Shampoo, shower gel, facial cleanser... why do cleansing products foam?

As water alone is insufficient to remove the oily dirt accumulated from daily life on the scalp and skin, cleansing products, regardless of their formulation, contain washing agents known as surfactants. These amphiphilic molecules, responsible for cleaning, are polarised with a hydrophilic head that has an affinity with water and a lipophilic tail that has an affinity with fats. Thus, they can attach to both water and fats, effectively removing dirt from the hair or skin during rinsing.

They are also the ones that create the foam. During hair washing or skin cleansing, as you massage your scalp or rub your skin, small air bubbles form, a result of an emulsion between water, dirt, and surfactants. Indeed, these bubbles form when air molecules are agitated and come into contact with the diluted product. Without external action, these air bubbles are unstable and burst quickly. However, with the action of surfactants, the air bubbles are encapsulated and stabilised, creating abundant foam.

"The more it foams, the more it cleans."

We often associate abundant foam with high performance/efficiency, especially when we observe that a conventional shampoo foams more when the hair is clean. However, the amount of foam produced has no correlation with its propensity to cleanse, even though in absolute terms, the more a product foams, the more it cleanses.

Indeed, it is not the case that a shampoo's effectiveness is determined by the amount of foam it produces. The foam is a result of the presence of surfactants. It is not a cleansing agent and does not serve as proof of effectiveness. Consequently, it is possible to have an effective shampoo that produces little foam, or conversely, a shampoo that produces a lot of foam but cleanses less effectively.

The volume of foam produced depends on the type of surfactant used. These are typically sulphated surfactants (e.g. sodium laureth sulfate, sodium lauryl sulfate, ammonium lauryl sulfate, etc.) that have this property of producing a generous foam. However, the more generous the foam, the greater the risk of skin drying out. Indeed, they have a tendency to weaken the skin's protective hydrolipidic film, making it more reactive and irritable.

On the other hand, criticisms are levelled at "natural" shampoos, those without sulfates, which use non-ionic or amphoteric surfactants derived from plant extracts (e.g. coco-glucoside, decyl glucoside, lauryl glucoside, sodium cocoyl glutamate, disodium cocoyl glutamate, sodium lauroyl glutamate, etc.), for producing less foam. However, these cleansing agents are gentler on the skin.

Nevertheless, foam is not without its uses. It plays a role in the practicality of cleansing products. It facilitates the distribution of the product on the skin, the scalp, between hair fibres and along strands, ensuring a thorough cleanse and avoiding excessive rubbing. This is an indicator that can be particularly useful for thick hair or hair with high density. Moreover, it also helps to use a reduced amount of product. To put it simply, a voluminous foam actually indicates that there is nothing left to clean, that there are no more impurities to remove.

Note : A shower gel or a facial cleanser will contain between 10 and 12% of surfactants in its formula, whereas a shampoo will have between 15 - 20% of surfactants, hence the fact that a shampoo will foam more.