Did you know you can balance a needle on top of water?
This is because water has something called, surface tension.
You may have experienced surface tension when you did a bellyflop in the pool that one time when you were young and you made sure to never do it again because you realized how much it hurt.
But as soon as you add a surfactant into the water, you can't balance the needle anymore. This is because the surface tension is now lowered.
The surface tension of water is also why you can't spread water well. If you notice when you drop water on a surface, it just bubbles up, it doesn't go flat. So this is why you can't clean with just water. It won’t wipe away all of the dirt.
Surfactants have an oil loving tail and a water loving head.
This is often represented by a tadpole.
The Head that is water loving can be described as;
polar
hydrophilic (water loving)
Lipophobic (oil hating)
The tail which is oil loving can be described as;
non-polar
lipophilic (oil loving)
hydrophobic (water hating)
So as you can see surfactants have these little molecules that want to do completely different things. One side wants to go to water and the other side wants to go to oil.
If you pour a surfactant into water, what happens, is all of the oil loving ends will align together and protrude out on the surface of the water or along the edges of the container to try and escape the water as the water loving heads are facing in towards the water. (as shown in the figure to the left)
The molecules doing this is what helps lower the surface tension of the water, which now allows us to spread water. This is why surfactants are also known as a wetting agent, surfactants help wet out water. This is why the needle can no longer balance on the water once we add the surfactant. The oil loving heads are all aligned on the surface of the water helping to keep the water flat and spreadable.
Surfactant is a word describing a broad range of different ingredients.
A Surfactant can be a foaming agent, solubilizer, emulsifier, wetting agent, etc. But they all are surface active agents, which is why they are all under the category Surfactant.
What is a Micelle?
Let's say we add so much surfactant to the water that there is no more room for the oil loving heads to go along the edges of the container or to protrude out of the top. What do the other oil loving tails do? This is when the oil loving tails actually come together to form this ball shape, and this is called a micelle. (see figure to the right)
This point where we have so much surfactant in water that they start creating micelles, this is actually called the critical micelle concentration (CMC) of the surfactant.
What is Foam and how does it work?
If you try to put air into water, you'll notice it doesn't really work. Again, this is due to water having surface tension. But once again, when you add a surfactant into the water this lowers the surface tension allowing air to now be incorporate into the water/surfactant mixture.
Some of the oil loving tails will go to that air bubble and completely surround the air bubble creating a micelle around the air bubble. They do this because there is no water in that air bubble, so the oil loving tails feel safe there. (see figure to the left)
This act is what helps stabilize foam in body washes, shampoos, face washes and other foaming products. This also helps explain why foam is just another type of emulsion.
I also want to mention that micelles aren’t just sitting still. They are constantly moving and breaking down and reforming.
Incorporating Fragrance Oil or Essential Oils into Foaming Products
We can also add a small amount of fragrance oil or essential oil into products like shampoos, body washes, and face washes that contain these cleansing surfactants because the surfactants contain oil loving tails. These oil loving tails can attach to the fragrance oil surrounding it creating a micelle around it, which causes the fragrance oil to be solubilized into the water. Note; these foaming surfactants will not be able to solubilize/emulsify a bunch of oil, only a small amount, maybe 1 - 4%.
I'm always asked, how to incorporate high amounts of oil into foaming products like shampoos, body washes, and face washes. This isn't something usually done for foaming products because oil suppresses foam and generally consumers like a lot of foam. Also the surfactant just washes away the oil, so really the oil doesn't do much except mild down the surfactant. Which you can mild down surfactants by using other ingredients that won't suppress foam such as refatting agents like glyceryl oleate.
There is a lot of innovation in the skincare world, so yes, it is possible to do it, but you will need specific ingredients to achieve stable results and your final product may not foam as well as a foaming product that doesn't contain oil. Personally one of my favorite face washes incorporates 7% oil into the formula. So as you can see, it can be done, but this is known as a milky cleanser. It doesn't foam as much and people often tell me it's like using a lotion.
Often times a clear gel face wash, body wash, or shampoo can not have oil incorporated into it because as soon as you do that, it turns milky and opaque. But, I have seen some clear gelled foaming products on the market that do claim to have carrier oils, but honestly who knows how much oil they are actually using in the product. They could also be using some unique ingredient us home crafters can't get our hands on. A lot is possible in the cosmetic formulating world as there is constant innovation going on. For example Lumorol K 5229 is an oil soluble surfactant. It can be mixed with carriers oils to make water free foaming products. So there are always exceptions to rules in the formulating world. I can just share with you general rules/guidelines to practice. This is why you should always read the formulating guidelines to your ingredients. They all have unique guidelines.
My source for this post comes from the book Discovering Cosmetic Science ; Page 19 - 25
The link I am using to this book is an affiliate link that I earn commission from, but I am not sponsored by the people who wrote or published this book. This book was recommended to me by @CosmeticYourWays on Instagram.
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