The Fat Side of the Cream I: Emollients

[Versione italiana]

In this article series we will explore some of the oils and butters used as emollients in DIY cosmetic products. We will learn something about their chemical structure and physical properties, and we will try to understand the formulators’ approaches to the choice and combination of such emollients in creams. 

Definition of Emollients 

Emollients are materials added to cosmetic formulas to soften and improve the feel of the skin. 

They are typically hydrophobic ingredients: basically, they are what we add to the oil phase of emulsions, that is, oils, butters, and esters. 

In cosmetic industry, emollients include hydrocarbons like mineral oil and paraffin. Here, we will mainly discuss vegetable oils and butters and some esters.

Chemistry of emollients

Emollients are typically hydrophobic substances, that is, ingredients that do not dissolve in water. Some are more polar than others, meaning that in their chemical structure they have hydroxyl groups or other polar moieties that can interact with water, but this doesn’t make them water soluble. 

The emollients I’m going to describe in this articles can be divided into vegetable oils/butters (mainly consisting of triglycerides) and vegetable-derived esters. 

Vegetable oils and butters: triglycerides 

Vegetable fats (oils and butters) are typically derived from parts of plants such as seeds, nuts, cereal grains, or fruits. They typically consist of mixtures of triglycerides, diglycerides, free fatty acids, and other components (1). What interests us the most is the triglyceride composition, which gives the oils most of their characteristics. 

Triglycerides are chemical compounds consisting of a molecule of glycerine attached to three fatty acids (2, 3). 

Fatty acids are long hydrocarbon chains (chains made of only carbon and hydrogen) that terminate with a carboxylic acid. The carboxylic acid is attached to the hydroxyl groups (OH) of glycerine in triglycerides. 

What differentiates triglyceride molecules is the type and combination of fatty acids. Indeed, fatty acids can differ for hydrocarbon chain length and for the presence or absence of double bonds between carbon molecules within the chain. 

  • Double bonds: some fatty acids have no double bonds (also called “unsaturations”) in their chain; they are called saturated fatty acids. Examples are stearic, palmitic, lauric and myristic acid. Some fatty acids have one double bond somewhere in the carbon chain and are called monounsaturated fatty acids. An example is oleic acid. Lastly, some fatty acids have more than one double bond and are called polyunsaturated fatty acids (PUFAs). An example is linoleic acid. The position of the unsaturation is specified by a number, that indicates where the unsaturation takes place with respect to the last carbon of the chain (the omega carbon). 
  • Chain length: fatty acids can have longer or shorter chains. For example, lauric and myristic acids have shorter chains than oleic acid.

The nomenclature of fatty acids takes into account both chain length and double bonds. We typically find fatty acids described with two numbers, with the first representing the number of carbons building the chain and the second one indicating the number of unsaturations of the chains. 

For example, stearic acid has 18 carbon atoms in the chain and no double bond: it is a 18:0 acid. Oleic acid has 18 carbon atoms in the chain and one double bond at carbon omega-9 (the 9th carbon from the end): it is an omega-9, 18:1 acid. 

The composition of vegetable oils can vary a lot in terms of fatty acids identitzìy. You can find some detailed compositions in references (4) and (5). Oils like linseed, safflower, grape, hemp, sunflower seed and wheat germ oils have a high amount of PUFAs, whereas rice bran, almond, peanut, rapeseed and olive oils have more monounsaturated acids. Coconut oil, palm oil and cocoa butter consists mainly of saturated acids. Shea butter also has a high quota of a saturated fatty acids (stearic acid), but also has a high percentage of a monounsaturated one (oleic acid). 

While the composition of the unsaturated-rich oils mentioned above is similar in types of fatty acids – although they vary in percentage – there are some vegetable oils that we use in cosmetics that are a bit different. Castor oil consists mostly of ricinoleic acid, a hydroxylated acid that is capable of hydrogen bond interaction with other ricinoleic acid molecules: this is at the core of the high viscosity of castor oil. Jojoba oil is also a different oil compared to the mentioned ones: it is actually a liquid wax, as it mainly consists of wax esters (but it is liquid at room temperature) (6).

Esters

Chemically speaking, esters are the product of reaction between a carboxylic acid and an alcohol. So, technically triglycerides would be esters, too. 

Anyway, many esters are used in cosmetics as emollients and many of them are vegetable-derived. They consist of a fatty acid chain that has reacted with an alcohol. The alcohol can have a short or long chain, too. Given the vaste choice among acids and alcohols that can be used to make esters, this group is very versatile and offers a wide range of tunable properties in terms of melting point, viscosity and consequent skin feel characteristics (7). 

Here are some esters you could have heard of:

  • Tocopheryl acetate
  • Ethylhexyl stearate
  • Isopropyl myristate
  • Caprylyl Caprylate/Caprate
  • Cetyl ricinoleate
  • Cetyl palmitate

Typically, these names made of two names with one ending with -ate indicate that the molecule is an ester. 

Other types of emollients mentioned in these articles

At the beginning, I said I would only speak about vegetable oils/butters and esters. It wasn’t true. There are some ingredients that are typically included in the “emollient” group and that are very common among hobbyist formulators, therefore I will include them as well in my lists. 

  • Caprylic-Capric Triglycerides: this triglyceride molecule is synthetic. It is made starting from caprylic and capric acids (from coconut oil). 
  • Alpha-tocopherol: alpha-tocopherol is not an oil. It is the prevalent form of vitamin E and it is present in several vegetable oils, from which it can be extracted. When purified, it consists of a very viscous fluid. It has antioxidant properties and can be used to limit oxidation of oils in a formula.
  • Dicaprylyl ether: I see this ingredient listed as “ester” sometimes, but actually it is an ether. Ethers are formed between two alcohols, they don’t have a carboxylic group. Dicaprylyl ether (or Dioctyl ether) is the ether formed by two molecules of capryl alcohol (or octanol).
  • Squalane: squalane is a hydrocarbon. It is the saturated version of squalene, a naturally occurring hydrocarbon. Squalene is very susceptible of oxidation, whereas squalane has a very good stability.

References

[Next article: Physical properties of emollients and impact on the final product]

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