Science of Craft: 001 What is Cotton?

I'm a hippy-dippy Whole Foods shopper. I buy healthy, organic, free-range, fair-trade, sustainable, recycled, recyclable, eco-friendly and ethical. I might be fooling myself that the higher prices I'm paying at Whole Foods or my local co-op are really giving me any benefit, but I'm hoping. It's hard to keep hoping though because I often find myself in some pretty bizarre company at these stores. I'll overhear folks parroting specious Food Babe claims about staying away from chemicals and ingredients with complicated names and it just makes me shake my head. Everything is made of "chemicals" dummy. Even that locally-sourced organic strawberry. Yummy, wholesome chemical goodness!

Cotton is made of chemicals too. Sugar, in fact. And no, I'm not getting confused by cotton candy, though the confection and the material your shirt is made of do have some things in common chemically. The chemical composition of cotton is important to understand if you're going to be working with the fiber. It determines the structure of the fiber and that influences how you manipulate it into thread or yarn, and finally into a textile. The chemistry of cotton determines the properties of cotton that in turn determine the application for which we use this fiber. Understanding the chemistry of cotton also helps us know how to dye this fiber and with what. I'm not a scientist, so I'm sure if I can understand why cotton has so many useful and frustrating properties, so can you.

Cotton's Chemical Composition

Do you remember learning about photosynthesis in high school biology? Someday I'll get more into it, but for now, all you need to know is that one of the products of photosynthesis is glucose, and plants string together a bunch of glucose molecules to make cellulose. When cereals advertise that they are high in fiber, cellulose is just one of the many kinds of plant fiber that you'll be sending through your poop-chute. All plants produce cellulose, hence why many plants, like bamboo and flax to name a few, can be processed into spinnable fiber. What makes cotton so special though, is that its bolls are made of over 90% cellulose in nice, long fibers, making it almost immediately ready to spin. But what actually is cellulose, and what makes it so suitable for spinning? Let's explore the chemical structure. Don't get scared, I'm going to try to make this easy. If you are a better chem-nerd than me however, I would kindly solicit any further details you can provide in the comments below, thanks in advance.

Sugars

Earlier I said that cotton is made of sugar, and if I remember correctly, you interrupted to tell me I must be confusing cotton with cotton candy. I understand your confusion. If you put cotton candy in your mouth, it dissolves. If your dentist shoves cotton balls in your mouth, it just stays there getting soggy and bloody. So what's the deal? Well, cotton candy and cotton balls are made out of different kinds of sugar in different arrangements, that's all.

All of the different kinds of sugar are really just carbohydrates that often like to form rings. Carbo is for the carbon atoms that are in sugar. Hydrate for H₂O that's in there too. That's it. Three elements, carbon, hydrogen, and oxygen. You put those together in different ways and it gives you different properties. Let us take a look at two of the many, many ways of putting those ingredients together.

The cellulose molecule: What cotton balls are made of

The sucrose molecule: What cotton candy is made of

Learning Break

Take out a scrap of paper and make a column for cellulose and a column for sucrose. We're going to be looking for similarities and differences. It's good for learning if you actually do this instead of just reading what I've already noticed. So, the following are the things you are going to be hunting for, using the two pictures above as reference:

• The number of oxygens (O), hydrogens (H), and carbons (C) in each molecule
• The number of oxygens bonded directly to hydrogens. They look like this: OH or HO
•  The shape of the molecules
• Any other differences or commonalities that jump out at you

When you are ready, highlight the list below to compare your's with mine:

• Depending on how you count, both molecules have 11 oxygens, unless you left out the one oxygen outside the bracket in cellulose. Oxygen is pretty electronegative, which means it's often reactive. You may be familiar with this property for oxygen's well known part in combustion (burning) and it's role in rusting metal.
• Cellulose has 20 hydrogens and sucrose has 22, but cellulose would totally have 22 hydrogens guys, if it wasn't on endless repeat. Those naked oxygens on the end, if they weren't bonded with another cellulose unit, would be hooked up with a hydrogen.
• Both molecules have the same number of carbons. In carbohydrates, carbon acts as the backbone to which all of the other elements or groups of elements attach to. What other elements and groups attach to the carbon determines the special properties of the entire molecule. But hang on a second, as we have inventoried, there really isn't that much of a difference of ingredients between cellulose and sucrose. What gives?
• Look at how those ingredients are arranged. Just like when you are baking, the same ingredients don't always produce the same results. In our case, cellulose has 6 OH groups and sucrose has 8. In cellulose, these spots are where the magic happens when it comes to dyeing. More on that in a future post.
• Furthermore, the shape of the molecules are different. Give or take a couple of details, the left half of both molecules are the same. But look at the right half. Cellulose repeats and then flips the left unit. There is a name for this unit, it's called glucose. The right half of sucrose has a pentagonal shape instead of an octagon. This unit is called fructose. To summarize, cellulose is made of repeated glucose units and sucrose is made of just a glucose and a fructose bonded together.
• You might have noticed some other things like how cellulose repeats its units. This means we call it a polysaccharide, or more generally, a polymer. Sucrose is made of just a couple of units so we call it a disaccharide.
• Maybe you also noticed that the oxygen that bridges the gap is up in cellulose, and down in sucrose. This link between units is called a glycosidic bond. Good eye. I have to confess though, of all of the ways of representing these molecules, the way I chose to draw them is not very accurate geometrically. None the less, the orientation of the oxygen in the glycosidic bond is very important, and I will go on and on about it in a later post when I discuss the physical structure of cotton.

So, did I miss anything? Did you notice a similarity or difference between cotton and candy that I didn't find? Kindly share in the comments below. I'll be talking about the chemistry of cotton dyeing soon, which is pretty good timing because I currently have picked all of the seeds out of my cotton bolls and am ready to start the process. If you're