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

1. Size of dye molecules

2. 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 + DD₂

D₂ + DD₃

D_n-1 + DD_n

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. 1_H and 19_F NMR

Reasons of dye aggregation in dyebath

1. Dyes are consists of

   1. Hydrophobic aromatic portion

   2. 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.