» What is Dye
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
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.
Evolution of Dyes
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
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.
How can the color of the Dyes be altered?
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.
Classification of Dyes
Dyes can be classified in various ways, each class has a
very unique chemistry, structure and particular way of bonding. Some dyes
can react chemically with the substrates forming strong bonds in the
process, and others can be held by physical forces. Some of the prominent
ways of classification are given below:
- By area and method of application
- Chemical classification - Based on the nature of their respective
- By nature of the Electronic Excitation (i.e., energy transfer
colorants, absorption colorants and fluorescent colorants).
- According to the dyeing methods
- Anionic (for Protein fibre)
- Direct (Cellulose)
- Disperse (Polyamide fibres)
US International Trade Commission has advocated the most popular
classification of dyes. This system classifies dyes into 12 types, which are
||Cotton, cellulosic and blended fibres
||Cotton, cellulosic and blended fibres
||Cotton, cellulosic fibre
||Cotton, cellulosic, blended fabric, paper
||Cellulosic fibre and fabric
||Wool, silk, paper, synthetic fibres, leather
||Printing Inks and Pigments
||Silk, wool, cotton
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