What is Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber.

Dyes are colored, ionising and aromatic organic compounds which shows an affinity towards the substrate to which it is being applied. It is generally applied in a solution that is aqueous. Dyes may also require a mordant to better the fastness of the dye on the material on which it is applied.

At the very basic level the use of color in identifying individual components of tissue sections can be accomplished primarily with dyes. Although there are other means, Dyes are however, the largest group that can easily be manipulate to our liking. Dyes are applied to numerous substrates for example to textiles, leather, plastic, paper etc. in liquid form. One characteristic of dye is that the dyes must get completely or at least partially soluble in which it is being put to. The rule that we apply to other chemicals is similarly applicable to dyes also. For example certain kind of dyes can be toxic, carcinogenic or mutagenic and can pose as a hazard to health.

The real breakthroughs in the history of dyes came in 1856 when a teenager who was experimenting at his makeshift laboratory in home made a certain discovery that acted as a sort of launching pad for the modern chemicals industry.

William Perkin an 18-year-old student was working on chemical synthesis of natural products. In a classic case of serendipity, the young William Perkin chanced upon his now famous 'Aniline Mauve' dye while he was attempting to synthesize quinine, the only cure for malaria. Perkin named his color Mauveine, after the French name of non-fast color which was made of natural dyes. So "Mauve" (a basic dye) was the first synthetic dye stuff. Mauve was a derivative of coal tar. It was the first mass-produced dye, that was commercially available and the idea was born that a color could be made in the factory. It was indeed a revolution.

It was in 2600 BC when earliest written records of the use of dyestuffs were found in China. The preparation and application of dyestuffs is one of the oldest forms of human activities. Evidences of which were found by Excavation at archeological sites where ancient fabrics were unearthed. There is also mention of it in the Bible and other works of classical antiquity.

What makes the Dyes colored?
Chomophores make the dyes proficient in their ability to absorb radiation. Chromophores act by making energy changes in the delocalised electron cloud of the dye. This alteration invariably results in the compound absorbing radiation within the visible range of colors and not outside it. Human eyes detects this absorption, and responds to the colors.

Dyes are basically ionising and aromatic compounds, they have Chromophores present in them. Their structures have Aryl rings that has delocalised electron systems. These structures are said to be responsible for the absorption of electromagnetic radiation that has varying wavelengths, based upon the energy of the electron clouds.

Electrons may result in loss of color, their removal may cause the rest of the electrons to revert to the local orbits. A very good example is the Schiff's reagent. As Sulphurous acid reacts with Pararosaniline, what happens is that a Sulphonic group attaches itself to the compound's central carbon atom. This hampers the conjugated double bond system of the Quinoid ring, and causes the electrons to become localised. As a consequence the ring ceases to be a Chromophore. As a result, the dye becomes colorless.

The color of the dyes are altered by the Modifiers. The Color modifiers of methyl or ethyl groups are responsible for any alteration in the dyes; they alter the energy in the delocalised electrons. There is a progressive alteration of color by adding a particular modifier. For example: Methyl Violet Series.

The following diagram explains what happens to the color of the dyes when modifiers are added.

Step A : When no methyl group is added the original dye Pararosaniline as it is called is red in color.

Step B : As Four Methyl groups are added the reddish purple dye Methyl Violet is obtained.

Step C : With the addition of more groups a purple blue dye Crystal Violet is obtained. It has in it six such groups.

Step D : Further addition of a seventh methyl group the dye that is obtained is called Methyl green.

What gives the Dyes Solubility and Cohesiveness?
The presence of an auxochrome in the chromogen molecule is essential to make a dye. However, if an auxochrome is present in the meta position to the chromophore, it does not affect the color.

Auxochrome, the only substance responsible for providing solubility and cohesiveness to dyes. An auxochrome is a group of atoms attached to a chromophore which modifies the ability of that chromophore to absorb light. Examples include the hydroxyl group (-OH), the amino group (-NH₂), and an aldehyde group (-CHO).

Auxochrome has the ability to intensify colors. It is a group of atoms which attaches to non-ionising compounds yet has the ability to ionise. Auxochromes are of two types, positively charged or negatively charged.