essential fatty acids.

Polyunsaturated fatty acids are added to cosmetics in the belief that they improve dry, seborrheic skin with disturbed lipid barrier and for dry scalps with brittle and dull hair, but they are prone to oxidation and must be protected with an antioxidant.

Although fatty acids are used in their free state in cosmetics, they are more useful for the formation of esters. Fatty acid esters are the result of a reaction between the acid and an alkyl alcohol. The first to be extensively used in cosmetics were isopropyl myristate and isopropyl palmitate. They are colourless and odourless hydrophobic liquids added as an alternative to hydrocarbon oils because they are much lighter and less greasy. Now there are many esters available. Esters of straight-chain fatty acids and lower alcohols are effective for dissolving oil-soluble ingredients to provide a light touch during application. Diesters such as diisopropyl adipate have a higher polarity and are effective as solubilisers for oil-soluble UV absorbers. Esters of higher alcohols such as myristyl myristate and cetyl palmitate are waxy solids and are added to cosmetics as consistency regulators and emulsion stabilisers.

In response to the natural trend, it may be that individual fatty acids are not separated but used as a composite mixture to form salts, thus potassium cocoate is the potassium salt of coconut oil. Esters can be quite complicated: sucrose tetrastearate triacetate is based on sucrose and vegetable fatty acids, and can be used as a lipidic phase modifier to influence the rheological and sensorial properties of oils and natural butters. PEG-n glyceryl cocoate, where n is the number of molecules of polyethylene glycol in the substance, are a relatively new class of water-soluble fatty acid esters used as emulsifiers and super-fatting aids. The more ethoxylated the material the more water-soluble it becomes.

Thus, fatty acids and their many derivatives are an essential cosmetic ingredient.