Surfactants are one of the most important groups of raw materials in the production of preparative cosmetics. They are present in products performing a large number of different functions, and what is particularly interesting is the fact that they can be an active substance in the product, but also an auxiliary substance that makes the product more acceptable to the consumer.
Today we answer the question of everyone who wants to know more - what are surfactants anyway?
The word surfactant comes from the English word - surface active agent , which means a surface active substance. The surface on which the surfactants act is the surface on which the two immiscible phases of the composition touch. We are most often talking about the oil and water phase. As is known, oil will never mix with water, because oil (like all other similar raw materials) is non-polar, while water is a polar substance. It should be kept in mind that like mixes with like, i.e. two polar substances will mix, two non-polar substances will also mix, but polar and non-polar never independently. And this is exactly where surfactants come into play, which in their structure contain both a polar (hydrophilic, "water-loving") and non-polar (hydrophobic, "fat-loving") part. Due to this structure, the surfactant molecules are "arranged" along the surface where the two phases touch, reducing the surface tension and enabling the "mixing" of the two phases.
You may have just asked yourself, why is mixing put in quotes?
The answer is simple, because we cannot go against certain natural principles. Likewise, we cannot magically mix the oil and water phases that normally do not mix. What actually happens is that the surfactant molecules are oriented so that their hydrophobic tails (usually longer straight chains of carbon atoms) are oriented towards the oil phase, and their hydrophilic heads (usually small polar groups) towards the water phase. In most cosmetic products, this orientation of surfactant molecules creates micelles. In the simplest terms, micelles are spheres whose outer shell is a layer of surfactant, with an oil phase in the interior of the sphere, and a water phase in its outer environment. The situation can, of course, be reversed, depending on the composition of the product.
Emulsification is the most well-known, but it is not the only role of surfactants that contributes to the appearance and a more pleasant experience when using the product. Solubilization is very similar to emulsification. The solubilization process will remove the turbidity caused by the incomplete dissolution of a particular component and thus make a particular product more appealing to the eye. Surfactants are often used as substances that will improve the spreadability of the product because they act as "wetting agents", i.e. they reduce the angle between the contact surfaces of the skin/hair and the drops of the preparation and thus increase the contact surfaces. Less known is the fact that they are also used in shampoos and shower gels to give the product a pearl finish , i.e. a pearly reflection. Such an appearance is given by the surfactants that are mixed into the preparation, but their particles remain large enough to reflect light. Given that micelles are very often charge-carrying structures thanks to the fact that the hydrophilic heads of surfactants are positively or negatively charged, it is easy to manipulate the density of the preparation containing them. If all the micelles in the preparation are identically charged (which they usually are), they will "repel" each other and the preparation will be thinner. By adding certain salts, it is possible to neutralize this charge, and the micelles will come closer to each other, and the product will become denser. The role of surfactants as antimicrobial agents should not be neglected either. Given that their structure is similar to the phospholipid layer of bacterial membranes, they can destroy it and thus ensure the microbiological stability of the preparation.
In addition to the aforementioned roles that affect the aesthetics of the product, it is not uncommon for surfactants to play a functional role in the preparation. This is most often the case with shampoos and hair conditioners, shower gels, makeup remover products, and facial cleansing gels. Surfactants act as detergents in this case. Most of the "dirt" on our body that we want to remove consists of the lipid ("fatty") layer secreted by our sebaceous glands and solid particles from the environment that cling to our skin or hair with weak attractive forces. The latter mentioned impurities will mostly be removed by the water itself, but the lipid layer tends to stay on the skin or hair. In that case, the surfactants will "surround" these lipid structures in the manner already described and "trap" them into structures that water will bind from the outside and remove from the hair or skin. In addition to cleaning, each surfactant is described by its foaming properties. Air bubbles trapped in a thin layer of liquid are actually the main "ingredient" of the foam, and surfactants act at the boundary of these two phases, preventing the thin layer of liquid from "breaking".
There are four main groups of surfactants.
Anionic surfactants are negatively charged and are known as one of the best cleaners and emulsifiers. Sodium lauryl sulfate (SLS) is one of the representatives of the group, but anionic surfactants carry the stigma of irritating components. Irritating potential exists, which is why the use of sulfates in products today has been reduced to a minimum.
Cationic emulsifiers are the opposite of anionic ones, i.e. they have a positive charge. They are most often used as agents that nourish and regenerate (eg in hair conditioners) because due to their positive charge, they can bind to negatively charged protein structures and smooth the hair, which makes it look healthy. Most often, these are various nitrogen compounds, but they are rarely used on the skin because they can irritate.
Nonionic surfactants (have no charge) and amphoteric surfactants (depending on the pH of the preparation they can be positively or negatively charged) are most often used in products for sensitive skin or gentle facial cleansers because they have the smallest, and one might say negligible, irritating potential. Nonionic surfactants are often characterized by a large number of polar –OH groups attached to long chains, which is why they are more often used as solubilizers, but they can also be used as cleaning agents in gentle preparations such as children's shampoos or washing gels for sensitive skin. Amphoteric surfactants are often used as secondary detergents due to their good foaming properties, but in gentler preparations they assume the role of primary detergent because they have satisfactory cleaning properties.
Although they have recently received the status of "bad" and "dangerous" raw materials in cosmetic products, surfactants are more than useful, and with proper selection and use, they are also harmless. Combinations of different surfactants are most often used in products, and the share of each group will depend on the target group of consumers, i.e. the type of skin for which it is intended.
LITERATURE:
- Rieger MM et al. Surfactants in cosmetics. New York, Marcel Dekker Inc., 1997
- What are surfactants? A Formulators Guide. 2017, https://chemistscorner.com/what-are-surfactants-a-formulators-guide/?fbclid=IwAR3j962z0aFNB7JiysNuZcq_fzUk4HL4tnd_dzv1SQtwdqOcdpXEix2tJy0 , accessed 16 July 2019
- Types of surfactants in cosmetics. 2017, https://chemistscorner.com/types-of-surfactants-in-cosmetics/?fbclid=IwAR3VdxQkL9oIibwMi49OGrLSIoPh7uIaPukdgfdDi9JLwkUHwyAf2sBCAss , accessed 17 July 2019
- 10 Things Surfactants Do In Cosmetics. 2017, https://chemistscorner.com/10-things-surfactants-do-in-cosmetics/?fbclid=IwAR3BTEnzlzetFCAUoMPAx5SLYPH9i_5ZDqSyszlPpzkQowhkt3wViA0wMgM , accessed 16 July 2019