Humectants can greatly help or hurt your hair's moisture levels. Understanding the science behind humectants will help you use them to your advantage.
Humectants are materials used in products to moisturize dry or damaged hair. They promote moisture retention by attracting water molecules from the local environment and binding them to specific sites along their structure.
Adsorption vs Absorption
Absorption is the process whereby a substance passes into the bulk of a material and are dissolved uniformly throughout. The solution cannot be easily separated into the original two substances.
Adsorption describes the process whereby atoms or molecules are attracted to and adhere to the surface of a material, usually via forces weak enough that they can be easily separated. Some humectants can adsorb several times their weight in water!
In humectants, this happens via hydrogen bonding - a very important phenomenon based upon polarity of specific atoms.
Molecules are made of combinations of different atoms. Sometimes the atoms have significant differences in their 'electronegativity', generating a charge separation where part of the molecule is more positive and the other is more negative. When this happens, even though the overall charge on the substance is neutral, the molecule has distinct polarity. Imagine a magnet, with its positive end and negative end, and how they can link together end-to-end.
Molecules like water (H2O) have polarity due to the oxygen being much more electronegative than the hydrogen atoms. The mildly negative oxygen atom is attracted to the mildly positive hydrogen atoms in other water molecules, and they move close to one another and form a bridge, called a hydrogen bond. Each oxygen can form a hydrogen bond with two hydrogen atoms. The triangular geometry of water molecules allows them to stack together into a complex three-dimensional array.
Humectant molecules have polar hydroxyl groups (-OH) that also favor hydrogen bonding. When applied to hair in a styling or conditioning product, they can attract water from the environment around them and bring it into close contact with the hair.
Humectants and Hair
When humectants bring water into contact with the hair, some can diffuse into the shaft of the hair. This can add suppleness and softness to slightly dry hair. It can make hair more bouncy and help it retain curl. Hair can recoil more easily from mechanical stress and is less likely to break.
In high humidity conditions, humectants may attract too much water to the hair from the wet environment. This can cause the hair shaft to swell, the cuticle to become ruffled, and hair to lose its shape and become big and frizzy. Porous hair suffers from this problem more so than non-porous hair. Some humectants may develop a sticky texture in these conditions as well. This is not pleasant!
In extremely low humidity conditions, humectants may draw water out of the interior of the hair shaft and cause dryness and possible damage or breakage. Use caution!
Humectants Can Help
Make hair feel softer
Make hair bouncier
Make hair more elastic and less brittle
Protect hair from dry weather and wind
Humectants Can Harm
Can create frizzy, sticky hair in high humidity
Can dry out hair in low humidity
Can cause permanent damage to hair in either extreme condition
Diols and Triols
Hexanediol or -triol beeswax
Humectants of biological origin
Sugars and modified sugars
Elastin, collagen, silk, keratin
Isoceteth-x, Isolaureth-x, Laneth-x, Laureth-x, Steareth-x
PEG-x (polyethylene glycol)
Structural formulae for some common humectants: Glycerin, propylene glycol, sorbitol, glucose, and sodium PCA (from left to right, top to bottom) All images except Sodium PCA courtesy of Wikipedia. Sodium PCA image courtesy of chemblink.com.