CH 2020/2290 Materials

CH 2020/2290 Dyes and Dyeing (adapted from Organic Chemistry: A Short Course, H. Hart, L. E. Craine, D. J. Hart, and T.K. Vinod 13th ed. Houghton-Mifflin, Boston, 2012.)

Materials From the Chemicals Hood: From the Side Shelf: Malachite green Sodium carbonate (3) 13-fiber Test Fabric strips Congo red Tannic acid

Introduction Why are dyes colored? Dyes contain chromophores (“color bearers”), which are unsaturated functional groups, such as those in Figure 1, that absorb certain wavelengths of electromagnetic energy, usually in the ultraviolet region. When a chromophore is attached to a system of pi conjugated electrons, the wavelength that the chromophore absorbs increases with increasing size of the conjugated system. This can bring the wavelength absorbed by the chromophore into the visible region. What color the dye appears is the complementary color of the color which is being absorbed by the chromophore; for example, if a chromophore absorbs green light (500 nm), the observer will see a red color from the dye.

Figure 1. Examples of chromophores

Auxochromes (“color helpers”), such as those in Figure 2, contain a lone pair of electrons which can be donated into a conjugated system. The delocalization of electrons imparted by the resulting resonance, shown in Figure 3, has the effect of lengthening the conjugated system, and increases the wavelength at which the chromophore absorbs.

Figure 2. Typical auxochromes

Often dyes are affixed to the substance being dyed through anchoring groups, which are functional groups which can bind to the substance through ionic, covalent, or hydrogen bonding. Carboxylic acids (-COOH) or sulfonic acids (-SO3H) are common anchoring groups.

Figure 3. Auxochrome donation to the conjugated system

Fabrics consist of fibers, either natural or synthetic, which are composed of polymers. The wide variety of functional groups and combinations thereof which comprise these polymers leads to a diverse array of substrates with which dyes can interact. Four classifications of synthetic dyes are direct, mordant, developed, and vat dyes, depending on how the dye interacts with the fiber. Direct dyes bind to the fiber without need of extra reagents. Mordant dyes are indirectly bound to the fiber in that the fiber first binds to a linker molecule or ion, called a mordant, which then binds to the dye. Developed and vat dyes depend upon the dye being synthesized in the presence of the fabric such that the dye product is insoluble in the solution containing the fabric and becomes trapped in the fibers.

Figure 4. Dye structures

In this experiment, you will use direct and mordant dying techniques and the dyes malachite green and Congo red (Figure 4). Figure 5 shows the direct interaction of malachite green and Congo red with fibers and the indirect interaction of malachite green with a fiber through a tannic acid mordant.

Figure 5. Interactions of dyes with fibers The Test Fabric strip consists of 13 different fibers, whose names and structures are given in Figure 6.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Acetate rayon

SEF (Monsanto’s Modacryclic)

Arnel bright

Cotton

Creslan

Dacron 54

Dacron 64

Nylon 6.6

Orlon 75

Silk

Polypropylene

Viscose Rayon

Wool

Figure 6. Test Fabric Fibers

Procedure (a) Malachite Green In a 250 mL beaker, dissolve about 0.1 g malachite green in 100 mL water. Heat the solution on the hot plate until boiling. Add a Test Fabric strip and allow the fabric to soak in the boiling solution for two minutes. Using forceps, remove the fabric and dunk it several times into a 400 mL beaker containing 300 mL clean water to rinse away any unbound dye. Note the colorfastness on each kind of fabric in the strip using the following scale: 1 no color 2 slight/barely any color 3 noticeable color 4 considerable color 5 severe/heavy color

(b) Congo Red In a 150 mL beaker, dissolve about 0.1 g Congo red in 40 mL water. Add 0.1 g sodium carbonate and heat the solution on the hot plate until boiling. Add a Test Fabric strip and allow the fabric to soak in the boiling solution for two minutes. Using forceps, remove the fabric and dunk it several times into a 400 mL beaker containing 300 mL clean water to rinse away any unbound dye. Note the colorfastness on each kind of fabric in the strip.

(c) Tannic Acid Mordant In a 150 mL beaker, dissolve about 0.1 g tannic acid in 50 mL water. Heat the solution on the hot plate until boiling. Add a Test Fabric strip and allow the fabric to soak in the boiling solution for one minute. Remove the fabric strip and press it between paper towels to dry off any excess tannic acid solution. Add the strip to a boiling solution of malachite green for two minutes. Using forceps, remove the fabric and dunk it several times into a 400 mL beaker containing 300 mL clean water to rinse away any unbound dye. Note the colorfastness on each kind of fabric in the strip.

Waste Disposal The Congo red and malachite green solutions may be poured into the Basic Waste container. The tannic acid solution should be poured in the Acidic Waste container.