THE CHEMISTRY OF DYES

Difference between dye and pigment:

Dyes have an affinity for the substrate (fibre) they are applied to. Dyes are either
soluble or dispersible in a solvent (the particles of dispersed dyes are essentially aggregates of
a few molecules).
Pigments have no affinity for the substrate they are applied to and tend to present as
an insoluble suspension in a drying 0.1 or other resinous vehicles. Pigment particles also tend
to be of the order of 1 m in diameter.

Basis of colour
We say something is coloured if it reflects or absorbs electromagnetic radiation in a narrow
region – the visible spectrum. This is the only region in the entire electromagnetic spectrum
which our eyes can detect. The effective range is 380 – 720nm. The ultraviolet region starts
below 360nm, and the infrared region starts above 780nm.
Hue: The hue refers to the major wavelength or wavelengths reflected from the materials.
Different wavelength indicates different hues. Approximately 150 hues can be detected in the
visible spectrum.
Brightness: The brightness of any substance depends on the amount of reflected light.
Strength: The strength of a coloured surface is inversely proportional to the amount of white
light the surface reflects. This is because white light dilutes the wavelengths which give the
surface its hue.
Colour fastness: This is the ability of the dye to resist fading. The causes of fading are uv
radiation from sunlight, and washing.
Fibre properties: The knowledge of the functional groups on different fibres can help give
an indication of the type of dye that should be used, as well as suggest ways of developing
new dyes. This will be discussed later in detail later.

The different colours of white light

Sunlight is reflected by atmospheric water, producing band of red orange, yellow,
green, blue, indigo and violet (rainbow). If a light source is deficient in any colour band, the
light appears to be coloured in the complementary colour. The table below shows the
wavelength, the corresponding colour, and its complementary colour:
Wavelength range (nm) Colour absorbed Complementary colour
(colour seen)
400 – 435 Violet Green – yellow
435 – 480 Blue Yellow
480 – 490 Green – blue Orange
490 – 500 Blue – green Red
500 – 560 Green Purple
560 – 580 Yellow – green Violet
580 – 595 Yellow Blue
595 – 605 Orange Green – blue
605 - 750 Red Blue – green

The figure 1 above show the effect on white light reflected off a solid object

Absorbs
Blue Looks yellow

The figure 2 above shows the effect on white light transmitted through a solution or other

transparent articles

The effect of molecular energy levels

This section will examine what exactly causes an object to be coloured.
 Molecules contain at least two atoms and these are capable of certain limited motions
relative to each other. The internal energy of a molecule (excluding translational energy) may
be written
Total energy = E = E rotation + E vibration + E electron
Molecular rotation is responsible for the absorption of radiation in the far infared. A
combination of molecular rotation and vibration causes absorption in the near infrared. The
absorption in the visible and ultra violet regions, with which dyes and related substances are
concerned is electronic in origin. Colourless organic substances absorb radiation in the UV,
relatively high energies being required to raise the molecule from to ground state E0 to the
excited state E1.

∆E = E1 – E0 = hv =

Where v = frequency
= wavelength of absorbed radiation
h = plank’s constant. C = speed if light
In saturated aliphatic compounds the electrons concerned with bond formation are tightly
bound and ∆E is thus large. In ethylene CH2 = CH2, ∆E is about 160k cal/mole and absoption
occurs in the UV at 1750 A0. As more and more unsaturated linkages occur in the molecule,
mas moves towards longer wavelengths.
For any substance to be coloured its molecule must contain mobile electrons which
can be raised from ground to excited state.
The several ways in which molecules can appear to be coloured can be summarized thus:
 Simple molecular excitation, such as in a neon tube, may cause the appearance of
colour. This is due to rotation and/or vibration of the molecules.
 Transition metal complexes are coloured due to the distortion of the metals d –
electron shell caused by ligands surrounding the metal ion.
 Electronic motion in conjugated organic systems, and charge transfer.
 Colour in crystalline solids arises from band theory – the blurring of many orbitals
throughout the solid. Solids are only coloured if the gap between the Highest occupied
molecular orbital (HOMO, the Fermi level) and the lowest unoccupied molecular
orbital (LUMO) is small enough.
 Colour due to refraction, scattering, dispersion and diffraction these are all due to the
geometrical and physical dimensions of a solid or a solution
What affects the colour quality of the dye
 The hue of a dye depends on the wavelength it absorbs. Since the wavelengths the dye
absorbs depends on its structure we can see that any change which affects the π-system will
affect the hue. A structural change which causes the absorption band, to move to longer
wavelengths (i.e yellow > orange > red - > violet > blue - > green is called a bathochromic
shift. The reverse shift towards shorter wavelengths is known as a hypsochromic effect.
Theory of Witt. O.N (1876)
Witt regarded the dye molecule as a combination of an unsaturated kernel with certain groups
called chromophores, such a combination being called a “chromogen” and one or more
characteristic substituent groups called auxochromes, the function of which was to intensify
colour and to improve the affinity of the dye for the substrate (fibre, yarn, cloth, plaster or in
fact any material which is to be coloured).

Examples of chromophores are:

 N=N– azo group
 NO nitroso group
 NO2 nitro group

-NH2, - NHMe, - NMe2, - generally as cations e.g. = +NMe2Cl

SO3H, - OH, - COOH - often as anions e.g. – O-, - S03-
Example is azobenzene

The above compound is an “unsaturated benzenoid” no auxochrome is present, however, and

although coloured, the substance is useless as a dye ‘having no aptitude for imparting colour
to a substrate. But if the substance contain an auxochrome as substituent such as dimethyl
amino group – NMe2, it will be strongly coloured as is used as a dye
Classification of dyes
Dyes can be classified in two ways:
a. According to its chemical structure
b. According to how it is applied to materials.
The first is of more interest to chemists who want to know what makes the molecule the
colour it is while the second is of interest to the dyer who need to know which dye is suitable
for the material they need to dye and the resulting colour.
The colour index
This is published by the society of dyers and colourist. Every commercial dye and
pigment in it is given a C.I. Generic name, which includes its application class, its hue and a
number which indicates its chronological discovery for example:
CI Acid violet 43 (CI 60730), CI vat Red 41 (CI 73300)
STRUCTURAL CLASSIFICATION
In this method, dye molecules are grouped according to shared structural groups, for
example, the azo dyes, which have the general structure:

Azo dyes are the most important of the dye classes with the largest range of colours. All azo
dyes contain at least one –N=N- group. The next most important dye class contains carbonyl
functions (- C = O). This group includes anthraquinones.

Classification according to application

Before the chemistry of dyeing and printing is discussed, the substrates for dyes will
be discussed. These are made up of both natural and synthetic fibres.
Natural fibres
Cellulosic: Cotton consists of almost pure cellulose; this is the most abundant of all organic
polymers, natural or otherwise. Cellulose is a linear polymer of glucose molecules.

The – OH group, size and configuration of the glucose units, are of great importance in
physics and chemistry of cotton dyeing.
Protein: wool is a protein fibre and is a polymer having as its structural units about 21
different - amino acids. This can be represented diagrammatically as:

The monomeric units of protein fibre are

R.CH NH2 Rʹ-CH. NH2
CO2H CO2H etc.
The R, Rʹ contain free amino (– NH2) groups or free carboxyl (– CO2H) groups that didn’t
take part in the formation of peptide bonds. These functional groups provide basic and acidic
centres important in wool dyeing.
Synthetic fibres
There are three major types:
Polyamide: example of this is Nylon 6, 6 obtained by condensation between adipic and
hexamethylene diamine

Polyester: This is a condensation reaction between Terephthalic acid and ethyleneglycol

Polyacrylonitrile: This is obtained from the polymerization of acrylonitrile, CH2 = CH2.CN

Dyeing
Dyeing is in general carried out in aqueous solution. The process of attaching dye to the fibre
is adsorption.
There are four kinds of forces by which dye molecules are bond to the fibre
(1) Ionic force (2) hydrogen bonds (3) Van der Waals’ forces and (4) covalent linkages.
Ionic forces: This is interaction between positive centre in a fibre and negative centres in a
dye molecule and vice versa. A good example of this is the dyeing of wool.
A “free” amino group and a free carboxyl group in wool can be represented thus:

Most wool dyes are the sodium (or other metal) salts of sulphonic acid, dyeing being carried
out in the presence of acid e.g H2SO4 represented by HX:

The action f the dye, designated NaD

Hydrogen bonds: This results from the acceptance by a covalently bond hydrogen atom of a
“lone - pair” of electrons from an election donor atom. e.g.

This bind does exist between dye and fibre.

Examples of election donor groups are:

Examples of acceptor groups (i.e their hydrogen atoms)

Examples of electron acceptor groups (I.e their hydrogen atoms):

Hydrogen bond is not thought probable with cellulose, since the affinity through hydrogen
bonding for water molecules of the amorphous areas of the fibre is so great that the dye
molecule is not able to displace these water molecules. The affinity of a dye for cellulose
must be explained on other grounds.
Van der Waal’s forces: These are weak forces existing between atoms or molecules of all
substances. In the dyeing process they are the result of 2nd – order wave mechanical
interaction of orbitals of dye and fibre molecules.
Covalent linkages: These are actual chemical bonds between dye and substrate molecules.
They occur by chemical reaction between a “reactive” dye molecule and for example a
hydroxyl group of a cotton fibre

Fastness properties
The ability or otherwise of a dye, in association with a given substrate to withstand the
various agencies in processing or use is called its fastness properties.
Colour fastness: This means the resistance of the hue of textiles to the different agencies to
which they may be exposed during manufacture and subsequent use.
Light fastness: This is assessed on a scale of eight. 1 is the least fastness while 8 the best.
A specimen of dyed or printed textile is exposed to daylight along with eight dyed
wool standards. If the specimen has faded more than wool standard rating 6 but less than
rating 4 them the light fastness of the dye is assessed at 5.

Classification of dyes according to application

i. Sulphur dyes: are used mainly for cellulosic fibres dyeings having moderate all-round
fastness. They are applied on the fibres in water soluble form using a sodium sulphide
and reoxidized to shade on the fibre by contact with air.
ii. Azoic dyes: The term is used for a system of producing an insoluble azo dye in situ i.e.
on the fibre. A coupling component is first impregnated into the fibre which is later
brought into contact with a solution of the second intermediate (diazo component)
leading to the formation of insoluble dye.
iii. Ingrain dyes: This applies to all types of dyes formed in situ on the substrate by the
development or coupling of intermediate compounds which are not themselves true
finished dyes. This includes Azoic and oxidation dyes.
iv. Acid dyes: They find their application in the textile filed on wool; they can also be used
on silk, polyamide and regenerated protein fibres, paper and leather. These dyes are
applied from a dye liqour containing H2SO4, formic, or acetic acids, neutral and slightly
alkaline dye bath are used occasionally. Chemical types (i.e. dye composition) involved
are azo, anthraquinone, triarylmethane, azine, xanthene, ketonimine, nitro and nitroso
compounds. The dyes include those giving very bright hues and a wide range of
fastness properties from very poor to very good.
v. Mordant dyes: These dyes cover dyes sold under such names as mordant dyes, chrome
dyes, metachrome dyes after chrome dyes, chrome printing colours. This group doesn’t
include the basic dyes which are dyed on tannin – antimony mordanted cotton, a
mordants being a substance, e.g. tannic acid, with which cloth (cotton) must be treated
before being dyed, the dye otherwise having no affinity for the fibre.
Mordants act as fixing agents to improve the colour fastness of some acid dyes which have

the ability to form complexes with metals ions. Mordants are usually metal salts; alum [KAL

(SO4)2. H2O] was commonly used for ancient dyes but there is a large range of other metallic

salt mordants available. Each gives a different colour with any particular dye, by forming an

insoluble complex with the dye molecules.