A humectant is a substance that attracts water. Typically used for moisturizing and conditioning hair and skin. It is also used to improve stability of formulas exposed to air. They are used in many products, including food, cosmetics, medicines, and pesticides. When used as a food additive, a humectant has the effect of keeping moisture in the food.

In cosmetics, humectants are used to hydrate the skin, hair, or nails. Some common humectants include:

• Glycerin
• Hyaluronic acid
• Sorbitol
• Lactic acid
• Urea
• Propylene glycol
• Butylene glycol

The CTFA dictionary defines humectants as ‘‘cosmetic ingredients intended to increase the water content of top layers of the skin’’. Humectants are hygroscopic substances generally soluble in water. These ‘‘moisture attractants’’ maintain an aqueous film at the skin surface. The primary used humectant in personal-care products is glycerin; it tends to provide heavy and tacky feel which can be overcome by using it in combination with
other humectants such as sorbitol.
Less expensive than glycerin, propylene glycol is the second most widely used humectant in cosmetic and toiletry products; it reduces viscosity of surfactant solutions and tends to depress the foam.

Low–molecular weight polyethylene glycol (PEGs from about 10 to 200 PEG units), amino acids and other constituents of skin natural moisturizing factors like sodium PCA and sodium lactate are also applicable for use in surfactant-based skin-cleansing products.

Humectants are not substantive to the skin and are easily rinsed-off after cleaning. Consequently, skin-feel improvement is not obvious to perceive and their efficacy in terms of skin moisturization is difficult to document. Glycerin, propylene glycol, 1,3-butylene glycol, or sorbitol are typically used in body washes, bubble baths, shower gels, or soaps to prevent the dessication of the product itself and the formation of a dry layer at the surface. They also ensure stability and clarity of liquid cleansers at cold temperatures. Few substantive humectants can be mentioned. They are cationic in nature, which makes them absorbing to the negatively charged skin surface. In the quaternized polyalkoxylated methyl glucose derivative (lauryl methyl gluceth-10 hydroxypropyldimonium chloride), the hydrophilic moiety delivers humectant properties; the hydrophobic chain at the cationic end of the molecule ensures both substantivity and skin conditioning.

Chitosan-PCA is another example. Chitosan is a polycationic (at acidic pH) high–molecular weight polymer produced by deacetylation of chitin, the major constituent of
invertebrate exoskeletons. Combining chitosan with pyrrolidone carboxylic acid (PCA) leads to a highly substantive, film-forming humectant material.

Source: Handbook of Cosmetic Science and Technology - André O. Barel, Marc Paye, Howard I. Maibach