A humectant /hjuːˈmɛktənt/ is a substance that is used to keep things moist. It is the opposite of a desiccant, whose purpose is to keep things dry. Humectants are hygroscopic substance. It is often a molecule with several hydrophilic groups, most often hydroxyl groups, but amines and carboxyl groups, sometimes esterified, can be encountered as well; the affinity to form hydrogen bonds with molecules of water is crucial here.
A humectant attracts and retains the moisture in the air by absorption, allowing the water to be used by other substances or by plants. It differs from a desiccant which also attracts moisture but adsorbs it instead of absorbing it i.e. condenses the water onto the surface as a layer of film.
When used as a food additive, the humectant has the effect of keeping the foodstuff moist. Humectants are sometimes used as a component of antistatic coatings for plastics. Humectants are also found in many cosmetic products where moisturization is desired, including treatments such as moisturizing hair conditioners and also commonly used in body lotions.
Humectants are also used in topical dosage forms to increase the solubility of the active ingredient, to elevate its skin penetration and increase its activity time. Humectants also elevate the hydration of the skin to minimize the dehydrating effect of some active ingredients like corticoids.
Examples of humectants include:
- propylene glycol (E1520) as well as hexylene glycol and butylene glycol
- glyceryl triacetate (E1518)
- vinyl alcohol
- sugar polyols such as glycerol, sorbitol (E420), xylitol and maltitol (E965)
- polymeric polyols like polydextrose (E1200)
- quillaia (E999)
- lactic acid
- aloe vera gel
- MP Diol
- alpha hydroxy acids like lactic acid
The chemical compound lithium chloride is an excellent humectant, but is toxic.
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