Dyes - What exactly are they ?

What are dyes?

Dyes are coloring substances, which unlike the pigments, are dissolved in liquids. They are subdividable according to their origin, their application and structure. They have been known for thousands of years, and as early as 2500 BC, the indigo plant was grown and used for dyeing. In ancient times, one used especially indigo and purple, but also alizarin, henna, kermes, saffron and turmeric. By 1834, colors were extracted from coal tar, and around 1843 the methods of isolating them were greatly expanded. Around 1856, the first synthetic dye, discovered on the basis of Witts dye theory, the purple mauveine. The true purple is now used only for religious purposes, since its extraction from the purple snail is expensive[1].

What are colors and how are they made?

The colors we see are only a very small part of the electromagnetic spectrum. The human eye sees only a wavelength range from 380 (violet) to 780 (red) nm. The UV range (ultraviolet) lies in front of the visible light range, while the infrared range lies after the visible light, both invisible to the human eye. All bodies are colored when they absorb a certain part of the light and reflect another. If a body reflects all wavelengths, it will appear white, as all colors cancel each other out. If a body absorbs all wavelengths, it appears black, since hardly any reflections are absorbed by our eyes. Depending on which wavelengths are absorbed and reflected, the body receives a color for the human eye. Metals are usually shiny and reflective because they scatter light less than e.g. a salt[2].

Why are dyes colored?

Organic dyes get their color through their structure and functional groups. This is described by the Witts dye theory. It states that molecules consist of chromophores, delocalized π-electron systems, + M-effect auxochromic groups, and -M-effect anti-auxochromic groups. All these properties give a substance its coloring. Chromophores are molecules with conjugated double bonds, ie double bonds with a bond distance to each other. For example, in benzene, and therefore in all aromatic compounds. Auxochromic groups are functional groups which have a lone pair of electrons which can donate to the molecule, e.g. Hydroxy or amino groups. They enhance the coloring of a substance. Anti-auxochrome groups are those which have a double or triple bond and therefore increase the electron density. For example, Nitro and carboxylic acid groups. They reduce the coloring of the substance. The mixture of chromophores, auxochromic and anti-auxochromes was cited by Witts as the reason for the coloration, and based on this theory, the first synthetic dye mauvein could be prepared [3].

Where do dyes come from, and what are they used for?

Dyes are often used in the textile industry to dye textiles, so jeans have always been dyed with the dye "indigo". Indigo is a so-called vat dye which has a water-insoluble colored and a water-soluble non-colored form. They are e.g. also used as so-called emulsion paints for paints. There are also a large number of different applications and structures. The best known are the azo, methine, and nitro dyes. They are also used in food and consumer goods. These dyes are titled E numbers from E100 to E163 + E180, and are used in many foods and cosmetics. In food and cosmetics, they are usually to beautify the products, or to give a more appetizing appearance. There is also the distinction between natural and synthetic dyes. Natural ones are e.g. Indigo, Hena, Alizarin, Crocetin, Basiline, Chlorophyll, Carmine, Curcurmin and Riboflavin. Synthetic are e.g. Tatrazine, Quinoline Yellow, Erythrosine, Brilliant Black BN or Litholubin BK. Natural dyes are usually poor in color strength, lightfastness, gloss and durability[4].

What do dyes in cosmetics do?

As with food, they are there to beautify the product and to make it more attractive. Dyes such as chlorophyll (the natural leaf green), or other of its derivatives, mostly with copper instead of magnesium in the center, serve to make many products greener. In addition, many plant extracts still contain their own dyes, and so dye the product without the addition of dyes[5].

Are dyes dangerous?

Many dyes are considered safe, but they can lead to allergies and pseudoallergies. So some people are sensitive to certain dyes. Also, the azo dyes are suspected to cause hyperactivity and attention deficit in children. This was the result of a study in 2007 [6], which, despite some doubts, convinced EFSA to lower the ADIs of 3 of these substances. (ADI = Allowed Daily Intake, EFSA = European Food Safety Authority). In addition, foods containing these dyes must carry the warning "May impair activity and attention in children". The coloring Chinolingelb is forbidden in the USA. The use of azo dyes in food has fallen sharply since the decision of the EU Commission in 2010, as such a warning makes marketing more difficult [7].

Can we just not use or consume dyes?

Only very rarely, since even a product labeled "without dyes" also contains dyes of e.g. May contain fruit juices, which need not be explicitly declared as dyes. But by no means all dyes have negative properties, many are considered completely safe. Dyeing and explicitly avoiding dyeing is difficult, and sometimes superfluous. If one sticks to certain limits, and does not feed exclusively on finished products, but often cooks with fresh ingredients, then one can reduce the dye consumption a good deal[8].

Sources:

Wikipedia [1][2][3][4][5][7][8]

Study on azo dyes [6]

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