What is the difference between a pigment and a dye?

Pigments and dyes are typically distinguished by their difference in solubility. Indeed, it is customary to consider that dyes are soluble substances in the medium in which they are introduced, while pigments are finely divided crystalline solids that present themselves in the form of insoluble powders. This difference in solubility impacts the colouring process, which varies for dyes and pigments.

Thus, dyes are coloured substances that dissolve during the application process and impart colour solely through the selective absorption of light. Pigments, on the other hand, allow a solution, textile or cosmetic product to be coloured by selective absorption and/or scattering of light. Unlike dyes, whose properties are almost exclusively defined by their chemical structure, the properties of pigments also depend on the physical characteristics of their particles, such as their size and shape.

Definitions:

• Selective Absorption : The principle of selective light absorption is based on the molecular structure of dyes and certain pigments. The double bonds present in their structure allow them to absorb certain wavelengths of visible light, which causes a electronic transition, that is, the movement of an electron from one energy level to another. This electronic transition is the origin of the perceived colour.

• Light Scattering : a colouring method unique to pigments, light scattering is based on the ability of pigments to reflect visible light. This property depends on their physical properties, that is, the size and shape of the particles that make up the pigment powder.

Beyond solubility, there are other differences between pigments and dyes, starting with their chemical nature. Pigments are primarily inorganic salts and oxides, such as iron and chromium oxides, typically dispersed as crystals or powder in an application medium. Pigments also tend to be more resilient than dyes. Indeed, they are very durable and stable to heat and light.

Furthermore, dyes are organic compounds, whereas this is far from systematic for pigments. Indeed, it is estimated that 95% of pigments are inorganic molecules. As a reminder, a compound is organic if and only if it includes carbon atoms in its structure.

Despite the clues provided above, it is not always easy to distinguish between pigments and dyes in the cosmetic field, and there is indeed no official distinction between these two compounds in Regulation (EC) No 1223/2009 of the European Parliament and the Council. In fact, in this regulation, all dyes and pigments are grouped under the generic term "colourant", defined as follows: "substances that are exclusively or mainly intended to colour the cosmetic product, the whole body or certain parts of it, by absorption or reflection of visible light; precursors of oxidation hair dyes are also considered as colourants".

The Regulation (EC) No 1223/2009 permits the use of approximately 150 pigments and dyes in cosmetic products, as listed in Annex IV. There are 4 regulatory divisions:

• The colouring agents permitted in all cosmetic products.

• Colouring agents are permitted in all cosmetic products except those intended to be applied around the eyes.

• Colouring agents are exclusively authorised in products that are not intended to come into contact with the mucous membranes.

• Colouring agents are only permitted in products that have brief contact with the skin, that is to say, rinse-off products.

These pigments and dyes are listed in the directive by their colour index number, or Colour Index (CI), and are thus named in the INCI ingredient lists of products. The colour index number provides valuable information about the origin of the colouring agents. Indeed, the mineral dyes and pigments have a CI starting with CI 77XXX, with the exception of carbon (CI 77266). Notable examples include CI 77891 White (Titanium Dioxide), CI 77492 Yellow (Iron Oxide) or CI 77007 Blue (Lazurite). The natural pigments and dyes are identified by their CI starting with 75XXX, such as CI 75370 Carmine Red (Cochineal) or CI 75810 Green (Chlorophyll). The nomenclature is then more diverse for organic dyes and pigments, such as CI 19140 Yellow (azoic), CI 14700 Red (azoic) and CI 60730 Purple (anthraquinone).

• Regulation (EC) No 1223/2009 of the European Parliament and of the Council. Official Journal of the European Union (2009).

• GÜRSES M. S. & al. Dyes and Pigments: Their Structure and Properties. Dyes and Pigments (2016).

• GARCIA JARES C. & al. Analysis of Dyes in Cosmetics: Challenges and Recent Developments. Cosmetics (2018).

• HEFFORD R. J. W. Colourants and dyes for the cosmetics industry. Handbook of Textile and Industrial Dyeing (2011).