We hear a lot about the humectants. But many of us may not know what they are and how they work.
Humectants are used in skin and hair-care products to promote moisture retention. These hygroscopic compounds posses a chemical structure that attracts water from the atmosphere and binds it to various sites along the molecule.
There are many different types of molecules that are effective humectants, but the chemical similarity they share is having multiple alcohol (hydroxyl) or similarly hydrophilic sites (such as ethers or ammonium groups) available for hydrogen bonding with water molecules. The strength of the hydrogen bonding between humectants and H2O improves moisture retention by minimizing water loss due to evaporation.
These ingredients are used in many products, especially those designed for consumers with dry, damaged hair. While they can provide benefit in certain circumstances, it is important to understand how they work and under what circumstances they might provide less-than-optimal results. When considering whether to use humectants, the most important piece of the puzzle is how the weather can impact your hair and the performance of these molecules.
Effects of Relative Humidity on Hair and Humectants
The laws of thermodynamics have a daily influence on our hair. There is a driving force in nature for everything to be in a state of equilibrium. Molecules that exist in an area of higher concentration in contact with an area of lower concentration will move toward the area of lower concentration in an attempt to reach an equilibrium state where the concentration is the same for both areas. For example, dry hair exposed to a very humid environment will become gradually immersed with water molecules, as they diffuse from the air (an area of higher concentration) into the hair (an area of lower concentration). Curly hair is especially susceptible to this type of diffusion because it is naturally more porous than straight hair.
Hair exposed to very dry air without protection can lose its moisture, develop an unpleasant texture and can become unruly, flyaway, and frizzy. It also can become more prone to breakage and split ends. Conversely, unprotected exposure to excessive moisture and humidity can swell the hair cortex, causing the cuticle scales on the exterior of the hair shaft to become ruffled and giving hair a coarse, unpleasant texture. Clearly, neither scenario is desirable.
In extremely low-humidity conditions, such as a cold, dry winter air, there is no appreciable amount of water in the air for the humectant to attract to the surface of the hair. In this particular type of climate, the best one can hope for with most traditional humectants is for them to prevent evaporation of water from the hair into the environment. Also, under these circumstances, there is some risk of the humectant actually removing moisture from the cortex of the hair itself, creating the problem it was intended to prevent.
That's why in dry climates it is important to use conditioning products which rely on strong moisturizers rather than traditional humectants. However, it is interesting to note that new humectants are being developed that perform well even in low humidity (such as hydroxypropyl bis-hydroxyethyldimonium chloride and dihydroxypropyltrimonium chloride).
In high-humidity conditions, such as summertime in the southeastern United States or the tropics — where the relative humidity can easily reach or exceed 90 percent during the day — there is a tremendous amount of moisture in the air. This can be disastrous for curly hair. If curly hair is dry and damaged, it is very porous, and easily absorbs water from the air. In high-humidity conditions, this can cause curly hair to swell so much that cuticles are raised, making the surface of the hair very rough. These cuticles can then become entangled with cuticles of adjacent hairs and create a huge, tangled mass, which is prone to breakage. Also, curly hair swollen by excess water can lose its curl pattern, creating the dreaded summer frizz.
Clearly, products heavy in humectants will only exacerbate problems with humidity-induced frizz. Some humectants can also develop a sticky feeling when they become saturated with water, which is certainly an undesirable characteristic for hair. Thus, in tropical and subtropical climates, it is essential to maintain well-moisturized hair that is in good condition (which will be less prone to absorbing water from the hair). But it is preferable to use products containing fewer humectants or humectants with less hygroscopic capacity.
Due to the limitations and pitfalls of humectants in extreme weather conditions, it is apparent that these ingredients probably function best for curly-haired consumers in moderate climates, with temperatures and humidity in the medium range.
Next month, we will discuss some of the different humectants in more detail.
Examples of HumectantsDiols and Triols
Humectants of biological origin
Sugars and modified sugars