The basis of a cosmetic product is water and oil. Well, it's true that if we're content with that, we end up with 2 very distinct phases since they don't mix. This is where surfactants come into play: they allow the aqueous phase (water) of the product to be mixed with the oily phase! With this, the product immediately looks better. All cosmetic products with a more or less homogeneous appearance therefore contain surfactants: shampoos, creams, gels… In short, it is difficult to ignore them.
This article was updated on 04/08/2023Mode of action of surfactants
Generally, within a cosmetic product, we have two ingredient profiles: compounds which consider that fat is life, they are called lipophiles, and those who are a little choosy and who prefer water, i named hydrophiles. As we know that you like complexity, know that lipophiles are also described as hydrophobic and hydrophiles as lipophobic, but the latter term is rarely used. Surfactants do not get wet, they will like fat as much as water, we call them amphiphiles. They thus have 2 polarities: the lipophilic part is apolar (with a neutral electric charge) while the hydrophilic part is polar (with an electric charge).
Schematically, the hydrophilic part is represented by a spherical head and the hydrophobic part by a very thin body. This amphiphilic character allows them to be placed right between the water and oil interface., and to lower what we call theexisting free energy, the one who is responsible for the tensions between them. The surfactants therefore have their tail firmly planted in the oil while the hydrophilic heads remain nicely on the water side. By creating various connections, they will reduce the voltage between the two phases. For chemists, these are hydrogen and ionic type bonds for the head, and hydrophobic and Van der Waals type bonds for the tail. If you disrupt the balance between water and oil and shake everything, droplets will form, just like in vinaigrette. The surfactants are then arranged in small spheres to form these droplets, which are more commonly called micelles. When the droplets are finally formed, the surfactants haven't finished the job. They stabilize them by reducing the pressure gradient at the interface, and by creating electrostatic repulsions between them. And There you go ! It is on this principle that your laundry is based, for example: while the hydrophobic tails will cling to the grease stain, the hydrophilic heads will promote detachment.
Physico-chemical properties of surfactants
The different categories of surfactants
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anionic surfactants (carboxylic acid salts, lipoamino acids, lipo-oligopeptides, sulfonated derivatives and sulfated derivatives): they carry a negative charge. These are the most common, they are inexpensive and have good detergent and foaming activity. However, they are known to be drying! They are found in particular in cleaning products.
Examples: Sodium coco sulfate (SCS), Sodium cocoyl isethionate (SCI), sulfated castor oil, Sodium lauryl sulfoacetate (SLSA), Sodium Lauroyl Sarcosinate
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cationic surfactants (quaternary amonium): they carry a positive charge. Overall, they are relatively poorly supported by the skin. They help to coat the hair because they combine well with keratin. They are, however, low in detergent and low foaming and tend to weigh down the hair.
Examples: BTMS
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amphoteric or zwitterionic surfactants (betaines, amino acid and imidazole derivatives): they will be either cationic or anionic depending on the pH of the environment in which they are found (practical!). They are rather well tolerated by the skin, and do not sting the eyes.
Examples: Cocamidopropyl Betaïn, Babassu foam (Babassuamidopropyl betaine)
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nonionic surfactants (polyoxyethylenes, alkanolamides, oligopeptides): they carry no charge. These are the most expensive but the sweetest! They have good detergent activity, they are good dispersants, but they hardly foam.
Examples: Ethoxylated alkylphenols, Ethoxylated alcohols, Decyl glucoside, Cetyl alcohol, glutamates, Lauryl glucoside, Coco glucoside
Currently, amphoteric and nonionic surfactants are the most used, alone or in synergy. Despite their much higher price, they have better biocompatibility. Plus, they are always active, regardless of the pH! If we generally combine several surfactants together for better effectiveness, anionic and cationic surfactants do not really get along, they form a complex and precipitate when combined.
Properties of surfactants
- detergents : they facilitate the elimination of impurities by removing them
- foaming : they promote the dispersion of a gas in a liquid, and thus allow foam to form
- wetting : they reduce the tension between a liquid and a solid. They therefore allow the product to be spread better on the skin.
- dispersants : they allow the solubilization of substances by forming micelles.
- emulsifiers : they reduce the tension between two liquids to promote the formation of a homogeneous mixture
- stabilizers : they stabilize the interface between the droplets of the dispersed phase and the dispersing phase, or they limit the meeting of the droplets by creating a repulsion between them.
The hydrophilic-lipophilic balance of emulsifiers
- 0 to 3: anti-foam
- 3 to 6: water-in-oil emulsifier
- 7 to 9: wetting
- 8 to 16: oil-in-water emulsifier
- 13 to 15: detergent
- 15 to 18: solubilizer
Recognize surfactants in cosmetic products
Surfactants to avoid
For reasons of toxicity for the skin or for the environment, certain surfactants should be avoided. Their manufacturing process involves the use of gases that are toxic to the environment, or their use causes harmful effects on long-term health.
Sodium Lauryl Sulfate (SLS), for example, is so irritating that it has become a reference for skin tolerance tests! Along with Sodium Laureth Sulfate (SLES), they are both accused of penetrating skin tissue to reach organs. They are then difficult to metabolize and, more seriously, can interfere with the endocrine system. PEGs, or polyethylene glycol, are polymers obtained by a heavy chemical process for the environment, because they require the use of toxic gases. On the other hand, they are non-biodegradable.
Surfactants known to be irritating
Cocamidopropyl Betaine
Cocamidopropyl hydroxysultaine
Disodium Cocoamphodiacetate
Disodium laureth sulfosuccinate
Disodium lauryl sulfosuccinate
Lauryl ether sulfate
Sodium cetearyl sulfate
Sodium coco sulfate (scs)
Sodium Dodecyl Sulfate
Sodium lauryl sulfoacetate (slsa)
Sodium lauroyl sarcosinate
Sodium laureth sulfate (sles)
Sodium myreth sulfate
The gentlest surfactants
Coco Glucoside: natural origin
Decyl Glucoside: natural origin
Disodium Cocoyl Glutamate: natural origin
Disodium coco-glucoside citrate
Glyceryl oleate
Laurdimonium hydroxypropyl hydrolyzed wheat protein
Lauryl Glucoside: natural origin
Babassu mousse: natural origin
Sodium Cocoamphoacetate: synthetic origin
Sodium cocoyl glutamate
Sodium cocoyl hydrolyzed wheat protein
Sodium cocoyl isethionate (sci)
Sodium cocoyl hydrolyzed wheat protein glutamate
Sodium Lauroyl Lactylate: natural origin
Sodium lauryl glucose carboxylate & lauryl glucoside
Sodium Lauroamphoacetate
Sodium lauroyl glutamate
Sodium Lauroamphoacetate
Sodium lauryl glucoside
Sodium lauroyl oat amino acids
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