Acetamide MEA: the amide humectant that holds water in hair care

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Definition. Acetamide MEA (N-(2-hydroxyethyl)acetamide, CAS 142-26-7) is a small 103 g/mol molecule formed by an amide bond between acetic acid and monoethanolamine. Liquid, hygroscopic and water-soluble, it is used in shampoos, masks and conditioners as a humectant and hair conditioning agent: it attracts and holds water at the fibre surface and in the conditioning film.

The INCI name can be misleading: here “MEA” stands for a monoethanolamide, not the free, alkaline ethanolamine used in hair colour. Hairswiss looks at its chemistry, its role in hair care and what the safety assessments say.

Key facts

  • Chemical name: N-(2-hydroxyethyl)acetamide, C4H9NO2, 103.12 g/mol
  • State: liquid at 20 °C (melting point 16 °C), hygroscopic, water-soluble
  • Functions: humectant, hair and skin conditioning, foam boosting, viscosity control
  • Reported concentrations (CIR 2015): 0.06-5% in rinse-off products, 0.03-8% in leave-on products
  • Safety: safe as currently used according to the CIR, in non-irritating formulas without nitrosating agents

What is Acetamide MEA, chemically?

The molecule combines three elements on a very short chain: a methyl group, an amide function (–CO–NH–) and a primary alcohol function (–CH2OH) at the end of the chain. There is no fatty chain. That is what sets it apart from its better-known relatives, Cocamide MEA and Lauramide MEA, which carry a chain of 12 to 18 carbon atoms and are used mainly as thickeners and foam stabilisers.

With no lipophilic part, Acetamide MEA behaves like a small polyol: it mixes with water in any proportion, stays liquid at room temperature and picks up moisture from the air. Suppliers therefore often deliver it as an aqueous solution.

Why does “MEA” not mean free ethanolamine?

In INCI nomenclature, “MEA” is used both for monoethanolamine salts and for monoethanolamine amides. In Acetamide MEA the ethanolamine nitrogen is tied up in an amide bond and is no longer basic. The molecule therefore has no alkalising role, unlike the ethanolamine used in ammonia-free hair colour, which opens the cuticle by raising the pH.

How does Acetamide MEA hold water on the hair?

A humectant works through hydrogen bonds. Acetamide MEA offers several: the hydroxyl group and the amide N–H are donors, while the carbonyl oxygen and the hydroxyl oxygen are acceptors. Each molecule can therefore bind several water molecules, which explains why it is hygroscopic.

On hair, the effect depends on air humidity. Keratin takes up and releases water according to relative humidity; a humectant deposited on the fibre surface or trapped in the film of a mask slows drying and keeps the fibre softer and easier to style. In very humid weather, however, any humectant can increase fibre swelling and frizz, which is why formulators pair them with cationic conditioners and film-forming fatty ingredients.

Acetamide MEA rarely works alone. In rinse-off treatments it sits alongside other humectants (glycerin, sorbitol, propylene glycol, panthenol) and cationic conditioning agents. Its small size and high solubility make it easy to add to a water phase without weighing down the texture.

What other functions does it have in a formula?

Ingredient databases based on the European nomenclature also list hair and skin conditioning, foam boosting, surfactant and viscosity-controlling functions. In a shampoo it can help stabilise the lather and adjust the consistency; in a mask or conditioner its main role remains surface moisture and a supple feel.

Is Acetamide MEA safe?

The US Cosmetic Ingredient Review (CIR) assessed it in 1993 and again in its 2015 report on ethanolamides (Fiume et al., International Journal of Toxicology). Conclusion: the ethanolamides, Acetamide MEA included, are safe in the present practices of use and concentration when formulated to be non-irritating. The panel adds that they should not be used in products where N-nitroso compounds can form, a precaution shared by all ethanolamides.

The data reviewed by the CIR:

  • reported concentrations of 0.06-5% in rinse-off products and 0.03-8% in leave-on products;
  • in humans, a 7.5% aqueous solution caused neither primary irritation nor sensitisation in 50 volunteers;
  • undiluted, it was a mild skin irritant in rabbits; in a guinea pig maximisation test it was not a sensitiser;
  • in four analysed lots, residual impurities were very low: 0.0006-0.0029% ethanolamine and 0.0006-0.0030% acetamide; no N-nitrosodiethanolamine detected (detection limit 0.05 ppm).

Checking residual acetamide makes sense: free acetamide is classified by IARC as possibly carcinogenic (Group 2B). The measured levels, however, are in the range of a few thousandths of a percent in the raw material, and are diluted further in the finished product.

Hairswiss takeaway

Acetamide MEA is a discreet but effective humectant: a small, hydroxylated, highly soluble amide that binds water through hydrogen bonds and adds suppleness without a greasy film. Despite its name, it should not be confused with free ethanolamine in hair colour, nor with the fatty ethanolamides used as thickeners. Its safety profile is well documented, provided nitrosating agents are avoided and formulas are kept non-irritating. On dry or porous hair it works best together with cationic conditioners and fatty alcohols, which limit the effect of ambient humidity.

Also on Hairswiss: Panthenol: provitamin B5 on the hair · Hyaluronic acid and hair · Dry hair

Which mask with Acetamide MEA is available on cliCHair?

As an example, Nika Fairy Silk Masque 500 by NIKA, available on cliCHair.ch, combines Acetamide MEA with other humectants (Propylene Glycol, Sorbitol, Glycerin, Panthenol), a cationic conditioner (Cetrimonium Chloride) and a fatty alcohol (Cetearyl Alcohol), together with Hydrolyzed Keratin, Triticum Vulgare (Wheat) Germ Protein and Butyrospermum Parkii (Shea) Butter. It is a rinse-out mask, left on the lengths and ends for 10-20 minutes, designed for dry or chemically treated hair.

→ Nika Fairy Silk Masque 500 on cliCHair.ch

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