Direct hair dyes are molecules that are already coloured and bind to the hair fibre without any oxidation reaction and without developer. Two families coexist in semi-permanent colour masks: cationic dyes, identified in the INCI by the prefix “Basic” (Basic Red 51, Basic Blue 99, Basic Yellow 87), which bind to the anionic sites of keratin through electrostatic attraction, and nitro dyes, identified by the prefix “HC” (HC Blue 2, HC Yellow 2, HC Blue 16), small neutral molecules that diffuse into the fibre and are held there by weak forces.

These two mechanisms explain why a colour mask never lightens, why the same mix gives different results depending on the starting base, and why the colour fades wash after wash. Hairswiss looks here at the chemistry of these molecules and its consequences in the salon.

What distinguishes a direct dye from an oxidation dye?

In oxidation colour, colourless precursors such as para-phenylenediamine react with hydrogen peroxide in an alkaline medium. The coloured molecules form inside the cortex and become too large to leave it: their entrapment is what makes the colour permanent, as explained in our article on the chemistry of professional hair coloration.

A direct dye follows the opposite logic. The molecule is coloured before application, undergoes no chemical reaction and is used without any oxidant. The direct consequence: without peroxide, melanin is not degraded. A direct dye adds colour; it never takes any away.

How do cationic dyes bind to keratin?

Keratin carries carboxyl and sulfonate groups that are negatively charged at the pH of hair formulations. Cationic dyes, by contrast, carry a permanent positive charge. The review published in Cosmetics in 2015 summarises the mechanism: the positive sites of the dye molecule bind to the negative sites of the fibre through an ionic bond.

It is the same principle of electrostatic attraction that allows conditioning polymers to deposit on hair, described in our article on Polyquaternium. The same review states that cationic dyes resist washing better than nitro dyes and that, because their molecules are of similar size, they bind and wash out evenly.

These substances are regulated by Annex III of Regulation (EC) No 1223/2009. For Basic Red 51, for example, the maximum concentration is 1.0% in non-oxidative hair dye products and 0.5% on the hair after mixing under oxidative conditions, with mandatory label warnings about allergic reactions.

How do HC nitro dyes work?

Nitro dyes are neutral aromatic amines or anthraquinone derivatives, highly polar but poorly soluble in water. They need a glycol-type solvent, which explains why Propylene Glycol or Glycerin so often appear in the formulas that contain them.

Having no charge, they do not anchor through an ionic bond: they diffuse into the fibre and are held there by Van der Waals forces. Their durability depends on their size: according to the review cited above, molecules with three aromatic rings wash out more slowly than those with a single ring.

Their regulation also covers purity. HC Blue 16 is permitted up to 3.0% in non-oxidative hair dye products, provided it is not used with nitrosating agents, its nitrosamine content stays at or below 50 µg/kg and it is kept in nitrite-free containers.

Why combine both families in one formula?

Formulators often combine nitro and cationic dyes to obtain a more even result. The principle described in the literature is complementary: the cationic dyes first occupy the anionic sites of the fibre, then the nitro dyes fill the remaining space by diffusion. The combination also widens the available colour range, since each molecule brings its own absorption band. For example, Gems 118 Graphite, available on cliCHair.ch, combines cationic dyes (Basic Blue 99, Basic Red 51, Basic Yellow 87) with HC nitro dyes (HC Blue 2, HC Blue 16, HC Yellow 2).

Why does bleached hair take up more colour?

Bleaching oxidises part of the cystine in keratin. This produces cysteic acid, which carries an anionic sulfonate group and is concentrated mainly in the cuticle. A study published in the Journal of Cosmetic Science showed that the higher the cysteic acid content of the hair, the greater the amount of cationic compounds adsorbed, and that oxidatively damaged hair adsorbs cationics more readily than reductively damaged hair.

In the salon, this shows up as porous ends that take the colour faster and more intensely than the roots. Uneven porosity gives an uneven result, even with a perfectly homogeneous mix.

Why does the starting base decide the shade obtained?

A direct dye removes no pigment: its colour adds to the colour already in the fibre. The result follows the rules of subtractive colour mixing, the same rules that allow blue-violet pigments to neutralise yellow tones. A blue dye on a yellow base turns greenish; a pastel shade only appears true to the chart on a very light, even base.

This is also why colour mask charts show the shades on a light strand: they show the dye alone, without interference from underlying lightening pigment.

What happens when a colour mask is diluted?

Diluting a colour mask with a colourant-free base lowers the concentration of coloured molecules in the mix. Less available dye means fewer occupied sites on the fibre and a lower optical density: the intensity drops while the molecules stay the same. This is how pastels are obtained from intense shades. An example of such a pigment-free base is Gems C Clear, available on cliCHair.ch.

The stronger the dilution, the less the dye covers the base: a highly diluted mix therefore needs an even lighter, more uniform base.

How long does a direct colour last?

Without a covalent bond, direct dyes gradually leave the fibre on contact with water and surfactants. The review published in Cosmetics puts the durability of a semi-permanent colour at three to six shampoos, compared with around twenty for demi-permanent colours, which combine direct dyes and oxidation precursors with hydrogen peroxide.

For the architecture of the fibre these molecules bind to, the cuticle and cortex are described in our article on hair biology.

What are the key points?

• Cationic dyes (Basic): positive charge, ionic bond with the anionic sites of keratin, better wash resistance.
• Nitro dyes (HC): neutral, polar molecules, diffusion into the fibre, retention by Van der Waals forces.
• Bleached hair: cysteic acid increases the anionic sites and therefore the uptake of cationic dyes.
• Starting base: without an oxidant nothing is lightened; the colour obtained is the sum of the base and the dye.
• Dilution: less dye, less intensity, same molecules.