The most frequently used humectants are diols and triols – compounds which contain two or three hydroxyl groups (-OH) that attract water molecules through hydrogen bonding
Last month we discussed how humectants work, and how the temperature and humidity can affect that process. We also discussed the effects of temperature and humidity on the hair itself, and how the use of products containing humectants can impact hair in various weather conditions.
This month, we will discuss some of the humectants found in hair-care products in more detail. While most humectants share the fact that they contain at least one hydroxyl group (-OH), they can be separated into several different categories, including alcohols, saccharides, provitamins, proteins, and many other types of molecules. We will discuss a few of the ones that are most commonly seen in hair-care products.
The most frequently used humectants are diols and triols — compounds which contain two or three hydroxyl groups (-OH) that attract water molecules through hydrogen bonding. These include glycerin, phytantriol, erythritol, and numerous varieties of alkyl diols and triols (propylene glycol, 1,2,6-hexanetriol, triethyelene glycol, etc.).
One common diol, and a very effective humectant, is propylene glycol. This ingredient is used in many personal-care products, as well as a variety of other applications. There has been a lot of debate about the possible health hazards of this diol.
The non-cosmetics-related application, which is the source of alarm, is its occasional use as an “anti-freeze.” Because of this, there have been implications that this ingredient has the toxicity associated with the more usual component of anti-freeze, ethylene glycol, which is quite toxic, even lethal at relatively small doses.
The truth is that propylene glycol has been substituted for ethylene glycol in many commercially available antifreeze formulations because it is much safer than ethylene glycol. The other important thing to note is that the word “anti-freeze" — used to strike fear into the heart of the educated consumer — is actually the technical word to describe a completely innocuous process, the lowering of the freezing point of a liquid. I am sure many of you are familiar with the application of salt to the streets and sidewalks to help prevent the formation of dangerous ice, which it does by lowering the freezing point of water. This is an example of a “safe” chemical being used as an “anti-freeze.”
My point is that one need not be alarmed by the chemical propylene glycol. While few chemicals are entirely without risk, propylene glycol is considered to be safe at the low concentrations used in personal care products and even food products, and is quite effective as a humectant.
Biologically Significant Humectants
Sodium PCA (sodium-2-pyrrolidone-5- carboxylate) is an interesting humectant that naturally occurs in human skin. It can attract and bind several times its own weight in water and is considered to be one of the primary moisturizers of skin cells. Panthenol is another very popular humectant for hair-care products. It is a provitamin of B5 (pantothenic acid), and is present in many living cells.
Another excellent humectant of biological origin is hyaluronic acid, a naturally occurring polysaccharide. This material is found not only in the epithelial cells of the skin, but also in connective and nervous tissue throughout the body. It is capable of binding incredible amounts of water molecules to itself, due to its polymeric sugar structure with many available hydroxyl sites along its backbone.
Other Molecules as Humectants
Simple sugars (monosaccharides) and sugar alcohols possess 5-6 hydroxyl sites available for hydrogen bonding with water, making them excellent humectants. Sorbitol, glucose, and fructose are all commonly found in hair-care products. Many other types of molecules are used as humectants, including (but not limited to) copolymers of silicone with polyols (usually PEG), PEG polymers (polyethers) of differing lengths, proteins, amino acids, short chain ethers, and certain vitamins.
New Advances in Humectants for Hair and Skin care
In recent years, Colonial Chemical, Inc. has developed two new molecules that perform exceedingly well at both low and high humidity levels. These two materials are known by the trade names Cola Moist 200 and 300P, and by the INCI names hydroxypropyl bis-hydroxyethyldimonium chloride and polyquaternium-71, respectively. Cola Moist 200 is a highly ionized small molecule, while Cola Moist 300P is a highly ionized polymeric molecule. Both are extremely hygroscopic (readily absorbs water), prevent dehydration of the hair, are water soluble, and moisturize very effectively. Their high degree of ionization is responsible for the excellent performance. Testing has shown these materials to outperform several traditional humectants by 3 to 6 times at low humidity. They have also been found to enhance the performance of humectants such as glycerin.
In conclusion, there are many humectants available for use in hair- and skin-care formulations. Even some of the simplest, such as glycerin and propylene glycol, can add benefit to a formulation. Some of the more expensive ones, such as hyaluronic acid, and the newer ionized humectants may provide superior benefits, especially in drier climates.
When selecting humectant-containing products, one must keep in mind the climate in which they live, how that impacts hair, and how they might expect a humectant to contribute to the overall performance of their own hair within the constraints of that climate. Scientists are continuously researching this area, and are developing new types of molecules in order to overcome some of the limitations of existing ones.
Examples of Humectants
- Diols and TriolsPropylene glycol
- 1,2,6 hexanetriol
- Butylene Glycol
- Dipropylene glycol
- Hexylene Glycol
- Triethylene glycol
- Capryl glycol
- Hexanediol or -triol beeswax
Humectants of biological origin
- Sodium PCA
- Hyaluronic acid
- Glycogen Sugars and modified sugars
- Polyglyceryl sorbitol
- Xylitol Hydrolyzed proteins
- Elastin, collagen, silk, keratin EthersIsoceteth-x, Isolaureth-x,
- Laneth-x, Laureth-x, Steareth-xPEG-x (polyethylene glycol)
- Silicone copolyols
Email your questions to Tonya.