Chemistry of Dyeing: Molecular Mechanisms, Mordants, and pH Effects

This essay delves into the chemistry of dyeing, providing a comprehensive overview of the molecular mechanisms involved in the process. It highlights the importance of dye characteristics, including chromophores and auxochromes, and describes various dyeing methods and the stages of dye transfer to fibers. The essay examines the role of diffusion, as described by Fick's theory, and the major types of interactions in dyeing, such as ionic, covalent, and hydrophobic interactions. It discusses the application of different dyes to natural and synthetic fibers, emphasizing the role of mordants as dye fixatives and their impact on color. Furthermore, the essay explores the crucial role of pH in the dyeing process, particularly its influence on dye adsorption and color intensity. It also discusses the effects of pH on different types of dyes, such as acid and reactive dyes, and provides examples of how pH changes can affect color tones, concluding with a summary of the chemical reactions associated with dyeing.

Running head: CHEMISTRY OF DYEING
CHEMISTRY OF DYEING
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The chemistry of the dyeing process is based on the characteristics of the dyes. These
include the ability of the dyes to carry out light absorption in the visible spectrum in the
wavelength range of 400-700nm, presence of at least a single chromophore, presence of a
conjugated system having alternating single and double bonds and exhibit resonance of
electrons. The lack of any of these characteristics results in inability to of the dye to show colour
(Kent, 2013). Dyes also consist of auxochromes that changes the colour of the dye and also
influences the solubility of the dyes. Some of the examples of auxochromes are amino or
hydroxyl groups, carboxylic acids, sulfonic acids, among others. Some examples of
chromophoric groups are anthraquinone, azo, nitro, methane, phthalocyanin, among others
(Shindy, 2016).
This essay will provide a description of the molecular mechanisms of the dyeing process,
role of mordants and the role of acidity on the final colour of the dye.
The process of applying color to fabrics are called dyeing. Reactive dyes combine with
the fiber directly, thereby giving rise to colorfastness. There are various methods of dyeing.
These are bale, batik, beam, burl, cross, jig, piece and random dyeing. The rate of transfer of
dyes to the fibers consists of four stages. These are convectional diffusion of the dye to the
surface of the fiber within the dye bath, molecular diffusion of the dye through the hydrodynamic
boundary layer, adsorption of the dye on the outer surface of the substrate and subsequent
molecular diffusion of the dye into the fiber. This process is called sorption. The particle size
distribution therefore affects the kinetics of the dyeing process (Lewis & Rippon, 2013 ). The
diffusion of the dyes is described by the theory of diffusion as proposed by Fick. The diffusion of
the mobile particles or the dyes is considered to be a random walk at the molecular level. The
number of particles that diffuse through the cross-section of the fiber per unit time is called flux

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and it is proportional to the concentration gradient. The diffusion co-efficient also called
diffusivity is a measure of the ease with which the mobile dye particles pass through a particular
medium (Shamey & Zhao, 2013). The major classes of interactions that take place in dyeing are
ionic, covalent and hydrophobic interactions. Ionic bonding involves the attraction of the
negative and positive charges. In the solution, acidic groups like sulfonic or carboxylic groups
lose a proton and in turn become anionic or negatively charged. Basic dyes like amines, on the
other hand accept protons and become cationic or positively charged. Covalent bonding involves
uncharged atoms and a stable configuration is achieved by either gain or loss of electrons. The
atoms donate electrons to the shared orbitals and in doing so the atoms remain bonded due to the
shared orbital. Co-ordinate bonds are a type of covalent bond, where the one atom donates all the
electrons, which are shared by both atoms, thereby generating the bonds. Covalent bonding
requires the presence of mordants. Hydrophobic stains or dyes bind to lipid rich regions
(Chequer et al., 2013).
Fibers can be natural or synthetic and different dyes are required depending on the
presence of different functional groups on the fibers. Acidic dyes contain –COOH or –SO3H
groups, which are attracted to the basic –NH groups present in the amide links of fabrics like
silk, wool or nylon. The direct dyes bind to fabrics by the process of hydrogen bonding and bind
to –OH groups present in cellulose fibers. The disperse or the vat dyes are insoluble in water and
are oxidized while in solution and are physically bound to the fibers. The reactive dyes bind to
hydroxyl or amine groups present on cotton fibers (Horrocks, 2018).
The mordants helps to act as dye fixatives that forms co-ordination complexes with dyes
and helps in attaching to the fabric. Mordants are chemicals, which usually consists of a metal
having a valency of 2 or above. However, other types of compounds can also act as mordants.

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Different mordants can give different colors to the same dye. The mordant thus allows a
chemical reaction to occur between the fabric and the dye (Prabhu & Bhute, 2012). In the textile
industries, mordants are used to fix the dyes on the fabrics. Metal mordants are polyvalent metal
ions that form co-ordination complexes with dyes (Manian, Paul & Bechtold, 2016). The
fundamental chemical reaction between a dye and a mordant involves the formation of 2 bonds.
These are a covalent bond between the metal atom and the hydroxyl oxygen. The other bond is
the coordinate bond formed between the metal and the double bonded oxygen referred to as
chelation. Varying the amounts of the mordants can result in changes the color f the dye.
Mordants are applied in 3 ways. These are pre-mordanting, post-mordanting and simultaneous
mordanting. In pre-mordanting process, the mordant is applied followed by the dye. In post-
mordanting, the mordant is added after the dye, while in simultaneous mordanting, both dyes and
mordants are added simultaneously. The addition of mordant to a dye solution results in sudden
and dramatic changes in the color of the dye solution. This results due to the fact that a metal
atom is incorporated into the delocalized electron system of the dye. The low energy levels of the
metals results in its incorporation in the delocalized electron system, thereby lowering the overall
energy. The absorbance of the dye and its color is therefore associated with the phenomenon
(Morales-Oyervides et al., 2017; Nptel.ac.in, 2018).
There are various factors that play an important role in the dyeing process, but the most
important is pH. pH plays a very important role on the adsorption properties of dyes. Controlling
the pH of the dye bath is essential as it affects the dyeing cycle. pH also play an important role in
the uptake of the dyes (Mondal & Islam, 2014). In acid dyeing, the low pH helps in the
generation of hydrogen bonds that helps in the binding of the acidic dyes to the protein fibers of
wool or silk. In the case of fiber reactive dyes, a high pH of the dye bath is essential as it helps to

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activate the cellulose fibers of the cotton fabrics, resulting in the formation of a cellulosate anion,
which in turn reacts with the dye molecule leading to the production of a permanent and strong
covalent bond, thereby intensifying the color (Pburch.net, 2018). For example, at acidic pH of 1-
3, anthocyanins are present as red flavylium cation, while with increase in the pH, the color
intensity of anthocyanin decreases due to the decrease in the concentration of the flavylium
cation. Thus, differences in pH from acid to alkaline gives rise to different color tones
(Wahyuningsih et al., 2017).
This essay therefore describes the chemistry of dyes and the chemical reactions
associated with the dyeing process. It also describes the role of the mordants, which is to
intensify the color of the dye. Moreover, pH also plays an important role as changes in pH results
in changes in the color intensities of the dye.

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Reference List
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