Cosmetic Surfactants - An Introduction for Cosmetic Chemists
I remember learning about surfactants in first year college chemistry. It covered one page of Zumdahl Chemistry, and we spent exactly 5 minutes covering it. It came up again briefly in Organic Chemistry and that was about it. Surfactants were just not important.
That’s why I found it strange to learn that in the cosmetic industry, surfactants are the cornerstone of nearly all formulation and cosmetic science in general. To be a great formulation chemist, you have to know the chemistry of surfactants.
Surfactants Reduce Surface Tension
There are various places you can find background information about surfactant chemistry such as here and here, but we’re going to focus on what is most important for cosmetic chemists to know.
The key thing to know about surfactants is that they are chemicals which have parts that are both hydrophilic (water loving) and lipophilic (oil loving). This molecular composition means they have the ability to reduce the surface tension when placed into solutions of oil and water. In fact, the word “surfactant” is a shortened form of the phrase “surface active ingredient.”
While it may make sense that surfactants reduce surface tension, you might be wondering what that means from a practical application standpoint. We’ll look at that next.
How do cosmetic surfactants work?
When surfactants are put into solutions, the molecules have a tendency to line up in a certain way depending on the solution composition, the concentration of the surfactant, and the temperature.
In a water solution with extremely low surfactant concentrations, the molecules tend to bounce around randomly without forming structures. But at the Critical Micelle Concentration (CMC) they arrange themselves in spherical structures called micelles. On the outer layer of the spheres are the hydrophilic parts of the surfactant molecule and on the inner layer are the lipophilic parts. It’s a bit like a cream filled donut.
Increase the surfactant concentration and more complicated structures result such as hexagonal cylinders, lamellar planes and liquid crystals. Since this is an introduction, we’ll stick with the micelle structures.
Surfactants uses cosmetics
Now that you know about micelles, you’re ready to learn practical applications of surfactants in cosmetics. Surfactants are useful for the following application.
5. Special effects
The useful thing about micelles is that they can help suspend oil in water. When a small amount of oily materials is put into an aqueous solution of surfactants, it will migrate into the center portion of the micelle. So, when you put a surfactant solution on a surface like hair or skin, the oil that is there will be drawn away from the surface and into the micelles. When the surfactant solution is rinsed away, the surface is clean.
For a graphical explanation of this process, see this short instructive video about surfactants (requires a Flash player).
Foam is another characteristic of surfactant solutions so you’ll need surfactants if you want your product to foam. Essentially, foam is the entrapment of air in liquids and the alignment of the surfactant molecules helps keep the foam stable. It should be noted that foam itself is not related to the ability of a product to clean. But consumers expect cleansing products to foam so as a cosmetic formulator, you’ll have to add foaming surfactants.
While cleansing cosmetics remove oils, many cosmetic products are design to add oily materials to the skin and hair. These ingredients usually can’t be applied directly because they have undesirable aesthetic characteristics in their concentrated form. For this reason, cosmetic chemists create emulsions using surfactants.
A full exploration of emulsion science is beyond the scope of this entry so suffice it to say, emulsions are semi-stable mixtures of oils and water. A surfactant, or emulsifier, is used to help blend and stabilize the mixtures.
In the simplest case, an emulsion formula is made by mixing an oil phase with a water phase and a surfactant. The micelles created by the surfactant entrap the oil in their centers and it remains suspended throughout the mixture. Products like creams and lotions are typically emulsions. When the product is applied to the skin or hair, the surfactant micelles break open and deliver the oily materials.
The problem with most emulsions is that they usually create opaque products. However, there are times when a cosmetic chemist wants a clear formula but still wants to blend an oil in a mostly water formula. Fortunately, there are surfactants that have the ability to create particles so small that light passes through them and the solutions remain clear. Molecules that do this are solubilizing surfactants. They are used to blend oily materials like fragrances or natural ingredients into clear solutions. An example would be a surfactant like Polysorbate 20.
Since surfactants often contain an “oily” part on their molecule, they have conditioning properties that can improve the feel and look of the surfaces of skin and hair. For them to work this way, the surfactants have to be left behind and also be non-irritating. This can be achieved through a leave-on cosmetic product or by using surfactants that can bond to surfaces through an electrostatic charge (more on this later).
In addition to the four properties discussed, surfactants have a number of other special effects that are useful to formulators.
For example, some surfactants have anti-microbial effects so they can be used as a preservative. Certain surfactants can be used to create an elegant, pearly effect in cosmetics to increase their aesthetic appeal. They can be used for thickening systems, reducing irritation and improving formula stability.
In part 2 of this surfactant series, we examine the different types of surfactants and how they are applied to cosmetic formulations.