Colour and protection: dyes, pigments, paints and coatings: Why things look coloured: chromophores, dyes and pigments
Understanding Chemicals / Module 8: Colour and protection: dyes, pigments, paints and coatings Part 1 of 5Why things look coloured: chromophores, dyes and pigments
Every colorant on the market, from the blue of denim to the white of a wall, works by removing some wavelengths of light and returning the rest. This part covers what makes a molecule coloured, what makes it stick to a fibre, and the one property that splits the business in two: whether the colorant dissolves.
1.1 Colour is the light a molecule does not absorb
Visible light runs from about 400 to 700 nanometres (a nanometre is a billionth of a metre). A substance looks coloured when it absorbs part of that band and reflects the rest; the eye sees the complement of what was taken out. Kent's table makes the point: a dye that absorbs blue light (435 to 480 nm) looks yellow, and one that absorbs green (500 to 560 nm) looks purple.
Freeman and Mock, writing in Kent, give four conditions for colour. The molecule must absorb between 400 and 700 nm, carry at least one chromophore (a colour-bearing group of atoms), have a conjugated system, and show resonance. Conjugation means a chain of alternating single and double bonds along which electrons can spread. Resonance means those electrons can sit in several equivalent arrangements, which stabilises the molecule.
The length of the conjugated chain matters most. Think of a guitar string: a longer string sounds a lower note. A longer run of conjugation absorbs at longer wavelengths. Vitamin A absorbs at 325 nm, in the ultraviolet, so it looks almost colourless. Beta-carotene has roughly twice the conjugated length and absorbs at 466 and 497 nm, which is why carrots are orange. Likewise the azo group (-N=N-) gives no colour between two small methyl groups, but strong colour when it links two aromatic rings, because it then sits inside a long conjugated system.
Aromatic rings, the six-carbon rings of benzene, naphthalene and anthracene, are the natural scaffolding: already conjugated, and easy to bolt groups onto. That is why the synthetic dye industry is built on aromatic hydrocarbons, first from coal tar and now from petroleum. Shreve reports that by the early 1980s under 1% came from coal.
| Wavelength absorbed (nm) | Colour absorbed | Colour seen |
|---|---|---|
| 400-435 | Violet | Yellow-green |
| 435-480 | Blue | Yellow |
| 480-490 | Green-blue | Orange |
| 490-500 | Blue-green | Red |
| 500-560 | Green | Purple |
| 560-580 | Yellow-green | Violet |
| 580-595 | Yellow | Blue |
| 595-605 | Orange | Green-blue |
| 605-700 | Red | Blue-green |
The colour seen is the complement of the band absorbed.
Source: Kent (2012), Ch 13, Table 13.5, pdf p. 526- Kent (2012), Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes · pdf p. 526
- Shreve (1984), Ch 39 Cyclic Intermediates and Dyes · pdf pp. 791-792
1.2 Chromophores give colour; auxochromes make it soluble and make it stick
Shreve describes a dye as having two working parts. The chromogen is the aromatic body carrying a chromophore, the group that creates the absorption band. The auxochrome regulates solubility and dyeing behaviour. Without both, Shreve says, the material is simply a coloured body, not a dye. Dryden writes it as a sum: chromogen plus auxochrome equals dye.
The common chromophores listed by Shreve and Dryden are the azo group (-N=N-), the nitro (-NO2) and nitroso (-NO) groups, the carbonyl group (C=O, central to anthraquinone dyes), carbon-carbon and carbon-nitrogen double bonds, and sulphur-containing groups. The chromophore is also the basis of one way of classifying dyes (azo, anthraquinone, triarylmethane, phthalocyanine); the Colour Index, the industry's catalogue, recognises 26 chemical classes.
The auxochromes are -NH2, -NR2, -OH, -COOH and -SO3H, all salt-forming. Amino groups make a dye soluble in acid; hydroxy, carboxy and sulphonic acid groups make it soluble in alkali. The sulphonate group (-SO3Na) is the workhorse: one or two of them make acid, direct and reactive dyes water-soluble and give them something to bond with on the fibre.
Chromophores differ greatly in strength, measured by the molar extinction coefficient: how strongly a given number of dye molecules absorbs light. Kent gives 5,000 to 7,000 for nitro dyes, 10,000 to 15,000 for anthraquinones, 30,000 to 60,000 for azo dyes and 50,000 to 80,000 for azo dyes built on heteroaromatic amines (rings containing nitrogen or sulphur). Strength is money: four times the extinction coefficient means roughly a quarter as many molecules for the same depth of shade. With easy synthesis, this is why azo dyes are by far the largest family. Anthraquinones survive because they are brighter and more lightfast, meaning they fade less in sunlight.
| Family | Extinction coefficient | Strengths | Weaknesses |
|---|---|---|---|
| Nitro | 5,000-7,000 | Low cost, good lightfastness | Yellow and orange only; weak colour |
| Anthraquinone | 10,000-15,000 | Bright, good lightfastness | Weak colour; less environmentally friendly synthesis |
| Azo (benzene-based amines) | 30,000-60,000 | Strong colour, easy synthesis, whole spectrum | Lightfastness varies with structure |
| Azo (heteroaromatic amines) | 50,000-80,000 | Bright and strong, simpler structures | Low reaction yields, higher cost, lower lightfastness than anthraquinones |
- Shreve (1984), Ch 39 Cyclic Intermediates and Dyes · pdf pp. 802-803, 806
- Dryden (1997), IVF Chemicals from Aromatics (including Dyes) · pdf pp. 550-551
- Kent (2012), Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes · pdf pp. 490, 500-501, 526-527
1.3 Dye or pigment: the dividing line is solubility
Kent's chapter on coatings draws the line cleanly. Organic pigments are insoluble solids. Dyes are coloured organic compounds that are soluble in the medium in which they are used. A dye enters a fibre molecule by molecule and is held there by chemical or physical forces. A pigment stays as a solid particle and needs a binder, a film-forming resin, to glue it to the surface. The same chemistry can sit on either side of the line: an azo molecule with sulphonate groups is a dye, while a similar azo molecule without them, or precipitated as a metal salt, is a pigment.
Pigments come in two families. Inorganic pigments, such as titanium dioxide, iron oxides and chrome colours, generally hide better, resist heat and do not bleed into solvents. Organic pigments give brighter, more saturated colour (higher chroma) and higher tinting strength. Kent calls organic pigments that are insoluble in their pure form toners, and those made insoluble by precipitation with a metal 'lakes'. Shreve adds that a lake is really a dyed inorganic pigment: the organic colour is precipitated onto clay, barytes, blanc fixe (precipitated barium sulphate) or aluminium hydroxide.
The markets differ too. Kent reports that about 80% of dyes go into textiles and 20% into paper, leather, food and similar uses; pigments go mainly into paints, inks and plastics. The Colour Index lists 9,475 colorants, 775 of them pigments, yet only some 1,500 to 2,000 dyes are commercially successful.
On the reader's site, the Dyes tile covers the soluble side and the Organic pigments tile the insoluble organic side, with Titanium dioxide and Carbon black as the two dominant pigments by tonnage.
- Colorants
- Dyes soluble in the medium; enter the substrate
- Textile dyes about 80% of dyes worldwide
- Paper, leather, food and other about 20%
- Pigments insoluble; need a binder
- Inorganic better hiding, heat stable
- Titanium dioxide
- Iron oxides
- Chromates and chrome greens restricted for lead
- Organic brighter, stronger tint, dearer
- Toners insoluble as made
- Lakes dye precipitated with a metal on a base
- Carbon black grouped with organic pigments by Kent
- Inorganic better hiding, heat stable
- Dyes soluble in the medium; enter the substrate
- Kent (2012), Ch 29 Pigments, Paints, Polymer Coatings, Lacquers, and Printing Inks · pdf pp. 1128-1129
- Kent (2012), Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes · pdf pp. 485-487, 510
- Shreve (1984), Ch 24 Surface-Coating Industries · pdf pp. 443, 455-456
See it in the table
H₂O WaterAl AluminiumTi TitaniumNaCl SaltTiO2 Titanium dioxideCB Carbon blackBz BenzeneDye DyesPig Organic pigmentsCheck your understanding
A dye absorbs light most strongly between 435 and 480 nm. What colour does it appear?
It absorbs blue light, so the eye sees the complement, yellow. A dye looks the colour of the light it does not absorb.Which statement best separates an organic pigment from a dye?
Both can share the same chromophores. The difference is physical: a dye dissolves and enters the substrate, a pigment stays a solid particle held by a binder.Why do azo dyes dominate the dye industry?
Kent gives azo extinction coefficients of 30,000 to 60,000 against 10,000 to 15,000 for anthraquinones, and notes their ease of synthesis. Anthraquinones are brighter and more lightfast, which is why they survive in some shades.