Dye

A dye is a colored substance that can dissolve in a liquid, called a solvent. This solubility is what makes dyes different from pigments. Pigments are substances that do not dissolve easily in most liquids. Because dyes can dissolve, they can chemically bind to the material they color. This allows the color to become part of the fiber. Most dyes are applied using an aqueous solution, which is a mixture of water and the dye. Sometimes, a substance called a mordant is added to the process. A mordant helps improve the fastness of the dye, which means the color stays on the fiber better and does not wash away easily.

Dyes create color by absorbing specific wavelengths of visible light. The visible spectrum ranges from 380 to 750 nanometers. The specific chemical structure of a dye determines which wavelengths are absorbed and which are reflected. This absorption is the reason we see different colors. If a dye is made insoluble by adding salt, it can become what is known as a lake pigment. This process changes how the color interacts with its environment.

Historically, people found dyes in the world around them. Dyeing textiles dates back to the Neolithic period. Archaeologists found dyed flax fibers in a cave in the Republic of Georgia that are 36,000 years old. In places like India and Phoenicia, dyeing has been a major activity for over 5,000 years. Early dyes came from animals, plants, or minerals with very little processing. Many dyes came from the plant kingdom, including roots, berries, bark, leaves, and wood. Some very rare dyes, like Tyrian purple and crimson kermes, were expensive luxury items. Plant dyes like indigo and saffron were also important for international trade.

A massive shift occurred in 1856 when William Henry Perkin discovered the first synthetic dye. He discovered mauveine, a purple dye, by accident. This event started a surge in the synthetic dye industry and in the field of organic chemistry. Many other aniline dyes followed, such as fuchsine and safranine. Today, most dyes are synthetic and made from petrochemicals. They are used because they are cheaper and have better color and resilience than natural dyes. This discovery also helped medicine. In 1891, Paul Ehrlich found that certain dyes could target specific cells. He used methylene blue to target the organisms that cause malaria.

Scientists classify dyes based on their chemical structures. Azo dyes are the most common and extensive class. They contain an azo group, which is a specific arrangement of atoms.

Almost all colors can be achieved with azo dyes, except for turquoise and a brilliant green. Anthraquinone dyes are another major group. By changing their substituents, or attached groups, they can produce colors from yellow to red and blue to green. Some anthraquinone dyes can be used as vat dyes or disperse dyes for synthetic fibers.

Other specialized groups exist to provide specific colors or strengths. Indigoid dyes belong to the carbonyl group and are often used as vat dyes. Indigo is the most famous example; it was once from plants but is now mostly made in factories. You can see this dye on blue jeans.

Metal complex dyes use metals like copper, chromium, or iron. These dyes have excellent fastness properties. Formazan dyes are structurally related to azo dyes and can form complexes with metals like nickel or cobalt.

Phthalocyanine dyes are copper or nickel complexes that are very good at resisting light damage.

Dyes are also classified by how they are used in technology. Sulfur dyes are water-insoluble and contain disulfide bridges. They are very cheap to produce and are often used to dye cotton. While they have good wash fastness, their colors are often more muted. Nitro dyes are small molecules used as disperse dyes for polyester. Finally, methine dyes use a system of double bonds to create color. These include cyanine dyes and styryl dyes. These different chemical paths allow humans to color almost any material in the world.