Kool-aid, Cotton, and Intermolecular Forces

Each year, I try out some experiments that connect to the annual National Chemistry Week (NCW) theme. The theme for NCW this year is “Forensics: Solving Mysteries through Chemistry: Focusing on the chemistry of fibers and forensics”. Based on this description, I decided to spend some time experimenting with dyeing fibers. Through various tests and readings, I learned that the process of dyeing fibers is intimately connected to intermolecular forces. So much so that I hope to create a laboratory exercise for my college students to dye several fabrics under varying conditions and use the concept of intermolecular forces to explain observations.

In particular I used red dye #40 (Figure 1, also known as allura red) found in strawberry Kool-Aid to dye eight different fabrics: acetate, cotton, nylon, polyester, acrylic fiber, silk, rayon, and wool.¹

Allura red

Figure 1: Chemical structure of red dye #40 (allura red)

In one such experiment, it was noted that allura red strongly dyes cotton, which is comprised of cellulose (Figure 2).

cellulose

Figure 2: Structure of cellulose, showing two glucose monomers.

The strong dyeing of cotton by red dye #40 can be explained by comparing the molecular structures of red dye #40 and cellulose. Inspection of the structure of red dye #40 shows that this dye has two negatively charged sulfonate (-SO₃^-) groups, and a single hydroxyl group. Cellulose, which is a polymer of glucose monomers, contains a large number of hydroxyl groups. Thus, one would expect strong hydrogen bonding between the hydroxyl groups on both the dye and cellulose molecules. In addition, strong ion-dipole forces potentially exist between the sulfonate groups on the red #40 and the hydroxyl groups on the cellulose chain. These intermolecular interactions do a good job of explaining why cotton is strongly dyed by red #40.

If you would like to see how I carried out my experiments, check out the video below. Within the video, I explain how I explain the extent of dyeing observed when all eight fibers are treated with the red dye #40. I’m not certain my explanations are entirely correct, so I’d appreciate hearing any comments you might have about my analysis. I would also appreciate if people would attempt to dye these various fibers under other conditions. I’ve tried a few variations on my own, like using McCormick food dyes or other flavors of Kool-Aid as sources of dyes. If you learn something particularly interesting, please share it with me! I’m hoping to include this new laboratory idea of mine next semester, and I would appreciate having several variations for my students to explore.

Video 1: Using Kool-Aid to Dye Fabrics: A Great Chemistry Experiment on Intermolecular Forces, Tommy Technetium YouTube Channel, Oct 4, 2016.

Reference:
1. A great laboratory resource for these investigations is sold by Testfabrics, Inc: a strip that contains eight different fibers: acetate, cotton, nylon, polyester, acrylic, silk, rayon, and wool on a single fabric. See: http://testfabrics.com/product-detail.php?id=TXpNM05BPT0=&pid=3374