Humectants - Cosmetic Formulating Basics

This month we are focusing on cosmetic raw materials. In this post we continue our look at functional cosmetic ingredients specifically on humectants.

During much of your time in college chemistry you learn about chemical reactions, molecular structure, and the methods for synthesizing ingredients. However, as a cosmetic formulator learning the functional properties of specific ingredients and what they do when incorporated into formulas is much more important. You don’t have to learn about how to make ingredients, you need to learn how they work.

We’ve previously discussed the 3 types of cosmetic ingredients and recently discussed at length emollients in cosmetics. In this post we will be focusing on another conditioning agent like emollients, humectants.

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 function in a different way. Basically, humectants attract and retain water.

The reason that humectants are hygroscopic (can attract and bind water) is because of their molecular composition. While there are both organic and inorganic materials that are humectants, organic humectants are primarily used in the cosmetic industry. And when I say ‘organic’ I mean organic in the way that chemists use the term.  Organic means an ingredient is a hydrocarbon.

Structure of humectants

The molecular structure of humectants vary 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 way humectants work is that they attract water from either the atmosphere or from the body and bind it to itself via hydrogen 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 a humectant it makes for formulations.

Ideal cosmetic humectants

Although many materials have this humectant property, not all of them work well in cosmetics. There are a variety of characteristics we look for when choosing cosmetic humectants. These include…

• High moisturize absorption over a wide range of humidities.
• 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

These days it may also be important for formulators that the humectant is sourced from a plant or other sustainable source.

There aren’t many ingredients that meet all of 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 most common humectant in cosmetics is glycerin or glycerol.  This is a molecule that has three -OH groups on it.  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 perhaps the most versatile humectant in cosmetics and comes closest 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 is another common humectant in cosmetics. It is non-toxic, low odor, low viscosity and compatible with many ingredients. Propylene Glycol doesn’t absorb quite as much water as glycerin however, it still is excellent for this application. It also doesn’t have the stickiness problem of glycerin and is less expensive. PG is similar to glycerin in chemical structure having three Carbon atoms. The main difference is that it only contain 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.  However, it is more expensive which is why it is not used as extensively as glycerin or propylene glycol.

Butylene glycol

This is a clear, low viscosity liquid that works well as a humectant. It is a 4 Carbon molecule that contains two -OH groups. Comparable humectancy to propylene glycol, it 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 (Polyethylene Glycol)

There is a whole range of polymeric humectants based on the polymerization Ethylene Glycol. PEGs with an average molecular weight of 200 - 2000 are the ones that exhibit useful humectancy. When formulating, the higher the PEG 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 makes for a good story. It is a highly effective humectant and can bind water 1.5 times better than glycerin. As humectants go, it is one of the best performing ingredients. However, its relatively higher cost has limited its application.

Formulating with humectants

The ability to attract and retain water makes humectants excellent for applications where you need to draw moisture to the surface. This means they work great for things like 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. They get washed down the drain by the rinse water during use. Many formulators continue to include high levels of humectants in things like body wash, shampoos, and hair conditioners despite the fact that they just get washed down the drain.

However, 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 look for raw material suppliers to start offering more “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.