A humectant // is any one of a group of hygroscopic substances used to keep things moist; it is the opposite of a desiccant. They often are a molecule with several hydrophilic groups, most often hydroxyl groups; however, amines and carboxyl groups, sometimes esterified, can be encountered as well (its affinity to form hydrogen bonds with molecules of water, is the crucial trait).
A humectant attracts and retains the moisture in the air nearby via absorption, drawing the water vapor into and/or beneath the organism/object's surface. By contrast, desiccants also attract ambient moisture, but adsorbs -- not absorbs -- it, by condensing the water vapor onto the surface, as a layer of film.
When used as a food additive, a humectant has the effect of keeping the foodstuff moist.
Humectants can be used in topical dosage forms to increase the solubility of a chemical compound's active ingredient(s), increasing the active ingredients' ability to penetrate skin, and/or its activity time. This hydrating property can also be needed to counteract a dehydrating active ingredient (e.g., soaps, corticoids, some alcohols, etc.). This is why humectants are common ingredients in a wide range of cosmetic and personal care products that make moisturization claims (e.g., hair conditioners, body lotions, face or body cleansers, lip balms, eye creams, etc.).
Examples of some humectants include:
- propylene glycol (E1520), hexylene glycol, and butylene glycol
- glyceryl triacetate (E1518)
- vinyl alcohol
- Sugar alcohols/sugar polyols: glycerol/glycerin, sorbitol (E420), xylitol, maltitol (E965)
- polymeric polyols (e.g., polydextrose (E1200))
- quillaia (E999)
- aloe vera gel
- MP Diol
- alpha hydroxy acids (e.g., lactic acid)
The chemical compound lithium chloride is an excellent -- but toxic -- humectant, as well.
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