Dyes and solvatachromism Flashcards
1
Q
Dyes
A
- λ of light absorbed depends on chemical structure
- Molecules of coloured organic compounds contain two parts:
1. An aromatic ring or a fused ring system with extensive electron delocalisation e.g.
look at ppt
- An extensive conjugated double bond system containing unsaturated groups, known as chromophores, e.g
azo carbonyl nitroso ethylene nitrito
look at ppt
2
Q
The intensity of colour can be increased by
A
- substituents containing lone pairs of electrons to the aryl ring ‘auxchromes’
look at ppt
– the entire structure of the colorant is called the chromogen
3
Q
approximate relative strengths of bonding between a dye and a fabric
A
look at ppt
4
Q
Classification of colorants
A
- The Colour Index International, is a comprehensive list of known commercial dyes and pigments and is updated regularly.
- Each colorant is given a Colour Index (C.I.) Name and Number e.g. C.I Acid Red 37
- Classification is by chemical structure and by method of application
- 25 structural classes according to chemical type.
- Most important are:
- azo dyes, b) anthraquinone dyes, c) phthalocyanines
5
Q
Azo dyes
A
- Azobenzene is the chromophore of azo dyes.
- Colour can be modified and intensity of the colour increased by varying the auxochromes
look at ppt
- The molecular structures of some azo dyes showing the auxochrome
6
Q
Anthraquinone dyes
A
- Based on anthracene
look at ppt
7
Q
Phthalocyanines
A
Phthalocyanines are made up of four
molecules of isoindole:
coordinate with metal atoms.
The most important, (2% of all colorants), are the copper phthalocyanines, used for their brilliant blue and green colours e.g. C.I. Direct Blue 86:
Sulphonic acid group helps water solubility
8
Q
Classification of colorants by methods of application
A
- To obtain the required shade the dyer makes mixtures of dyes and must ensure that these are compatible.
Dye transfer from solution to fibre is controlled by: - pH of the dyebath (for acid and basic dyes)
- an electrolyte (a solution of sodium sulfate or chloride)
- the temperature (within the range of ambient to 400 K)
- dispersing agents (surfactants) that produce a stable aqueous dispersion of dyes if they are of low solubility
9
Q
AUXILIARIES for TEXTILES
A
- • Antifoams e.g. silicones
- • Levelling and anti-migration agents
- • Wetting and deaerating agents
- • Lubricants and anticrease
- • Dispersing and sequestering agents
- • Detergent and washing agents e.g. non-ionic or anionic detergents
- • Antistatic agents
- • Dye Fixative
- • Corrosion inhibitors
- • Stabilizer ,pH buffer
- • Dye fixative stripper
- • Fire retardant
- • Printing auxiliaries
- • Oil and water repellents
- • Catalysts
- • Odour masking
- • Moisture management agents
- • Anti-slip agents
- • Elasticity control agents
look on ppt
10
Q
Example of direct dye for dying cotton
A
- Linear and planar structure binding to cellulosic chains in the fibre, via intermolecular (including hydrogen) bonding.
look at ppt
11
Q
Reactive dyes
A
- fast dyes that would covalently bond to fabrics rather than by weak intermolecular forces.
- During the 1950’s, chemists at ICI modified the structure of azo dyes by adding reactive groups (trichlorotriazane) to combine with the amino groups of proteins in wool.
- dye molecules reacted with both the amine and hydroxyl groups on cotton fibres under alkaline conditions (not so good for wool as it dissolves in alkali) with the elimination of molecules of hydrogen chloride
12
Q
basic dyes
A
- Earliest synthetic dye e.g. Mauveine
- The chromophore is present as a cation.
- Mauveine is a mixture of four related aromatic compounds differing in number and placement of methyl groups. There are now 12 related structures.
- ~100 basic (cationic) dyes whose colours span reds, yellows and blues, with bright strong shades.
- many are triarylcarbonium ions
- e.g. C.I. Basic Green 4 (Malachite Green)
13
Q
disperse dyes (group 1)-
A
- Hydrophobic and almost insoluble in water. Affinity for hydrophobic fibres, e.g.polyesters, and are applied as very fine dispersions in water. Mostly azobenzene compounds and can give colours across the spectrum. Some are anthraquinone.
- smallest dye molecules among all dyes
- Contain nitro, amine, hydroxyl substituents.
14
Q
Pigments
A
- used in the coloration of paints, printing inks, ceramics and plastics.
- finely divided solid essentially insoluble.
- The chromophores used in pigments are usually the same as those used in dyes but the pigments are large molecules and do not have solubilising groups.
- They contain groups that form intermolecular bonds that help to reduce solubilities.
- The larger the molecule, the more opaque the pigment.
- Organic pigments generally produce a higher intensity and brightness of colour than inorganic pigments such as chrome yellow (lead(II) chromate(VI)).
- pigments have wavelength-selective absorption.
- differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.
- inorganic pigments e.g vermilion (mercury sulfide) or cadmium yellow (cadmium sulfide)
- absorb light by transferring an electron from the negative ion (S2−) to the positive ion (Hg2+ or Cd2+).
- “charge-transfer” complexes with broad absorption bands that subtract most of the colors of the incident white light.
15
Q
azo chemistry organic pgiments
A
- yellow, orange and red shade areas e.g. a monoazo pigment
- Copper phthalocyanines provide the majority of blue and green pigments. Structurally complex but inexpensive to make e.g.
ppt
