Surfactants in cosmetic products: properties and uses

The base of a cosmetic product is water and oil. Well, it's true that if we are satisfied with that, we end up with two distinct phases because they do not mix. This is where the surfactants come into play: they allow to mix the aqueous phase (water) of the product, and the oily phase! With this, the product immediately has a better appearance. All cosmetic products of more or less homogeneous aspect contain surfactants: shampoos, creams, gels... In short, it is difficult to ignore them.

Mode of action of surfactants

Generally, inside a cosmetic product, there are two ingredient profiles: compounds that consider fat to be life, they are called lipophiles, and those that are a bit picky and prefer water,I named hydrophiles. As we know that you like complexity, know that lipophiles are also called hydrophobic and hydrophilic lipophobic, but the latter term is rarely used. Surfactants on the other hand do not get wet, they will like fat as much as water, they are called 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 looong, thin body. This amphiphilic character allows them to sit right between the water and oil interface, and lower what is called the existing free energy, the one responsible for the tensions between them. So the surfactants have their tails firmly planted in the oil while the hydrophilic heads stay nicely on the water side. By creating various bonds, they will decrease the tension between the two phases. For chemists, these are hydrogen and ionic bonds for the head, and hydrophobic and Van der Waals bonds for the tail. If the balance between water and oil is disturbed and the whole thing is shaken, droplets will form, just like in salad dressing. The surfactants then arrange themselves into small spheres to form these droplets, which are more commonly called micelles. When the droplets are finally formed, the surfactants are not finished with the job. They stabilize them by decreasing the pressure gradient at the interface, and creating electrostatic repulsions between them. And that's it! This is the principle behind your laundry detergent, for example: while the hydrophobic tails will cling to the grease stain, the hydrophilic heads will help it to come off.

Physicochemical properties of surfactants

The different categories of surfactants

  • 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 a 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

  • cationic surfactants (quaternary amonium): they carry a positive charge. Overall they are quite poorly supported by the skin. They allow the hair to be sheathed because they associate well with keratin. However, they are not very detergent or foaming and tend to weigh down the hair.

Examples: BTMS

  • amphoteric or zwitterionic surfactants (betaines, amino acid and imidazole derivatives): they are going to be either cationic or anionic depending on the pH of the medium in which they are found (handy!). They are rather well tolerated by the skin, and do not sting the eyes.

Examples: Cocamidopropyl Betaine, Babassu foam (Babassuamidopropyl betaine)

  • non-ionic surfactants (polyoxyethylenes, alkanolamides, oligopeptides): they carry no charge. They are the most expensive but the mildest! They have good detergent activity, they are good dispersants, but they hardly foam.

Examples: Alkylphenols ethoxylated, Alcohols ethoxylated, Decyl glucoside, Cetyl alcohol, Glutamates, Lauryl glucoside, Coco glucoside

Currently, amphoteric and non-ionic surfactants are the most widely used, either alone or in synergy. Despite their much higher price, they have a better biocompatibility. Moreover, they are always active, regardless of the pH! If several surfactants are usually combined together for better efficiency, anionic and cationic surfactants do not really get along, they form a complex and precipitate when combined.

Properties of surfactants

  • detergents: they facilitate the removal of impurities by unhooking them
  • foamers: they promote the dispersion of a gas in a liquid, and thus enable the formation of foam
  • wetting agents: they decrease the tension between a liquid and a solid. Therefore, they allow the product to be better spread on the skin.
  • dispersants: they allow the solubilization of substances by forming micelles.
  • emulsifiers: they decrease the tension between two liquids to help form 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

Although emulsifying surfactants are amphiphilic, the hydrophilic vs. lipophilic battle is not completely won. Each one can give of its person, one can thus have a hydrophilic pole more important than the lipophilic pole, and conversely! To measure this character, we use a method called hydrophilic-lipophilic balance (HLB). Each surfactant is noted on a scale ranging from 0 to 20 according to their solubility in water. From 0 to 10, we will have surfactants rather lipophilic (or hydrophobic), and from 10 to 20 surfactants rather hydrophilic. Thus, the higher the HLB, the more hydrophilic the surfactant. Thanks to this scale, it is also easier to classify surfactants:

  • 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

Recognizing surfactants in cosmetic products

Surfactants to avoid

For reasons of toxicity for the skin or the environment, some surfactants should be avoided. Their manufacturing process implies the use of toxic gases for the environment, or their use induces harmful effects on the health on the long term.

Sodium Lauryl Sulfate (SLS), for example, is so irritating that it has become a benchmark for skin tolerance tests! Together with Sodium Laureth Sulfate (SLES), they are both accused of penetrating the skin tissue to reach the organs. They are then difficult to metabolize and, more seriously, can interfere with the endocrine system. The PEGs, or polyethylene glycol, are polymers obtained by chemical process heavy for the environment, because they require the use of toxic gases. On the other hand, they are not biodegradable.

Surfactants known to be irritants

Ammonium Lauryl Sulfate

The mildest surfactants