Introduction
In this blog we will be discussing the different types of surfactants, how and why they are used in cosmetic formulation. We will give some common examples and make differentiations between the different types of surfactants and how although many cosmetic ingredients are chemically surfactants we categorise them differently in cosmetic formulation. We will also be taking a look at the different types of charge surfactants have and why this is relevant when formulating cosmetics.
What are surfactants?
Surfactants, a contraction of the term surface-active agents, are amphiphilic (dual-natured) molecules because they possess both hydrophilic and hydrophobic groups. The general structure of a surfactant molecule is shown in the figure right. A surfactant molecule posses both a polar “water-
loving” head group attached to a non-polar “water-hating” (or “oil-loving”) tail. (University of Bristol, 2024).
Why are these used in cosmetics?
Surfactants function in cosmetic products as cleaning agents. Its molecular composition makes it effective in cleaning products such as shower gels and shampoos. The non polar or oil loving tail bonds to dirt, whilst the polar water loving tail allows it bond to water washing away the dirt resulting in clean hair or skin. Emulsifiers are also surfactants as they too need both water loving and oil loving properties in order for water and oil to bond together. But usually when you hear the term surfactants in cosmetic formulation it is referring to the group of cleaning agents. Whilst emulsifiers technically are also surfactants, in cosmetic formulation they fall under their own category of emulsifiers.
Solubilisers are also surfactants as they too have oil loving and water properties. Their hydrophilic (water loving) head is bigger than their, hydrophobic (oil loving) tail, for this reason they are used in formulations that require a small amount of oil to mix with a larger amount of water. Polysorbate 20 is an example of a solubiliser this is commonly used to create cosmetic products such as toners or hair mists. But similar to emulsifiers whilst they are also chemically surfactants in cosmetic formulation they fall under their own category of solubilisers.
Superfatting agents are another example of surfactants. They are used in cosmetic formulation for their suitability in foaming products and the moisturising properties they impart. Using straight lipids such as oils in wash off products are not desirable because they reduce foam and the cleaning ability of the product. For this reason superfatting agents are preferred, PEG 40 castor oil is an example of a commonly used superfatting agent in cosmetic formulation. Again whilst they are chemically surfactants in cosmetic formulation they fall under their own category of superfatting agents.
To summarise when we are talking about surfactants in cosmetic formulation we are exclusively referring to the group of materials used for their cleaning properties.
Here are some common examples of surfactants used in cosmetic formulation:
Decyl Glucoside
Cocomidopropryl Betaine
Caprylyl / Capryl Glucoside
Coco Glucoside
Sodium Laureth Sulfate
Sodium Lauryl Sulfate
How much surfactants should you use in cosmetic products?
Whenever you purchase a cosmetic ingredient it should come with a safety data sheet. On the safety data sheet the manufacturer stipulates recommended usage rates for the ingredients. Cross reference these with reports on Cosmetic Ingredient Review and SCCS opinions that will tell you regulatory safety limits for ingredients depending on the type of product being formulated and its intended use, you don’t need to read the entire report just skip to the conclusion and you should find what your looking for.
Surfactants and charge
Surfactants have three different types of charge this is important as this helps us determine the mildness of the surfactant.
Anionic surfactants: Anionic surfactants, the most common of which are the alkyl sulfates, are really the primary ingredient used in cleansing products. They are strongly positively charged surfactant ions. Examples include sodium lauryl sulfate and ammonium lauryl sulfate (ALS). They have really high irritancy profiles and are usually paired with cocomidopropyl betaine to boost the mildness of the finished product. Sometimes anionic surfactants are modified to make them less irritating. For example, ALS is commonly “ethoxylated” by reacting it with ethylene oxide to produce ammonium laureth sulfate. This additional chemical processing makes the final product significantly less irritating and slightly more water soluble.
Cationic surfactants: These are positively charged surfactant molecules. They are not used for cleansing formulas because they don’t clean, rinse, or foam as well, and they are more irritating—so they have a lot of drawbacks. They are also not compatible with anionics, so their positive benefits can't be obtained from formulations that also contain an anionic surfactant. That being said, cationics are great for conditioning. They are substantive during use and are the primary ingredients for rinse-off hair conditioners.
Non ionic surfactants: Nonionic surfactants are molecules that do not have a charge. When placed in a solution of water, the molecules do not dissociate as the previously mentioned surfactant molecules do. Salt also has no effect on whether these chargeless surfactants thicken or not. There are a variety of reasons to use non ionics in cosmetics. They are good foam enhancers (when used with anionics) and can reduce irritation. They also can thicken systems and provide a conditioning effect. Additionally, they are very good for solubilising fragrances and other natural oils in formulating. Gentle cleansers such as baby shampoos are based on non ionics, the most common of which is PEG-80 sorbitan laurate. Nonionic surfactants are also the primary surfactants used to create emulsions. The reason these surfactants aren't used as the primary cleansing surfactant in most formulas is that they don't foam nearly as well on their own and are significantly more expensive. Overall, non ionics do not work as well as anionics in shampoos.
Amphoteric surfactants: Amphoteric surfactants can have both a negative charge and a positive charge, depending on the pH. These materials are also referred to as zwitterionic materials, and they include ingredients such as cocamidopropyl betaine, cocoamphopropionate, and sodium lauraminopropionate. These three ingredients are probably the most commonly used amphoteric surfactants in cleansing products, particularly in shampoos. Amphoterics are used because they have good detergency and are less irritating than the anionics. They also can help thicken a formula and have a positive effect on foam, as they make the bubbles smaller and feel creamier. The main drawback to using them is that they are significantly more expensive and, on their own, don’t really foam well enough to produce a good shampoo. (Romanowski, 2015).
Conclusion
In this blog we have discussed what surfactants are, and even though many ingredients used in cosmetic formulation are surfactants such as emulsifiers, solubilisers and superfatting agents, we don’t refer to them as surfactants. We concluded that the term surfactant in cosmetic formulation refers to ingredients with cleaning abilities ones that we would find in shower gels or shampoos. We took a look at some common examples of surfactants in cosmetic formulation as well as discussing different types of charge and how this affects the performance, their compatibility and when they are commonly used. Hopefully this blog has helped demystify what surfactants are and provided clarity on the different types and how and why we use them in cosmetic formulation.
References
International Products Corporation. (2022). An Easy Guide to Understanding How Surfactants Work. Accessed [17/01/2024]. Available at: https://www.ipcol.com/blog/an-easy-guide-to-understanding-surfactants/#:~:text=The%20hydrophilic%20head%20of%20each,%2C%20nonionic%2C%20cationic%20or%20amphoteric.
Romanowski, Perry. (2015). An introduction to cosmetic technology. Accessed [15/01/2024]. Available at: https://www.aocs.org/stay-informed/inform-magazine/featured-articles/an-introduction-to-cosmetic-technology-april-2015?SSO=True#:~:text=There%20are%20anionic%20surfactants%2C%20which,surfactants%2C%20which%20are%20positively%20charged.
University of Bristol. (2024). What Are Surfactants. Date Accessed [15/01/2024]. Available at: https://www.bristol.ac.uk/chemistry/research/eastoe/what-are-surfactants/