What is the purpose of surfactants?

Also known as surfactants, tensioactives are amphiphilic organic or organometallic compounds. From a structural perspective, they possess a polar head attached to a long non-polar tail. This characteristic gives tensioactives useful hydrophobic and hydrophilic functionalities in cosmetic formulation. From a chemical standpoint, these molecules work by lowering the surface tension of the solvent. To do this, tensioactives aggregate at the interfaces between water and substances insoluble in water, forming micelles. Their hydrophilic part allows them to bind to water molecules while they can bind to fats via their hydrophobic part.

A surfactant is a molecule that enables the binding of two liquids that do not usually mix, much like water and oil.

An emulsion is termed 'normal' when the oily phase is dispersed in the form of droplets surrounded by the micelles of surfactants, whereas it is referred to as 'inverse' when it is the aqueous phase that forms these droplets, encapsulated within the heart of the micelles. The 'normal' emulsion is the most common in cosmetics. There are four categories of surfactants:

• Anionic surfactants : When dissolved in water, these surfactants produce a negatively charged active surface group. Anionic surfactants were the first to be used in cosmetics and still make up the majority of surface agents today. They can act as foaming agents, antistatic agents, dispersants, detergents, emulsifiers or even stabilisers in formulations. Sodium lauryl sulfate (SLS) and ammonium lauryl sulfate (ALS) are the most commonly used anionic surfactants in cleaning products. However, they are criticised for their irritating effects.

• Cationic surfactants : Cationic surfactants form positively charged micelles. They are not used in cleansing formulas but they make good conditioners and are frequently found in hair products. A unique feature of cationic surfactants: they are not rinsed off by water. Indeed, as the skin is negatively charged, when a formulation with a cationic agent is applied to its surface, the two form an ionic bond. Cetrimonium bromide (CTAB) and benzalkonium chloride are among the most commonly used cationic surfactants.

• Non-ionic surfactants : This type of surfactant carries no charge. They are often used in conjunction with anionic surfactants as they reduce the irritation associated with the latter. Non-ionic surfactants can also thicken formulations. The Tween series, fatty alcohols, and PEGs are the most common non-ionic surfactants.

• Zwitterionic or amphoteric surfactants : These particular surfactants can carry either a positive or negative charge, depending on the pH of the environment in which they are dissolved. They are primarily used in shampoos, conditioners, and cleansing products and are often less irritating than anionic surfactants. Examples of surfactants in this category include cocamidopropyl betaine and babassuamidopropyl betaine.

In a cosmetic formula, surfactants can provide different effects depending on their type, concentration, and hydrophilic-lipophilic balance (HLB). This scale, ranging from 0 to 20, assesses their water solubility, that is, the significance of their hydrophilic pole compared to their hydrophobic pole. Depending on their HLB, surfactants can be classified as follows:

Thanks to their unique physicochemical properties and diversity, surfactants can perform numerous functions in cosmetics.

• Surfactants stabilise cosmetics.

  One of the primary roles of surfactants is to enable the mixing of two normally immiscible liquids, such as water and oil, in order to achieve a stable emulsion, which is essential in cosmetics. The emulsifying properties of surfactants stem from their amphiphilic structure, allowing them to bind to both water and oily textures and keep them together.

• Surfactants can help to extend the lifespan of a product.

  Emulsion stabilisers play a key role in the longevity and stability of a cosmetic product by preventing phase separation. They can act in two ways: by strengthening the interface between the droplets of the dispersed phase and the dispersing phase, which makes the breakdown of the emulsion more difficult, and by inducing an electrostatic repulsion between the droplets, thus preventing their coalescence. This allows oil or water droplets to remain separate and suspended in the mixture, thereby ensuring a better shelf life for cosmetics.

• Surfactants can facilitate the removal of impurities.

  The amphiphilic configuration of surfactants allows them to capture impurities present on the skin's surface, such as sebum, dust or makeup residues, which are often hydrophobic. By binding to these impurities with their lipophilic end and to water with their hydrophilic end, detergents enable their dissolution in water and removal during rinsing. This process relies on the formation of micelles, spherical structures where impurities are trapped in their hydrophobic centre.

• Surfactants can form foam.

  Foaming agents are a subgroup of surfactants responsible for the formation of foam when the product is agitated or applied to the skin. Their primary function is to encapsulate a gas, usually air, within a liquid, to create stable bubbles. More specifically, foaming agents stabilise the foam and slow down its collapse. The formation of foam from a cleansing product is often perceived as a sign of effective cleaning, although its presence is not directly linked to the efficiency of the detergent.

• Surfactants enable the solubilisation of substances by forming micelles.

  Dispersing agents are also used to ensure a uniform distribution of insoluble particles in a liquid. For instance, they can help to keep pigments and sunscreens suspended and prevent them from settling at the bottom of the product. This property of certain surfactants again relies on the formation of micelles capable of trapping lipophilic or hydrophilic substances at their centre.

• Surfactants facilitate the spreading of cosmetic products.

  By reducing the surface tension between cosmetics and the skin or hair, wetting agents allow products to spread more effectively over the skin or hair surface. This not only facilitates their application but also improves their distribution and, indirectly, their effectiveness.

Which surfactants can be found in Typology skincare products?

At Typology, as a precautionary principle, we exclude potentially irritating surfactants, such as the sodium lauryl sulfate. We preferentially use caprylyl/capryl glucoside, coco betaine, coco-glucoside, decyl glucoside, sodium lauroyl sarcosinate and lauryl glucoside, which are gentle surfactants for the skin and scalp and possess the various properties mentioned above.

• PRADA-RPDRIGUEZ D. & al. Surfactants in Cosmetics: Regulatory Aspects and Analytical Methods. Analysis of Cosmetic Products (2018).

• KUMAR S. & al. Use of surfactants in cosmetics and cleansers: a review. International Journal of Multidisciplinary Educational Research (2013).

• BAKI G. Introduction to Cosmetic Formulation and Technology. Wiley (2023).