Of all the ingredients you could use to formulate cosmetics, surfactants are perhaps the most important. They are used for a wide range of applications such as cleaning surfaces, delivering conditioning materials, making materials compatible, creating foam, killing microbes, and more. To be a complete cosmetic chemist you’ll have to know about the chemistry of surfactants. In this post we will begin with an introduction to surfactants.
The Polarity of Molecules
To understand surfactants it’s first important to understand a property of all chemicals, polarity. We explained this a bit in our post on cosmetic emulsions but basically compounds can be classified as polar and non polar. Non-polar molecules like oils have enough electrons in them to make them stable. Polar molecules, like water, have an imbalance in electrons which make them attracted to other imbalanced molecules. The important part to understand here is that Polar molecules are attracted to other polar molecules while nonpolar molecules are attracted to other nonpolar molecules. Or as chemists like to say,
Like attracts like
This means that things like Water and Oil will not mix.
It is a bit more complicated since there is a range of polarity values and some polar molecules aren’t compatible with other but the old adage works fairly well in most situations.
If you are formulating a cosmetic product you could create a product using only nonpolar molecules and it would be fairly easy to blend them together. In fact, products like body butters, balms, salves and ointments are typically simple blends of nonpolar molecules. You could also formulate with only polar molecules and make things like moisturizing mists, moisturizers, and leave-in conditioners.
The problem is that single note formulas like these are extremely limited in the benefits you can deliver and they don’t usually look, feel or smell good. The best cosmetic products are a blend of Polar and Nonpolar materials.
But as we said above, these materials don’t mix. That is unless you have another material that is able to help. That’s where surfactants come in.
What are surfactants?
Surfactants are materials that are made up of both polar and nonpolar parts. That means a part of the molecule will be compatible with polar molecules while another part of the molecule will be compatible with non-polar molecules. There are four technical terms you might hear when someone talks about surfactant molecules
- Hydrophilic - the “water loving” or polar part of the molecule
- Hydrophobic - the “water hating” or nonpolar part of the molecule
- Lipophilic - the “oil loving” part of the molecule (same as hydrophobic)
- Lipophobic - the “oil hating” part of the molecule (same as hydrophilic)
If you examine a surfactant molecule like Sodium Lauryl Sulfate you’ll see that one end of the molecule is hydrophilic and the other end is hydrophobic.
This dual compatibility is what makes surfactants useful for cosmetic formulators.
…dual compatibility is what makes surfactants useful for cosmetic formulators.
The term Surfactant is a portmanteau of the words Surface Active Agent. In a mixture of ingredients, surfactants are attracted to the surface between the polar molecules and the nonpolar molecules. At the surface they are active which means they can arrange themselves in different structures than just a single molecule floating around.
The exact structure a surfactant takes on depends on the other materials in the mixture and the concentration of the surfactant. The most simple structure is known as a micelle which is essentially a micro sized sphere in which the non-polar ends of the molecules turn inward while the polar ends of the surfactants turn outward. Here is an example of what a micelle would look like under the proper circumstances.
There can be more complicated structures depending on the composition of the surfactant mixture but we’ll focus on micelles for the moment.
Since micelles have an oil loving part in the middle and a water loving part on the surface, this lets you to put other types of oils inside the structure of the micelle. Essentially, allowing you to create a stable mixture of polar materials with non-polar materials.
Surfactants make it so that your cosmetic formulations are not limited to only-polar blends or only-nonpolar blends. They open up a wide world of possibilities. With special surfactants called emulsifiers you can create creams and lotions and all kinds of different types of cosmetics. You can take functional non-polar molecules like Emollients or Occlusive Agents and blend them with polar Humectants to get an elegant feeling skin moisturizer.
Of course, surfactants can do much more than that. In the next part of this series we’ll take a look at all the things that surfactants can do.