dyes and solvatochromism Flashcards
dyes
• λ of light absorbed depends on chemical structure
molecules of coloured organic compounds contain two parts
- An aromatic ring or a fused ring system with extensive electron delocalisation
- An extensive conjugated double bond system containing unsaturated groups, known as chromophores,
how can the intensity of colour be increased
by substituents containing lone pairs of electrons to the aryl ring “auxochromes
what is the entire structure of the colourant called
chromogen
see pp for
approximate relative strengths of bonding between a dye and a fabric
classification of colourants
- The Colour Index International, produced by the Society of Dyers and Colourists, in Bradford, 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.
what are the 3 most important structural classes according to chemical type
azo dyes
anthraquinone dyes
phthalocyanines
azo dyes
• Azobenzene is the chromophore of azo dyes.
• Colour can be modified and intensity of the colour increased by varying the auxochromes
see pp for diagrams
anthraquinone dyes
• Based on anthracene
phthalocyanines
• 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:
classification of colourants by methods of application
• 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
fabric dying machine
dyestuff and auxiliaries can be injected into the machine by the high pressure-dosing pump
auxiliaries for textiles
- 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
see pp for
important dye and fibre table
direct dye for dying cotton
linear and planar structure binding to cellulosic chains in the fibre, via intermolecular bonding
reactive dyes
- 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
basic dyes
- Earliest synthetic dye e.g. Mauveine
- Discovered by William Henry Perkin at age of 18
- 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)
disperse dyes group 1
- 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.