humectant
A humectant is the ingredient that keeps a cream, gel or paste from drying out and going crusty — a water-attracting additive that acts like a sponge holding moisture inside the product and on the skin. It is why a gel stays soft in the tube instead of cracking into a dry crust.
Technically a humectant is an excipient that retains moisture by attracting and holding water, slowing evaporation from semisolid preparations and from the skin surface they are applied to. Common humectants include glycerol (glycerin), propylene glycol, sorbitol and polyethylene glycols. They are hygroscopic — they draw water from the surrounding air — which keeps the product supple and helps maintain the hydration of the skin's outer layer.
There is a concentration sweet spot. At sensible levels a humectant keeps a preparation smooth and moisturising, but at high concentrations or low humidity a strongly hygroscopic agent can actually pull water out of the deeper skin, and large amounts of propylene glycol can sting or irritate. Humectants are therefore balanced against the rest of the formula rather than simply maximised.
Glycerol added to a toothpaste or to an aqueous gel stops the surface drying and cracking when the tube is left open, keeping the product smooth and spreadable.
A water-attracting additive keeps a semisolid from drying into a crust.
Some humectants double as cosolvents or sweeteners — glycerol and propylene glycol, for example — so one excipient can hold moisture, help dissolve a drug and add body all at once.