What are surfactants?

Tensides (surfactants, surface active substances) are one of most important groups of raw materials in skincare production. Their role in products is to perform a large number of different functions. What makes them especially interesting is the fact that they can be the active substance in the product, but they can also be excepients or added substances that make the product more acceptable for the consumer. Our dear associate Lucija Božičević answers the question – what are in fact surfactants?    

The term surfactant is short for surface active agent. Surfactants operate on a surface where two phases of a product touch each other, but do not mix. These phases are most often oil and water phases. As is well known, oil will never mix with water since oil (just as all similar raw materials) is a nonpolar and water is a polar substance. One must keep in mind that substances that are similar mix together, meaning that two polar substances will mix just like two nonpolar substances, but a polar and a nonpolar substance will never mix together on their own. That is where surfactants come in, since their structure includes both a polar (hydrophilic “likes water”) and nonpolar (hydrophobic “likes fat”) region. Due to such a structure surfactant molecules “line up” along the surface on which the two phases touch each other, reduce surface tension and enable the “mixing” of those two phases. But why have we placed the word mixing under quotation marks, you might be asking yourselves…

The answer is simple. It’s because there are some natural principles that we just cannot go against. We cannot magically mix oil and water phases that usually don’t mix. What really happens is that the surfactant’s molecules turn in such a way that their hydrophobic tails (most commonly longer straight chains of carbon atoms) point towards the oil phase and their hydrophobic heads (most commonly small polar groups) point towards the water phase. In most cosmetic products such an orientation of surfactant molecules creates micelles. Most simply put, micelles are pellets whose outer covering is the surfactant layer, the inside of the pellet contains the oil phase and the water phase is in its external surroundings. The situation can of course also be reversed, depending on the composition of the product. In the reverse case the hydrophilic regions are oriented towards the interior of the micelle that holds the water phase, while hydrophobic tails are turned towards the micelle’s surroundings which is the oil phase.

Emulsification is the most well-known but by no means the only role of surfactants which contributes to the appearance and a more pleasant experience of use of the product. Very similar to emulsification is also solubilisation. The process of solubilisation will eliminate the murkiness caused by the incomplete dissolution of a certain component and in that way will make the product more pleasant to the eye. Surfactants are also often used as substances that will improve the spreadability of the product since they act as wetting agents, that is, they reduce the angle between the skin/hair contact surfaces and the drops of product, thereby making the contact surfaces larger. A lesser known fact is that they are also used in shampoos and showering gels to create a pearl finish of the product. Such an appearance is created by those surfactants that are mixed into the product but their particles remain large enough to be able to reflect light. Since micelles very often have a charged structure due to the fact that the surfactants’ hydrophilic heads are positively or negatively charged, it is easy to manipulate the density of the product which contains them. If all micelles in the product have an identical charge (which they most often do) they will repel each other and the product will have a thinner consistency. By adding certain salts it is possible to neutralise that charge and the micelles will be drawn towards each other, making the consistency of the product denser.  Since their structure is similar to that of the phospholipid layer of a bacteria’s membrane, they can break through it and in that way ensure microbiological stability of the product.

In addition to the already mentioned roles which have an effect on the aesthetics of the product, it is not uncommon for surfactants to also have a functional role in the product. This is most often the case in shampoos and conditioners, makeup removal products and face cleansers. In that case surface active substances act as detergents. Most of the “dirt” that we want to remove from the surface of our bodies are made up of a lipid or fatty layer secreted by our sebaceous glands and solid particles from the environment that get attached to our skin or hair by weak attractive forces. The latter impurities will mostly be removed by water itself but the lipidic film has a tendency to linger on the skin and hair. In that case surfactants will, as already described, “surround” those lipidic structures and “capture” them into structures that water will bind onto itself from the outside and remove from your hair or skin. Apart from its cleansing properties, each surfactant is also described according to its foaming properties. Air bubbles trapped in the thin liquid film are in fact the foam’s main “ingredient” and surfactants operate on the edges of these two phases preventing the thin liquid film from “breaking up”.   

There are four main groups of surfactants and substances are classified into these groups according to their respective charges. Anionic surface active substances are negatively charged and are known as one of the best cleansers and emulsifiers. Sodium lauryl sulphate (SLS) is one of the representatives of this group and anionic surfactants bear the stigma of irritating components. They certainly do have the potential to irritate which is why use of sulphates in products today has been reduced to a minimum.

Cationic emulsifiers are opposite of anionic and have a positive charge. They are most often used as repairing and conditioning products (such as hair conditioners) because due to their positive charge they can bind to negatively charged protein structures and smooth hair down making it look healthy. They are mostly various nitrogen compounds and they are rarely used on the skin because they can be irritating.

Nonionic surfactants (carrying no charge) and amphoteric surfactants (which depending on the product’s pH can have a positive or negative charge) are most often used in products for sensitive skin or gentle cleansers since they possess the lowest, and one could even say negligible irritating potential. Nonionic surfactants are often characterised by a large number of polar –OH groups in long chain formations, which is why they are more often used as solubilising agents. However, they can also be used as cleansing agents in gentle products such as children shampoo or wash gels for sensitive skin. Amphoteric surfactants are often used as secondary detergents due to their good foaming properties, but in gentler products they assume the role of primary detergent since they have satisfactory cleansing properties.

Even though lately they have acquired the status of “bad” and “dangerous” raw materials in cosmetic products, surfactants are more than useful and if selected and used properly, they are harmless. Different surfactants are most often combined in a single product and each of their respective shares will depend on the consumer target group and the skin type for which the product is intended.

REFERENCES:

  1. Rieger MM et al. Surfactants in cosmetics. New York, Marcel Dekker Inc., 1997
  2. What are surfactants? A Formulators Guide. 2017, https://chemistscorner.com/what-are-surfactants-a-formulators-guide/?fbclid=IwAR3j962z0aFNB7JiysNuZcq_fzUk4HL4tnd_dzv1SQtwdqOcdpXEix2tJy0, accessed 16 July 2019
  3. Types of surfactants in cosmetics. 2017, https://chemistscorner.com/types-of-surfactants-in-cosmetics/?fbclid=IwAR3VdxQkL9oIibwMi49OGrLSIoPh7uIaPukdgfdDi9JLwkUHwyAf2sBCAss, accessed 17 July 2019
  4. 10 Things Surfactants Do In Cosmetics. 2017, https://chemistscorner.com/10-things-surfactants-do-in-cosmetics/?fbclid=IwAR3BTEnzlzetFCAUoMPAx5SLYPH9i_5ZDqSyszlPpzkQowhkt3wViA0wMgM, accessed 16 July 2019

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