Dyes are the colouring material that colour commodities of our day to day use. Hardly there is any industry where dyes are not used commercially. From Plastic toys for children to that jazzy t-shirt of yours, every where there is application of dyes. The page tries to give a comprehensive knowledge about the dyes, its sources and the various types that are used successfully.
What is Dye
By definition Dyes can be said to be coloured, 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 colour 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 atleast 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.
This is a very common question that occurs in everybody's mind. The answer to which is explained by the presence of a substance called Chromophore in the dyes. By definition dyes are basically aromatic compounds. 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.
It is actually because of this reason that chromophores do not make dyes coloured. Rather it makes the dyes proficient in their ability to absorb radiation. Chromophores acts 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 colours and not outside it. Human eyes detects this absorption, and responds to the colours.
Another possibility is that if the electrons are removed from the electron cloud, it may result in loss of colour. Removing electrons 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 pararosanilin, 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 colourless.
To conclude chromophores are the atomic configurations which has delocalised electrons. Generally they are represented as carbon, nitrogen, oxygen and sulphur. They can have alternate single and double bonds.
The answer lies in the Modifiers. Colour modifiers like methyl or ethyl groups can actually alter the colour of dyes. They do so by altering the energy in the delocalised electrons. It has been found that by addition of a particular modifier there is a progressive alteration of colour. An example can be given for methyl violet series.
The following diagram explains what happens to the colour of the dyes when modifiers are added.
Step A: When no methyl group is added the original dye Pararosanil as it is called is red in colour.
Step B: As Four Methyl groups are added the reddish purple dye Methyl Violet is got.
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 got is called Methyl green.
The answer to this riddle lies in substance called Auxochrome. Moreover the Auxochromes has also the ability to intensify colours. 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.
There are several ways for classification of dyes. It should be noted that each class of dye has a very unique chemistry, structure and particular way of bonding. While some dyes can react chemically with the substrates forming strong bonds in the process, others can be held by physical forces. Some of the prominent ways of classification is given hereunder.