Ethanolamine (monoethanolamine, MEA, INCI name Ethanolamine) is a primary amine with an alcohol group, formula HOCH₂CH₂NH₂ (61.08 g/mol), used as the alkalising agent in so-called “ammonia-free” oxidative hair colours. Like ammonia, it raises the pH of the colour mix to swell the fibre and activate the hydrogen peroxide; unlike ammonia, it is barely volatile, which removes the pungent smell but changes how it behaves on the hair.

The switch is therefore more than a new label. It changes how much alkaliser is needed, what stays in the fibre after rinsing and how the service should be finished. Hairswiss sums up what chemistry says and what studies show.

Why does an oxidative colour need an alkaliser?

A permanent colour combines precursors (for example toluene-2,5-diamine), couplers and hydrogen peroxide. The reaction takes place in alkaline conditions, usually around pH 9 to 10, for three reasons. The high pH swells the fibre and lifts the cuticle scales, so small molecules can reach the cortex. It favours the formation of the perhydroxyl anion (HOO⁻; the pKa of hydrogen peroxide is close to 11.6), mainly responsible for lightening melanin. Finally, it allows the precursors to oxidise and couple into larger coloured molecules, which remain trapped in the cortex.

For the general chemistry of hair colour and the role of precursors, see the Hairswiss articles on the chemistry of professional hair coloration and on para-phenylenediamine.

Ethanolamine and ammonia: what are the chemical differences?

• Basicity: the pKa of the conjugate acid is 9.50 for ethanolamine and 9.24 for the ammonium ion. Both bases buffer in the same pH range.
• Volatility: ammonia boils at −33 °C and escapes from the mix during processing; ethanolamine boils at 170 °C and its vapour pressure is only about 64 Pa at 20 °C. It practically does not evaporate.
• Odour: the pungent smell of conventional colours comes from released ammonia. Ethanolamine has a faint, ammonia-like odour that is noticeable mainly at close range.
• Molar mass: 61.08 g/mol against 17.03 g/mol for ammonia. To reach the same pH, a larger mass of ethanolamine is therefore needed.

For the chemistry and precautions specific to ammonia, see the Hairswiss article on ammonium hydroxide in hair dyes.

Does ethanolamine lighten as much as ammonia?

Not at the same amount. Ammonia that evaporates takes part of the alkalinity with it: the pH tends to fall during processing. Ethanolamine instead stays in the mix and in the fibre. To achieve the same lightening of melanin, formulators have to use higher percentages of ethanolamine than of ammonia, as noted in a study published in 2014 in the Journal of Cosmetic Science by a Procter & Gamble team (Bailey, Zhang and Murphy).

Is an ammonia-free colour gentler on the fibre?

Not necessarily. In the study by Bailey and colleagues, colours with ethanolamine at the upper limits of use were compared with ammonia colours using three methods: measurement of cysteic acid by infrared spectroscopy (a marker of keratin oxidation), observation of the cuticle by scanning electron microscopy and measurement of protein loss. All showed more damage with ethanolamine, up to 85% more in the most extreme case. The authors relate this result to the fact that non-volatile ethanolamine can remain in the hair after rinsing.

These figures concern formulas at maximum concentrations: they do not automatically apply to every ammonia-free colour, whose dosages and pH vary. They do show, however, that “ammonia-free” describes the alkaliser used, not a level of gentleness. The proven benefit is olfactory comfort during the service, for the client as well as for the colourist.

What does European regulation say?

Ethanolamine falls under Annex III of Regulation (EC) No 1223/2009 (entry 61, monoalkylamines, monoalkanolamines and their salts). The conditions are a minimum purity of 99%, a maximum secondary amine content of 0.5% in the raw material, a maximum nitrosamine content of 50 µg/kg, no use with nitrosating systems and storage in nitrite-free containers. These rules aim to limit the formation of nitrosamines. Like other components of oxidative colours, it can also cause contact allergy, which a dermatologist can investigate with a patch test.

What to keep in mind in the salon?

• Follow the manufacturer’s mixing ratio and developer volume: with a non-volatile alkaliser, excess alkalinity does not disappear on its own.
• Rinse thoroughly, until the water runs clear, to remove as much residual alkaliser as possible.
• Finish with an acidic post-colour treatment, which brings the hair surface back towards its natural pH and helps the cuticle close.
• On sensitised or porous hair, choose the lowest developer volume compatible with the result, and carry out the allergy alert test 48 hours before each application.

In short, ethanolamine allows an oxidative colour without the smell of ammonia, with the same chemical logic of alkaline pH. Its low volatility, which is its advantage, also calls for more care when rinsing and at the end of the service.