Water constantly evaporates from the deeper layers of the skin, an effect known as transepidermal water loss (TEWL). Since dry skin is brittle and rigid, any increase in the water content of skin is believed to improve the skin quality. The ability to retain moisture depends on the lipid bilayer between the corneocytes.
Emollients prevent evaporation of water from the skin by forming an occlusive coating on the surface of the stratum corneum. TEWL is normally about 4–8 g/(m²⋅h). A layer of petrolatum applied to normal skin can reduce the TEWL by 50–75% for several hours.
Humectants also have an emollient effect, but they act differently, by drawing water into the stratum corneum.
Petrolatum (White Soft Paraffin) is probably the most effective emollient. Other popular emollients are castor oil, cetyl alcohol, cetearyl alcohol, cocoa butter, isopropyl myristate, isopropyl palmitate, lanolin, liquid paraffin, polyethylene glycols, shea butter, silicone oils, stearic acid, and stearyl alcohol.
Emollient cosmetics may additionally contain antioxidants, ceramides, emulsifiers, fragrances, humectants, penetration enhancers, preservatives, and solvents. Some products are marketed as having anti-wrinkle and skin enhancement effects. Many plant and animal extracts have been claimed to impart skin benefits, with little scientific evidence.
Although various lipids or emollients have been used for anointing throughout history, this use hardly counts as cosmetical in today's sense. Scientific cosmetic chemistry exists only since 1920.
Emollients are used for the treatment of certain skin ailments, such as psoriasis, ichthyosis vulgaris, xerosis, and pruritus in atopic dermatitis. More often, they are bases or vehicles for topical medication, such as in Whitfield's ointment. They are often combined with keratolytic agents, such as salicylic acid and urea.
Excess use of emollient may cause mild folliculitis, especially in adolescent girls. It may also exacerbate fungal skin infection (dermatophytosis). Subcutaneous injection of emollient may cause panniculitis.
Emollients should be used after bathing or daily.
- Arza Seidel; et al., eds. (2013), Kirk-Othmer Chemical Technology of Cosmetics
- Tony Burns; et al., eds. (2010), Rook's Textbook of Dermatology (8th ed.)