Aggregation of Dyes:
A full understanding of the dyeing process requires a knowledge of the molecular state of the dye in the dyebath, because many of the problems associated with the dyeing that can be explained by the aggregation or colloidal state of the dye. It is well known that most dyes are aggregated to some extent in aqueous solution and that the degree of aggregation depends on factors such as concentration, temperature and the presence of electrolytes.
Dye aggregation is the process by which dye molecules or ions associate in solution to form clusters or aggregates that comprise initially two molecules/ions which subsequently can grow in size to by accretion of more dye molecules/ions to form higher aggregates. The aggregation of dyes in solution leads to lower rates of diffusion of the molecules from the bulk solution to the fiber surface.
Aggregation occurs in a stepwise manner (i.e. monomer → dimer → trimer, etc.) and can eventually result in the formation of a colloid. Such association can occur between similar dye ions/molecules or, in the case of the use of dyes in admixture (as is routinely employed in typical dyeing processes), between ions/molecules of different dyes. The aggregation of dyes often is accompanied by marked changes in the color of a dye in solution. In addition, as aggregation affects both the size and aqueous solubility of dye molecules, the phenomenon can have a marked effect on the nature and extent of both dye diffusion and, therefore, dye adsorption.
Main problems during dyeing is 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:
- Size of dye molecules
- 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.
Dyestuff aggregation can be observed with all classes of dyes:
Driving forces for aggregation can be:
- Hydrophobic interactions, for example, in case of presence of longer aliphatic groups in the dye molecule
- Hydrogen bonding between dye molecules and dye–water molecules
- Aromatic interactions (π–π interactions) between benzene rings and annealed rings
- Van der Waals forces in particular when dye molecules arrange parallel or in layered structure
Electrostatic repulsive forces:
Charged ionic groups (mainly sulphonate groups) that are present in the molecules to improve dyestuff solubility also lead to a electrostatic repulsion between dyestuff molecules. Thus the number of sulphonate groups and their position in the dye molecule will be of influence for tendency of dye molecules to aggregate.
There are some advantages to aggregation of dye molecules in the fiber, as long as uniform distribution of dye throughout the fiber can be achieved. The light fastness and wet fastness of dyes that aggregate in the fiber are very good. The improvement varies with the dye application class and the type of fiber being dyed. Table 1 shows the general aggregation properties of the various dye application classes.
Table 1: Aggregation behavior of dye application classes.
Measurement of Dye Aggregation:
- X-ray diffraction
- Measurements of diffusion coefficients
- Activity of counter (Sodium)
- Light scattering
- Evaluation of colligative properties
- Visible light adsorption
- H and 19F NMR
Reasons of Dye Aggregation in Aqueous Solutions:
👉Dyes are consists of:
- Hydrophobic aromatic portion
- 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.
👉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.
👉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.
👉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.
👉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.
👉Calculation shows that below concentration of 10-5 mole/L various higher aggregates appear, giving a polyassociated system.
👉Higher ionic strength, ionic dye aggregation becomes more dominant.
Prevention of Aggregation:
- By raising the temperature of dyebath
- Liberation and existence of monomers by circulations or stirring and keep concentration below 10-5 mole/L of dye.
- An Introduction to Textile Coloration: Principles and Practice By Roger H. Wardman
- Textile Chemistry By Thomas Bechtold and Tung Pham
- The Coloration of Wool and other Keratin fibers Edited by David M. Lewis and John A. Rippon
- Physico-chemical Aspects of Textile Coloration by Stephen M. Burkinshaw
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