Aggregation of dyes
Aggregation of dyes
Dye-dye self association in solution is called dye aggregation, which is important phenomenon where dye molecules or ion takes part. In general, the term aggregation is used for dye-dye interaction and dye association for interaction of dyes with other compounds e.g. polymers.
Generally dye molecules form aggregation in aqueous solution at room temperature and to an extent which depend on
i. Size of dye molecules
ii. No of solubilizing groups in the dye molecules
In dye aggregation multiple equilibria need to be considered i.e. diametric, trimetric etc, aggregates are formed
D + D
D2
D2 + DD3
Dn-1 + DDn
Diagrammatical explanation
Dyes generally remain or tend to remain scattered in powder form but in aqueous solutions individual dye molecules stack one on top of other e.g. aggregate
Dye aggregation prevents the dye molecules from diffusion into the fiber pores and hence causes dye wastage as dyes are absorbed in monomeric form which decreases with dye aggregation.
Measurement of dye aggregation
1. Conductometry
2. Calorimetry
3. Polarography
4. Solubility
5. Sedimentation
6. Fluorescence
7. X-ray diffraction
8. Measurements of diffusion coefficients
9. Activity of counter (Sodium)
10. Light scattering
11. Evaluation of colligative properties
12. Visible light adsorption
13. 1H and 19F NMR
Reasons of dye aggregation in dyebath
1. Dyes are consists of
i. Hydrophobic aromatic portion
ii. Polar groups (OH, amino etc.) for water solubility and charged groups (sulfonic or positive charged groups) for rendering molecule water soluble
When dye molecules dissolved in water a new interface is created between the hydrophobic portion and water. Dye can reduce the size of the interfacial water by overlapping of the hydrophobic areas and there will be a tendency to aggregate.
2. Usually linear and planar dye molecules should tend to stack one molecule upon another with the ionized groups arranged so as to give minimum free energy condition causes aggregation.
3. Dyes with long aliphatic chains form micelles of a spherical form in which the flexible chains associate in the interior with the sulfonic acid groups exposed on the surface of sphere.
4. Aggregation of dimer is more obvious as aromatic ring system have maximum overlap (van der waals forces) because the distance between the anionic charges is larger (minimum electrostatic repulsion).
As dye concentration increases there will be an increased tendency for trimers, tetramers etc. to be formed.
5. Aggregation is also expected from the unusual structure of water. When the interface is formed on dissolution of the dye molecule, the water molecules adjacent to the hydrophobic portion form an ‘iceberg’ type structure accompanied by a reduction in entropy. When the dye molecules aggregate not only will energy be gained from the reduction on the interfacial energy but also an increase in energy will rise from the melting of the iceberg structure.
6. Calculation shows that below concentration of 10-5 mole/L various higher aggregates appear, giving a polyassociated system.
7. Higher ionic strength, ionic dye aggregation becomes more dominant.
Prevention of aggregation
1. By raising the temperature of dyebath
2. Liberation and existence of monomers by circulations or stirring and keep concentration below 10-5 mole/L of dye.
Labels: Theory of Dyeing
Dyeing Kinetics
The actual dyeing theory mathematically can be obtained from kinetics of dyeing or dyeing equilibria. The dyeing phenomena is found in principle of dyeing curve. The factors for uniform color and optimization of dye all are related to kinetic phenomena. Therefore kinetic dyeing is important in the dyeing process.
During dyeing process, two methods play a dominant role:
Dyeing kinetics: the rate of transfer of dye in solution (or dispersion) from the dye bath into substrate
Dyeing equilibiria: the position of sorption versus desorption after (theoretically) infinite time. Most of the equilibrium properties of dyeing system depend on three quantities:
· affinity
· heat of dyeing
· entropy change
The kinetic behavior of a dye is graphically depicted by rate of dyeing (or uptake) curve. The transfer consists of:
a. Convectional diffusion to the fiber surface occurring in dyebath.
b. Molecular diffusion through the hydrodynamic boundary layer
c. Adsorption at the outer surface
d. Molecular diffusion into the fiber (Absorption)
e. Anchoring of dye molecule
In the case of disperse dyeing (stage a) is preceded by the dissolution of disperse dye particles. Therefore the particle size distribution may influence the dyeing kinetics of disperse dye. In reactive dyeing, azoic dyeing, metallised dyes, vat and sulfuric acid esters of leucovat dyes, chemical reaction (stage e) takes place.
For the kinetics of over all dyeing processes stages a, c & d are important. Liquor circulation in the dye bath must be vigorous enough to ensure that stage a is short relative to stage d. The Sorption is faster than the preceding stages. Absorption leads to immbobilisation stage e. In equilibrium dyeing processes a complete immobilisation doesn’t take place.
The dyeing kinetic principle can be shown schematically
Dye in dyebath Convective diffusion Dye in boundary layer Molecular diffusion Dye on fiber surface Adsorption + Diffusion Dye in fiber surface Absorption dye
sorption Fixation Immobilized
Labels: Theory of Dyeing
Dye Fiber Interaction/ Anchoring system
Dye fiber interaction system can be divided into
1. Nonionic system
2. Ionic system
3. Reactive system
4. Hydrogen bond system
5. Other interactions
1. Nonionic system: PET, acrylic, polyamide etc.
2. Ionic system
o Fiber which possess charged group:
Anionic and cationic: Acrylic fiber (contain negatively charged sulfonic or –COOH group) and basic dyes
Wool, Silk, Nylons (contain charged –NH4+ groups) & acid dye
o Fiber which contain no charged groups
Anionic and Anionic: Cellulose is dyed with direct & vat dyes both of which carry negative charges. The dye is absorbed by virtue of its attraction to the fiber & in doing so it is accompanied by other ions of electrolytes e.g. Na+ & Cl-.
3. Reactive system: cellulose, wool and reactive dye
4. Hydrogen bond system
5. Other interactions: Van der Waals force
Role of fiber functional groups in dye fiber interaction systems
· Cotton: ionic system and covalent bond forces and H-bond
Cotton fiber has –OH groups, which is highly electronegative and is capable of hydrogen bonding. It is also capable of reacting with reactive groups of reactive dyes and form covalent bonds.
· Protein: ionic system
Wool fiber has –COOH and –NH2 groups which are capable of ionizing and at certain pH are positively or negatively charged. So it can be dyed with basic and acid dyes.
· Polyester:
contains –COOH, -OH as functional groups but don’t undergo ionisation, so it is not possible to dye them with ionic dyes. So nonionic system and hydrophobic interaction and Van der waals force exist.
· PAN: ionic and nonionic system
Contains –OSO3H, can be dyed with cationic/basic dyes
· Rayon: Ionic system
Contains –OH groups, -COOCH3 groups
Forces in dyeing systems
1. Electrostatic force
2. Hydrogen Bond
3. Covalent bond
4. Van der Waals force
5. Physical force
6. Hydrophobic interactions/Entropy factors
· Electrostatic force:
The forces have a range about 100A°.
Electrostatic force formed when the dye particle and fiber surfaces are oppositely charged. Such force exists in the dyeing of wool, silk, polyamides with anionic dyes (or fibers containing anion with cationic dyes). The polymers of these fibers contain amino and carboxyl groups depending on pH value in water, these groups are either neutral (-COOH, -NH2), cationic (-NH3+) or anion (-COO-).
· Hydrogen Bond:
When hydrogen atoms are united with strongly electronegative group element, the latter by attracting the electron of the hydrogen atom, gives to it a positive bias. This positively charged hydrogen atom may form bond with groups containing unshared pale of electrons. They are of short range 1A° to 5A° (0.1 to 0.5nm)
R-δ------H+δ … …. … …ö= C O---1.5-1.9Aº-----H …1Aº…… ö
Hydrogen bonds are formed because of extra attraction between such atoms. It is a weak type of bond. This bond may be intermolecular or intramolecular.
· Covalent bond
The covalent bond between carbon atom in most organic compound is very stable. They are of short range 1A° to 5A° (0.1nm to 0.5nm). Covalent bonds are formed when dyes react chemically with fibers. All reactive dyes form covalent bonds so fastness properties of such dyes are generally good.
· Van der Waals force
Van der waals forces are only effective for sorption of dyes to fiber molecules if the distance between the dye and fiber is very small. These are weak forces and depend on atoms being at certain relative position.
· Physical force
It is found that although –OH, -NH2, -N=N- and –CO groups might be responsible for attachment by hydrogen bonds to the fiber but this explanation is to a great extent discounted because the coordinating power of these groups is satisfied by chelation within dye molecules which is due to nonpolar or physical force.
· Hydrophobic interactions/Entropy factors:
It is found that increasing the no of aromatic rings or unbranched aliphatic chain makes a much greater increase in affinity than does the introduction of potential bond forming groups. This is assumed that the hydrophobic part of unbranched aliphatic chain dissolved in water because of ice-like structure of the water molecules in the immediate vicinity of hydrophobic molecules, which is of completely entropy factors
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