Colourants: Pigments and Dyes
Colourants: Pigments and Dyesby Jasmine Chan
Colourants are substances which impart colour to other materials. [1] They are chemical compounds that are coloured, black or white. Colours have always been used in all sorts of materials for practical and decorative reasons. Currently, there are over 9000 colourants which are created due to the large range of hues desired by society. [2] The primary forms of colourants are pigments and dyes - but what are their differences?
Pigments are a group of mainly insoluble compounds. They are usually applied in solution form, where the pigment is the solute, and another liquid is the solvent. [3] There are two types of pigments: organic and inorganic. Organic pigments are made from natural sources that can be obtained from plants and plant products. [4] Most pigments today are synthetically made, meaning that they are artificially produced which mimics the natural form of the pigments but with other substitutes. Natural pigments are derived from natural substances on earth; they are earthy, dull and nature-like. In order to obtain bright, bold colours, pigments can be modified synthetically to produce unnatural colours such as neon. [5]
Dyes are mostly organic compounds and are mainly soluble compounds. They can be used by its original form or its diluted form. [6] Whereas pigments are applied by layering itself on top of substances, dyes form strong chemical bonds with itself and the substance - causing the structure of the substance to alter. [5] Dyes are much smaller than pigments, which gives them their soluble properties.
There isn’t a fine line between the differences between dyes and pigments; their overall properties are generalised. Since organic pigments are chemically similar to dyes, there are cases where dyes can become pigments after use, but never vice versa. [2]
In this article, I would be looking into a few specific pigments and dyes, looking closely into their chemical properties and structures.
Carbon Black Pigment
One major group of carbon pigments are known as carbon black (PBk7, C.I. 77266) - a generic name for organic black pigments derived from large quantities by incomplete combustion of various hydrocarbons, or other organic matter. Carbon blacks have high hiding power(1) and are hence used in printer inks and paint. [7] Under a scanning electron microscope, carbon blacks are arranged as aciniform(2). Particles in carbon black can be arranged differently, causing different carbon blacks to have different properties. Particles may have different sizes (ranging from tens to hundreds of nanometres [8]) that can affect the opacity of the pigment, and dispersibility of the pigment. Carbon black are allotropes of carbon, but it has surface properties which cause various functional groups such as hydroxyl (-OH) and carboxyl (-COOH) groups to combine to its surface. [9] These functional groups increase the affinity of carbon black with other substances such as printer inks. [10]
Fig.2 Carbon black under a transmission electron microscope.
Source: Karl, Frank, et al. “Adventures in Carbon Black.” McCrone, 20 Apr. 2020, www.mccrone.com/mm/adventures-carbon-black/.
Fig.3 Example of the skeletal formula of a carbon black pigment with functional groups present on its surface.
Source: Tehrani, Mohammad J. Micromechanical Analysis of Strength of Polymer Networks with Polydisperse Structures. 2017.
Fig.4 Example of how three different carbon black pigments look next to one another due to the differences in functional groups on their surfaces.
Source: “Our Wiki: Everything about Carbon Black.” PentaCarbon, pentacarbon.de/en/wiki/.
Titanium White Pigment
Titanium white (PW6, C.I. 77891), commonly known as titanium dioxide (TiO2) is a popular inorganic pigment present in many products we use around us daily due to its whitening abilities such as sunscreen, cosmetics and gum. It is non-toxic, has a high boiling temperature of ~2900°C which makes it easy to use in everyday products. Titanium dioxide is found naturally on earth, especially on titanium-containing minerals which react with the oxygen in the air. It is known as the ‘whitest white’ pigment on earth due to its high refractive index, making it a bright substance. [11] As a photocatalyst(3), a redox reaction occurs when photons come into contact, allowing harmful substances such as wavelengths of ultraviolet to disintegrate into harmless substances. [12]
Fig.5 Example of titanium dioxide pigment.
Source: “Titanium Dioxide Pigment (Chloride Process).” The Cary Company, www.thecarycompany.com/kemox-rc-822-rutile-tio2-titanium-dioxide-pigment.
Source: “Photocatalyst Technology.” Database for Advancements in Science and Technology, 22 Nov. 2013, sciencedatacloud.wordpress.com/2013/11/22/photocatalyst-technology/.
Ultramarine Pigment
Ultramarine (PB29, C.I. 77007) is a sulphur-containing sodium aluminosilicate. They have a general formula of Na2OSAl2O3SiO2, and they appear intensely blue due to the unpaired electron in the sulphur radical anions (S3-). [13, 14] Ultramarine has a low hiding power and low tinting strength(4) and hence, it does not have a lot of other functions aside from acting as a colourant. [15] Ultramarine can be found organically from a gemstone known as lapis lazuli, and they can also be synthesised by a calcination process by a mixture of alumina, silica, sulphur and soda together at 800°C. [15, 16] Unlike the synthetic inorganic ultramarine, which is made of even-sized particles that can diffuse light evenly, the organic ultramarine pigment has a crystalline structure, which makes it appear more vivid. [15, 17]
Fig.7 A picture of lapis lazuli and ultramarine blue pigment.
Source: https://www.buyboosq.com/index.php?main_page=product_info&products_id=71846
Azo Dye
Azo dyes are a class of synthetic dyes which contain an azo group (-N=N-) and are mainly aromatic. They are also the largest group of dyes; 60% of the dyes are accounted as azo dyes. [20] Azo dyes can be classified into different sub-groups according to their number of azo bonds in the dye. For example, if it has one azo bond, it would be a monoazo, and if it has two azo bonds, it would be a diazo and so on. [21] The aromatic side groups present in the dye helps to stabilise the azo bond by making the bond part of the delocalised system. The extended delocalised system helps to absorb different frequencies of visible light, allowing azo dyes to produce an extensive range of colours. [20] Azo dyes are synthesised by a series of reactions known as diazotisation and coupling. During diazotisation, a primary aromatic amine reacts with a nitrous acid such as hydrochloric acid to form a diazonium salt at 0°C with the help of ice. The diazonium salt produced then acts as an electrophile with an electron-rich coupling component such as phenol to produce the azo dye. [21]
Fig.8 An example of the diazotisation process in the production of azo dyes, where a primary aromatic amine produces a diazonium salt.
Source: “Diazotisation.” Organic Chemistry, www.organic-chemistry.org/namedreactions/diazotisation.shtm.
Source: “Azo Coupling.” Wikipedia, Wikimedia Foundation, 3 Aug. 2020, en.wikipedia.org/wiki/Azo_coupling.
Sulphur Dye
Sulphur dyes are a group of non-ionic dyes which contain sulphur linkages. They are insoluble in water, but they can be dissolved in sodium sulphide solution. [22] Sulphur dyes are usually made from heating sulphur or sodium sulphide with aromatic amines or other organic compounds. These dyes can only create dull and dark shades such as blue and black. During the application of the sulphur dyes, they must be reduced by the help of a reducing agent such as sodium sulphide solution and then left to react with the oxygen in the air to be oxidised back to its original form. [23]
Fig.10 An example of the skeletal formula of a sulphur dye.
Source: “Sulfur Dye.” Wikipedia, Wikimedia Foundation, 7 May 2020, en.wikipedia.org/wiki/Sulfur_dye.
Conclusion
The world of colourants is incredibly fascinating. While dyes and pigments can impart colour on substances, some have other specific properties which make them unique and are globally used to make our surroundings more vibrant and colourful! Learning about some of the mechanisms of how dyes are imparted and produced, perhaps new hues of colourants may be produced in the future to further amplify our surroundings with joy and excitement. Who knows? Only time will tell.
Glossary
- Hiding power: the ability of a substance to hide the surface of a material
- Aciniform: a substance that has a grape-like shape.
- Photocatalyst: a substance which absorbs light to bring it to higher energy level and provides this energy to a reacting substance to start a specific chemical reaction
- Tinting strength: measures the minimum amount of colourant required to impart a perceptible colour to a specific amount of clear liquid or white paint, or the relative colour intensity of a specific amount of colourant mixed with a specific amount of clear liquid or white paint
Bibliography
[1] “Colourants: Dyes VS Pigments.” Winsor & Newton, www.winsornewton.com/na/articles/colours/spotlight-on-colourants-dyes-pigments/.
[2] Abrahart, Edward Noah, and J.B. Stothers. “Dye.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 5 Mar. 2019, www.britannica.com/technology/dye.
[3] The Editors of Encyclopaedia Britannica. “Pigment.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 25 Mar. 2019, www.britannica.com/technology/pigment.
[4] “Colouration with Pigments.” Fundamentals and Practices in Colouration of Textiles, Second Edition, by J. N. Chakraborty, WPI Publishing, 2015, pp. 202–213.
[5] Saitzyk, Steven. “Types of Pigments.” True Art Information, 30 June 2013, http://www.trueart.info/?page_id=520/.
[6] “Everything You Need to Know About Dyes and Pigments.” Meghmani, 25 Nov. 2019, www.meghmaniglobal.com/everything-you-need-to-know-about-dyes-and-pigments/.
[7] The Editors of Encyclopaedia Britannica. “Carbon Black.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 8 Feb. 2016, www.britannica.com/science/carbon-black.
[8] Tehrani, Mohammad J. Micromechanical Analysis of Strength of Polymer Networks with Polydisperse Structures. 2017.
[9] “Three Main Properties of Carbon Black.” Mitsubishi Chemical, 2006, www.carbonblack.jp/en/cb/tokusei.html.
[10] “Our Wiki: Everything about Carbon Black.” PentaCarbon, pentacarbon.de/en/wiki/.
[11] “What Is Titanium Dioxide?” TDMA, 18 Feb. 2020, tdma.info/what-is-titanium-dioxide/.
[12] “What Is a Photocatalyst?” BIOMIMIC, www.biomimic-c.com/en/service/biomimic_coat/photocatalyst/.
[13] “Inorganic Pigments for Plastics.” Vibron, 18 Sept. 2019, vibron.com.au/inorganic-pigments-for-plastics/.
[14] Seel, F. “Natural Ultramarine.” ColourLex, 26 Aug. 2020, colourlex.com/project/ultramarine-natural/.
[15] “ULTRAMARINE BLUE (C.I. PIGMENT BLUE 29).” CHEMICALLAND21, www.chemicalland21.com/specialtychem/finechem/ULTRAMARINE BLUE.htm.
[16] Jensen, Elizabeth L. “Ultramarine Blue Pigment, Synthetic.” McCrone, 11 May 2020, www.mccrone.com/mm/ultramarine-blue-pigment-synthetic/.
[17] MacEvoy, Bruce. “Synthetic Organic Pigments.” Handprint, 8 Jan. 2015,http://www.handprint.com/HP/WCL/pigmt1d.html/
[18] The Editors of Encyclopaedia Britannica. “Azo Dye.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 20 Nov. 2017, www.britannica.com/science/azo-dye.
[19] Allen, R. L. M. The Chemistry of Azo Dyes. 1971.
[20] “Azo Dyes.” FOOD-INFO, 2 July 2017, www.food-info.net/uk/colour/azo.htm.
[21] Benkhaya, Said, et al. Classifications, Properties, Recent Synthesis and Applications of Azo Dyes. 2020.
[22] Chakraborty, J. N. Handbook of Textile and Industrial Dyeing. 2011.
[23] Kiron, Mazharul Islam. “Introduction of Sulfur Dye: Properties, Classification, Application and Aftertreatment of Sulfur Dye.” Textile Learner, textilelearner.blogspot.com/2011/03/defination-classification-application_4761.html.
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