Humectant

A humectant is a substance that attracts and binds water. In skin care and formulation, it helps retain moisture in skin or products.

Hygroscopicity: The tendency of a substance to absorb water from its surroundings. It describes how humectants take up moisture from air or nearby materials.
Glycerol: A colorless, water-soluble polyol used as a solvent, sweetener, and moisture-retaining ingredient. It is a widely used humectant in skin care and many other formulations.
Water potential: A measure of water's potential energy that predicts the direction it will move. Water moves in response to gradients that humectants can influence.
Ceramide: A class of lipids found in cell membranes and abundant in the skin's outer barrier. Ceramides support the barrier's lipid structure rather than attracting water like humectants.
Water activity: A measure of the availability of water in a material, based on its vapor pressure. Humectants can lower water activity by binding water in a formulation.
Hyaluronic acid: A large sugar polymer found in connective tissues that can bind substantial amounts of water. Its water-binding properties make it a familiar humectant in topical products.
Osmosis: The net movement of water across a selectively permeable membrane toward higher solute concentration. It helps explain water movement across biological barriers, though skin is not a simple membrane.
Petrolatum: A semisolid mixture of hydrocarbons used on skin as a protective barrier. It reduces evaporation and is a classic occlusive, not a humectant.
Stratum corneum: The outermost layer of the epidermis, made largely of dead cells and lipids. Humectants in skin care act at or near this water-regulating outer layer.
Urea: A nitrogen-containing compound found naturally in skin and used in topical treatments. At suitable concentrations, it hydrates skin and can soften thickened areas.

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