Humectants are a functional ingredient for Cosmetics.
When formulating cosmetics such as our Instant Facelift it is extremely important to learn about what a specific ingredient will do when incorporated into a formulation. Humectants, along with emollients are the backbones of cosmetic formulation.
What are Humectants?
Humectants and emollients are used for similar reasons. Both are conditioning ingredients incorporated into cosmetics to improve the look and feel of skin and hair.
Because of their different molecular composition, humectants and emollients perform this perform in a very different manner.
Basically, humectants attract and retain water. The reason that humectants are referred to as being hygroscopic, (can attract and bind water) is as a result of their molecular composition.
Humectants may be from organic and inorganic materials, but it is organic humectants that are primarily used in the cosmetic industry.
Organic is in the terms used by chemists, a hydrocarbon.
The Structure of Humectants:
The molecular structure of humectants varies widely, but they all have multiple hydroxyl group (-OH) or other hydrophillic sites that can interact through hydrogen bonding with water molecules. For the most part humectants are non-polar molecules compatible with water.
The manner humectants work is that they attract water from either the atmosphere or from the body and binds itself via chemical element bonding.
This property known as hygroscopicity can be measured for any kind of humectant. The amount of water that a material can bind at a specific humidity is called the equilibrium hygroscopicity. It is determined by placing a known quantity of a material in a fixed humidity chamber then measuring the change in mass of the material. When this test is done on a common humectant like Glycerin you find that it will absorb 25% of its weight in water when exposed to 50% humidity. Under the same conditions another common humectant, Propylene Glycol will absorb 20% its weight in water. In general, the more moisture a humectant will absorb, the better it is for formulations.
Ideal cosmetic Humectants:
Although several materials have this property, not all them work well in cosmetics. There are a range of characteristics we consider in selecting cosmetic humectants. These include…
- High moisturize absorption over a wide range of humidity.
- A consistent moisture content even with changing humidity.
- Non-toxic and safe to use for cosmetics.
- Low odor and low color.
- Low viscosity to make compounding easier.
- Generally non-reactive with other cosmetic ingredients.
- Low cost and readily available.
In today’s cosmetics market it is more important that the humectant is sourced from a plant or other sustainable source.
There aren’t many ingredients that meet all these criteria but there are a few that come close. Here are some of the most common types of humectants used in cosmetics.
Common Humectants in cosmetics:
Glycerin – the foremost substance in cosmetics is glycerin or glycerol. The molecule has three -OH groups.
It is an odorless, clear liquid that can be derived from natural sources. While it can be found in nature it is primarily manufactured as a bi-product of chemical reactions with fats and oils. It can also be synthesized from petroleum sources.
Glycerin is probably the foremost versatile substance in cosmetics and comes nearest to being ideal. Its principle drawback is that at high levels is can feel sticky. When formulating with it you need to keep the levels low or find other materials to off-set the stickiness.
Propylene Glycol – this can be another common substance in cosmetics.
It is non-toxic, low odor, low viscosity and compatible with many ingredients. It doesn’t absorb quite as much water as glycerin but it still is excellent for this application. It also doesn’t have the stickiness problem of glycerin and is less expensive. Propylene glycol has a chemical structure like glycerin in in that is has three Carbon atoms. The main difference is that it only contains two -OH groups. The primary drawback to propylene glycol is that it is synthetically produced from petroleum processing. It also has a bad reputation suffering from misinformation about it on the Internet. But from a formulation standpoint, it is excellent.
Sorbitol – This is a 6 Carbon sugar that has 6 -OH groups. It can be derived from glucose so can maintain a natural story. It is more hygroscopic than glycerin and doesn’t suffer from the stickiness problem. But, it is more expensive which is why it is not used as extensively as glycerin or propylene glycol.
Butylene glycol – this can be a transparent, low viscosity liquid that works well as a humectant.
It is a 4 Carbon molecule that contains two -OH groups. It is similar in humectancy to propylene glycol and makes a good substitute if you are trying to move away from that material. It is also a more effective solubilizer than both Glycerin and Propylene Glycol.
PEG – there’s an entire vary of chemical compound humectants derived from the polymerization of Ethylene Glycol. PEGs with a molecular weight of between 200 – 2000 are best. A higher the molecular weight the less water soluble it will be.
Sodium PCA – This humectant is found naturally in human skin so it is often included as part of the Natural Moisturizing Factor. It is a highly effective humectant and can bind water 1.5 times better than glycerin. As humectants go, it’s amongst the most effective. However, its relatively higher cost has limited its application.
Formulating with humectants:
The ability to draw in and retain water makes humectants wonderful for applications wherever you wish to draw moisture to the surface.
This means they are very effective for skin moisturizers and hair conditioners. They are almost always soluble in water so they are compatible with most water-based cosmetic formulas. So when you formulate with them you include them in the water phase.
Of course, the fact that they are soluble in water means they will be of little use in rinse-off products. This is because they get rinsed away. Many formulators continue to include high levels of humectants in things like body wash, shampoos, and hair conditioners despite the fact that they get washed down the drain.
Yet, there is some merit to including these ingredients even in rinse-off products. They are not included for the functional benefit but to help keep the formula moisturized and stable. This is particularly useful for products that are delivered from pump packaging. A humectant can prevent the pump from getting clogged with dried out product.
With the movement towards natural ingredients suppliers are seeking a lot of “derived from nature” humectants.
Also, on the synthetic side there will likely be more polymeric humectant ingredients that are not based on ethylene oxide polymerization. Of course, it will be tough to beat glycerin.