Chemistry of Dyes

Project 1: Our Colourful Lives

Introduction:

Ever wonder how our everyday materials, like our clothes, our devices, our food, our home necessities can be altered into any different colours we wish them to be? In this research paper, we will discuss how our material things get coloured on the chemistry level and how exactly colors are created or where do they come from. This will also tell you some environmental impact it causes to our daily lives and in nature.

Properties of Dyes:

Pigments and colors are both used to color all kinds of substances and have been used for millions of years. We can differentiate dyes and pigments by how dyes disperse at a molecular level because they are soluble in the substrate, whereas pigments are insoluble and dispersed as particles. The colour of dyes is brighter than pigments, but are less reliable because of their low stability and are less permanent.

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Pigments are usually referred to as inorganic salts or oxides like chromium oxides ad iron. Pigments are coloured or colourless or fluorescent in inorganic or organic solids that are usually insoluble in, and chemically or physically unaffected by the medium in which they are combined with. They have different appearances depending on a specific absorption or scattering of light. Inorganic and organic pigment powders are fine crystalline solids, which cannot solute in medias like ink or paint. Pigments are hard to process and have poor colour brilliance and strength but can stand heat pretty well, lightfast, highly durable, migration fast, and solvent resistant.

Meanwhile, dyes are coloured substances that can mix into a solution in an application process and shows colour through selective light absorption. Dyes have properties that are described by their chemical structure. While, pigments have their particles so it can be described by its physical attributes. Using inorganic salts or natural pigments like animals, plants, or minerals are coloring substances which are commonly called dyes. Dyes are chemical compounds to use to give colour for things like food, cosmetics, clothes, plastic, etc., and for inks and artistic colours. Dyes could be classified as synthetic or natural dyes. Natural dyes are taken from animals, plants, or minerals, while synthetic dyes are based on petroleum compound. Dyes is the opposite of pigment which has an excellent colour and brilliance strength, and easy to process but also has poor heat, high immigration, poor durability and solvent stability.

Chemistry Structure of Dyes:

The absorption of electromagnetic radiation by a molecule in the UV and visible regions leads to electronic excitation and an electron moves from a lower to a higher level of electronic energy. The chromophore is known as a covalently unsaturated group which is accountable for the absorption in the visible or UV region. The colour appears when a compound absorbs light in the visible region (400–800 nm). That’s why, a chromophore might or might not give a compound color depending if the chromophore absorbs radiation in the visible UV region.

Because wavelength and absorption intensity depend on a lot of factors, there aren’t distinct rules for chromophore identification. A covalently saturated group that changes both the intensity and the wavelength of the maximum absorption when attached to a chromophore is known as auxochrome, e.g., halogens, NH2, SH, OH, etc. Auxochromes generally increase the value of λmax (maximum wavelength) as well as εmax (molar absorption coefficients) by extending the conjugation by means of a resonance. They are the colour enhancing groups. An auxochrome by itself can’t surpass 200 nm. When chromophore and auxochrome are combined it acts like a new chromophore with different values of λmax and εmax. An example is when benzene shows λmax 256nm and 200nm of εmax, while phenol shows λmax 270nm and 1450 of εmax. The OH group is therefore an auxochrome that prolongs the conjugation of the lone pair of electrons in the oxygen atom which increase the values of λmax and εmax.

The maximum absorption is shifted to a longer wavelength (lower energy) and typically to a higher intensity than an unconjugated chromophore, when two or more chromophoric groups are conjugated. A conjugated diene is low in energy when an electron moves from the highest occupied molecular orbit to the lowest unoccupied molecular orbit and causes a bathochromic shift (a shift of a band so the energy lowers or the wavelength becomes longer). The higher the values of λmax and εmax, the longer the conjugated system is. That is why it is easier to for a longer conjunction to be coloured under the visible region (400-800nm).

There are three main components of an organic dye molecule which are chromogen, chromophore, and auxochrome. Chromogen is a chemical compound that is coloured or that can be coloured by a compatible substituent. Chromophore is an electron-accepting. It is what give the appearance of colour. Auxochrome is an electron-donator. It influences its color, not like chromophore which determines what exact color or hue it creates. Both are in the conjugated system and are part of chromogen.

Dyes have their colour because they absorb light in the spectrum of visibility which is in the 400-800 nm, they have at least one chromophore which is a colour-bearing group, have a conjugated system, for example a structure with double and single bonds alternating, and last is that they show electrons resonance, a stabilizing force in organic compounds.

If one of these characteristics lacks a molecular structure, the color is non-existent. Most dyes also contain auxochromes which are colour helpers. Some examples are sulfonic acid, carboxylic acid, hydroxyl groups, and amino acid. Although they are not accountable for colour, getting mixed with it is able to change the colour of a colorant and are most often used to influence the solubility of the dye. The relationship between visible wavelength and the absorbed or observed colour is shown on Figure 1. Figure 1 shows the relationship between the visible wavelength and the absorbed / observed color. Figures 2–4 illustrate other factors that contribute to colour.

Fig. 1

Colour in organic dyes versus wavelength of light absorption

Fig. 2

Examples of chromophoric groups present in organic dyes

Fig. 3

Conjugated systems in Vitamin A (top) and β -carotene (bottom)

Fig. 4

A pair of resonance structures for Malachite Green (C.I. Basic Green 4)

For a colour-generating chromophore in organic compounds, there must be a chromophore in the conjugated system. This is illustrated by the examples in Figure 5, where the placing of an azo group between methyl groups creates a colourless compound, while a yellow – orange color is attained when the azo group is put between aromatic rings. Figure 3 shows Figure 3 shows the importance of a prolonged conjugate system. It is obvious that making the length of the conjugated system longer in Vitamin A to give β-carotene causes a bathochromic shift, ex. a darker color.

Fig. 5

Importance of having a chromophore within a conjugated system

What helps to achieve targeted colours is adding auxochromes, it also helps influence solubility. Adding more of an electron-donating to the azobenzene structure creates a bathochromic effect, which is a shift of a band so the energy lowers or the wavelength becomes longer. Bathochromic effect happens when an electron-donating (NH2) and electron-accepting (NO2) groups are put in conjugation. Nitro groups are beneficial in this which contribute to their prevalence in dispersed dye structures. More electron-attracting groups conjugated with electron donor makes a bathochromic effect. When alkyl groups are added to the N-atom, the effect of electron-donating on amino groups are boosted.

Fig. 6

Effects of substituent groups within an azo-dye system

Environmental Impacts:

With the way the fashion industry works to give different colours for our clothes is by having tons of water involved. For removing colour, cleaning, bleaching, dying causes pollution in textile wastewater. When water is involved, the used water ends up in water bodies like lakes, rivers, and others which causes aquatic environment damage. The main pollutants are recalcitrant organic, toxicant, surfactant, colored and chlorinated compounds and salts in the textile run-offs. Drinking water, irrigation, and recreation water are limited for use because of the oxygen deficiency from high toxic and mutagenic dyes that causes light penetration and photosynthesis to decrease. Our DNA could also get damaged when a so-called N-hydroxylamines are formed. Dyes are created to keep their colour and not degrade so they stay in our environment for a very long time. An example is, at pH 7 and 25°C, it is about 46 years for the half-life of a hydrolyzed dye Reactive Blue 19. It is confirmed by mutagenic activity that our river and drinking water contain C.I. Disperse Violet 93 (DV93) (DB373), C.I. Disperse Orange 37 (DO37) and C.I. Disperse Blue 373 which means that flotation, coagulation, flocculation, and pre-chlorination (effluent treatment) do not work as much as everyone would want to remove the dyes from water.

Conclusion:

Looking at our surroundings, there are colours everywhere. Everything that exists has colour and we can manipulate the colours to our liking. We can see different colours because each conjunction can absorb a different amount of radiation, they have different lengths, and have different amount of chromophore and auxophore. Chromophore is the most responsible for the given colour a paint or ink has. These colour giving chemicals doesn’t only give colour in our lives but they also cause damage to health and homes. They create harmful chemicals which contaminate our waterbodies that we use for daily cleaning or drinking. We can now understand how colours are implemented in dyes, how they are created, and what are the cons to using them.

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