Dyes and Pigments

Dyes are substances that can be used to impart color to other materials, such as textiles, foodstuffs, and paper. Unlike pigments, dyes are absorbed to a certain extent by the material to which they are applied. The colors from some dyes are more stable than others, however. A dye that does not fade when the material it was applied to is exposed to conditions associated with its intended use is called a fast dye. Contrariwise, a dye that loses its coloring during proper usage is referred to as a fugitive dye. Some of the conditions that could cause such a change in the properties of a dye include exposure to acids, sunlight, or excessive heat as well as various washing and cleaning procedures. Certain dyes may be considered both fast and fugitive, depending on the material with which they are used.

The process of dyeing is carried out in a variety of ways depending on the specific dye utilized as well as the properties of the material. Silk, wool, and some other textiles may, for instance, be directly dyed by simply dipping them into the colorant. Much more often, however, the use of a reagent known as a mordant is necessary to fix dyes to materials. A number of different compounds may be used as mordants, but metallic hydroxides of tin, iron, chromium, or aluminum are most common. Oftentimes, the color that a particular dye imparts is dependent on the mordant it is utilized with. Another method of dyeing involves the use of vats. For instance, the dye indigo begins as a colorless soluble substance that is dissolved in water in a vat before cloth is dipped into it. When oxygen from the air or another chemical added to the vat comes into contact with the indigo solution, an insoluble blue color results. Batik dyeing, a process that was invented during antiquity in Java, can be used with silks or cottons and involves the application of wax to the cloth before dye treatment in order to create unusual designs and color patterns.

A dye can generally be described as a coloured substance that has an affinity to the substrate to which it is being applied. The dye is usually used as an aqueous solution and may require a mordant to improve the fastness of the dye on the fibre. (In contrast, a pigment generally has no affinity for the substrate, and is insoluble)

Archaeological evidence shows that, particularly in India and the Middle East, dyeing has been carried out for over 5000 years. The dyes were obtained from either animal, vegetable or mineral origin with no or very little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood, but only a few have ever been used on a commercial scale.

The first man made organic dye, mauveine, was discovered by William Henry Perkin in 1856. Many thousands of dyes have since been prepared and because of vastly improved properties imparted upon the dyed materials quickly replaced the traditional natural dyes. Dyes are now classified according to how they are used in the dyeing process.

Acid dye - Water soluble anionic dyes that are applied to fibres such as silk, wool, nylon and modified acrylic fibres from neutral to acid dyebaths. Attachment to the fibre is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fibre. Acid dyes are not substantive to cellulosic fibres.

Basic dye - Water soluble cationic dyes that are applied to wool, silk, cotton and modified acrylic fibres. Usually acetic acid is added to the dyebath to help the take up of the dye onto the fibre. Basic dyes are also used in the coloration of paper.

Direct (Substantive) dye - Dyeing is normally carried out in a neutral or slightly alkaline dyebath, at or near the boil, with the addition of either sodium chloride (NaCl) or sodium sulphate (Na2SO4). Direct dyes are used on cotton, paper, leather, wool, silk and nylon. They are also used as pH indicators and as biological stains.

Mordant dye - As the name suggests these dyes require a mordant. This improves the fastness of the dye on the fibre such as water, light and perspiration fastness. The choice of mordant is very important as different mordants can change the final colour significantly. Most natural dyes are mordant dyes and there is therefore a large literature base describing dyeing techniques.
Vat dye - These dyes are essentially insoluble in water and incapable of dyeing fibres directly. However, reduction in alkaline liquor produces the water soluble alkali metal salt of the dye. In this leuco form these dyes have an affinity for the textile fibre. Subsequent oxidation reforms the original insoluble dye.

Reactive dye - First appeared commercially in 1956 and were used to dye cellulosic fibres. The dyes contain a reactive group that, when applied to a fibre in a weakly alkaline dyebath, form a chemical bond with the fibre. Reactive dyes can also be used to dye wool and nylon, in the latter case they are applied under weakly acidic conditions.

Disperse dye - Originally developed for the dyeing of cellulose acetate. They are substantially water insoluble. The dyes are finely ground in the presence of a dispersing agent then sold as a paste or spray dried and sold as a powder. They can also be used to dye nylon, triacetate, polyester and acrylic fibres. In some cases a dyeing temperature of 130 deg C is required and a pressurised dyebath is used. The very fine particle size gives a large surface area that aids dissolution to allow uptake by the fibre. The dyeing rate can be significantly influenced by the choice of dispersing agent used during the grinding.

Azoic dye - A dyeing technique in which an insoluble azo dye is produced directly onto or within the fibre. This is achieved by treating a fibre with a diazo component and a coupling component. With suitable adjustment of dyebath conditions the two components react to produce the required insoluble azo dye. This technique of dyeing is unique in that the final colour is controlled by the choice of the diazo and coupling components.

One other class which describes the role dyes have rather than their mode of use is food dyes. This is a special class of dyes of very high purity. They include direct, mordant and vat dyes. Their use is strictly controlled by legislation. Many are azo dyes but anthraquinone and triphenylmethane compounds are used for colours such as green and blue. Some naturally occurring dyes are also used.
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Pigments are the basis of all paints, and have been used for millennia. They are ground colored material. Early pigments were simply as ground earth or clay, and were made into paint with spit or fat. Modern pigments are often sophisticated masterpieces of chemical engineering. In biology, pigment is any color in plant or animal cells. Nearly all types of cells, such as skin, eyes, fur and hair contain pigment. Creatures that have deficient pigmentation are called albinos.

In the coloring of paint, ink, plastic, fabric and other material, a pigment is a dry colorant, usually an insoluble powder. There are both natural and synthetic pigments, both organic and inorganic ones. Pigments work by selectively absorbing some parts of the visible spectrum (see light) whilst reflecting others.

A distinction is usually made between a pigment, which is insoluble, and a dye, which is either a liquid, or is soluble. There is no well-defined dividing line between pigments and dyes, however, and some coloring agents are used as both pigments and dyes. In some cases, a pigment will be made by precipitating a soluble dye with a metallic salt. The resulting pigment is called a "lake".

Pigments are chemical compounds which reflect only certain wavelengths of visible light. This makes them appear "colorful". Flowers, corals, and even animal skin contain pigments which give them their colors. More important than their reflection of light is the ability of pigments to absorb certain wavelengths.

Because they interact with light to absorb only certain wavelengths, pigments are useful to plants and other autotrophs --organisms which make their own food using photosynthesis. In plants, algae, and cyanobacteria, pigments are the means by which the energy of sunlight is captured for photosynthesis. However, since each pigment reacts with only a narrow range of the spectrum, there is usually a need to produce several kinds of pigments, each of a different color, to capture more of the sun's energy.

There are three basic classes of pigments.

Chlorophylls are greenish pigments which contain a porphyrin ring. This is a stable ring-shaped molecule around which electrons are free to migrate. Because the electrons move freely, the ring has the potential to gain or lose electrons easily, and thus the potential to provide energized electrons to other molecules. This is the fundamental process by which chlorophyll "captures" the energy of sunlight.

There are several kinds of chlorophyll, the most important being chlorophyll "a". This is the molecule which makes photosynthesis possible, by passing its energized electrons on to molecules which will manufacture sugars. All plants, algae, and cyanobacteria which photosynthesize contain chlorophyll "a". A second kind of chlorophyll is chlorophyll "b", which occurs only in "green algae" and in the plants. A third form of chlorophyll which is common is (not surprisingly) called chlorophyll "c", and is found only in the photosynthetic members of the Chromista as well as the dinoflagellates. The differences between the chlorophylls of these major groups was one of the first clues that they were not as closely related as previously thought.

Carotenoids are usually red, orange, or yellow pigments, and include the familiar compound carotene, which gives carrots their color. These compounds are composed of two small six-carbon rings connected by a "chain" of carbon atoms. As a result, they do not dissolve in water, and must be attached to membranes within the cell. Carotenoids cannot transfer sunlight energy directly to the photosynthetic pathway, but must pass their absorbed energy to chlorophyll. For this reason, they are called accessory pigments. One very visible accessory pigment is fucoxanthin the brown pigment which colors kelps and other brown algae as well as the diatoms.

Phycobilins are water-soluble pigments, and are therefore found in the cytoplasm, or in the stroma of the chloroplast. They occur only in Cyanobacteria and Rhodophyta.
The picture at the right shows the two classes of phycobilins which may be extracted from these "algae". The vial on the left contains the bluish pigment phycocyanin, which gives the Cyanobacteria their name. The vial on the right contains the reddish pigment phycoerythrin, which gives the red algae their common name.

Phycobilins are not only useful to the organisms which use them for soaking up light energy; they have also found use as research tools. Both pycocyanin and phycoerythrin fluoresce at a particular wavelength. That is, when they are exposed to strong light, they absorb the light energy, and release it by emitting light of a very narrow range of wavelengths. The light produced by this fluorescence is so distinctive and reliable, that phycobilins may be used as chemical "tags". The pigments are chemically bonded to antibodies, which are then put into a solution of cells. When the solution is sprayed as a stream of fine droplets past a laser and computer sensor, a machine can identify whether the cells in the droplets have been "tagged" by the antibodies. This has found extensive use in cancer research, for "tagging" tumor cells.