Science and the Senses: Pigments Jasmine Chan (Year 12, Wu)
Pigments are everywhere. They can be found in the fabric of clothes that you are currently wearing, the hair on top of your head and even the food that you consume. As a person who loves colour, the realm of pigments fascinates me. In this article, I am going to introduce you to the basics of pigments, how they are classified and their biological applications. 1 INTRODUCTION TO PIGMENTS 1.1 What are pigments? Pigments are chemical compounds that are coloured, black or white [1]. In nature they usually have specific functions, but we make extensive use of pigments in many industries to add colour to objects. Pigments are usually insoluble solids but in order to change the surface colour of an object easily, a liquid form would be desired. Due to their insolubility, a solution cannot be formed and therefore, a mixture containing solid particles in a liquid is used. When the pigments are incorporated, they remain unaffected, chemically and physically [2, 3]. However, there are some cases where pigments are used in their solid form, such as eyeshadows in the cosmetics industry. 1.2 Organic and Inorganic Pigments Organic pigments are made from natural sources, such as plants. Organic means that one or more carbon atoms are present in the pigment molecules, whereas inorganic products do not contain any carbon atoms. Organic pigments also usually contain small amounts of sulphur and nitrogen atoms [4]. There are three types of organic pigments: ‘carbon’ pigments (eg. lamp black pigment which is made from the products of incomplete combustion of oils), ‘lake’ pigments (combining anionic dyes with metallic salts) and ‘non-ionic’ pigments (eg. azo pigments) [5]. The largest group of organic pigments is azo pigments which contain one or more azo groups (-N=N-) which form red, orange and yellow pigments [6]. Inorganic pigments are usually made from the oxidation of chemicals that are not sourced from plants. One example is ‘titanium white’; these white pigments are known as white extenders which not only provide opacity and lighten the colour of other substances but also improve their properties, such as durability, and strength [3, 4]. When comparing organic and inorganic pigments, inorganic pigments have a much larger average particle size than organic pigments which allows inorganic pigments to be more opaque. As organic pigments have a larger surface area to volume ratio, they provide higher colour strength (better ability to provide a colour of higher intensity to other
Figure 1: List of organic and inorganic pigments [7] 10
