If you leave French bread on the counter for a couple days, it will get stale and crumbly, however, if you do the same with banana bread, it remains moist. There are a few explanations for this but one of the main reasons is because banana bread contains a high level of sugars that are able to bind water so that it doesn’t evaporate. This phenomena, called hygroscopy, is characteristic of humectants which are ingredients used in formulation situations where you want to maintain water. There are numerous applications for humectants in formulating among which include skin and hair moisturization, product preservation, solubilization and more. Let’s look at these in a little more depth.
Moisturization
Humectants in skin and hair products are typically there for their conditioning effects because they can improve the look and feel of biological surfaces. They do this by attracting and maintaining water in skin and hair.
The reason humectants are hygroscopic is because of their molecular composition. While there are both organic and inorganic materials considered 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, a hydrocarbon. It has nothing to do with how the raw materials were farmed.
Structure of humectants
The molecular structure of humectants varies widely, but they all have multiple hydrophilic sites that can attract water molecules through the process of hydrogen bonding. For the most part, cosmetic humectants are polar molecules compatible with water and frequently contain -OH groups or -NH groups.
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. The level of 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 absorbs, the better a humectant it makes for formulations.
Ideal cosmetic humectants
Although many materials have humectant properties, not all of them work well in cosmetics. There are a variety of characteristics we look for when choosing cosmetic humectants. These include…
- High moisture absorption over a wide range of humidities.
- Consistent moisture content even with changing humidity
- Non-volatile so they remain on the surface to continue to work
- 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.
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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 it can feel sticky. When formulating with it you need to keep the levels low or find other materials to offset the stickiness.
Propylene Glycol – This is another common humectant 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 however, it still is excellent for this application. It also doesn’t have the stickiness problem of glycerin and is less expensive. Propylene glycol is similar to glycerin in chemical structure having 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. 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. 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 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 are most useful as humectants. When formulating, the higher the PEG molecular weight the less water soluble it will be.
Urea – Is a compound that is made up of a carbon atom double bonded to an Oxygen with a couple of -NH groups. The -NH groups allow for the hydrogen bonding and thus the humectant effect. Urea is also a biomolecule that is part of the Natural Moisturizing Factor (NMF) in skin.
Sodium PCA – This humectant is also found naturally in human skin in the NMF. For that reason, it makes a good marketing 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.
Hyaluronic Acid – This is a popular humectant polymer and a natural biomolecule that is created by the body. It is derived from sugars and many beauty brands and consumers love it as an anti-aging ingredient. While it can bind more efficiently than something like Glycerin, it is a film former and thickener which limits the amount you can use in a formula.
Amino acids, peptides and hydrolyzed proteins – These molecules are made up of -NH and -OH groups. Depending on the configuration, they may be able to act as humectants. Some amino acids that have been used for this purpose include glycine, leucine, and arginine. Various peptides work as humectants as do hydrolyzed proteins.
Aloe Vera – This is a popular humectant material derived from the aloe plant. Since there are so many variants available and growing seasons vary, it’s difficult for a formulator to know whether the particular sample you get will perform reliably. But the main components of aloe that create a humectant effect are the sugars both mono and polysaccharides.
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 minimal use in rinse-off products. This is because they simply 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 just get washed down the drain, and they can negatively impact foam.
Formula stabilization
There is some merit to including humectants 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. These ingredients can also reduce the cloud point of a system and help improve stability.
A humectant can also help trap water which results in a reduction of water activity. This in turn can inhibit microbial growth and boost the effectiveness of the preservative system. That is why ingredients like caprylyl glycol are often blended with preservatives.
Natural Humectants
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. On a cost per pound plus performance basis, Glycerin remains the best choice for most formulators to use.
Incidentally, sometimes cosmetic marketers, the media and consumers refer to humectants as “hydrators.” This isn’t a scientific term, but this presumably refers to the fact that the ingredients will attract water or add water to the skin. I mention it because it is a term you might hear from consumers or your marketing group.
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