My girlfriend threw a tie-dye party recently, which by all accounts was a much larger success than the tie-dye party I threw a year or two ago. My party mainly involved me alone in my kitchen, tie-dying my socks and underwear with cheap Rit dye that I bought from the grocery store. Here is what my socks ended up looking like (notice all the bleeding that occurred):
My gf's party differed from mine in three crucial ways: 1-other people were present; 2-she printed off patterns so we knew how to make those awesome spirals and stuff; 3-she used expensive "fiber-reactive dyes" that tend not to bleed or fade. Here is how some of our best items turned out (notice the white whites, and vibrant colors):
Unfortunately I have not been able to find the structures of Rit dyes or the "fixative", Retayne, often used with dyes such as Rit, so I cannot explain exactly how those work. What I can explain is how fiber-reactive dyes work...they react with the fiber, doy!! !!! !!!!!
Fiber-reactive dyes such as Procion MX are meant to be used with cotton fabrics. Cotton is made mostly of the biopolymer cellulose. Here is what cellulose looks like:
Notice how there are lots of "OH" groups on the molecule. Those are called "hydroxyl" groups, which are made of one oxygen atom bound to one hydron atom and one other atom (usually a carbon atom). These are the same type of unit found in water (H-O-H). Those hydroxyl groups are the reason that cotton soaks up water so well, since hydroxyl groups love to hang out with other hydroxyl groups. Those cellulose hydroxyl groups are where the dye molecules attach themselves to the fabric.
Here is a representative example of a fiber-reactive dye molecule. A collection of millions of molecules like the one below would look red:
The fiber-reactive part of the dye is on the left side, and is called a "dichlorotriazine" ("two-chlorines-three-nitrogens", pretty simple, huh). The oxygen atoms on the hydroxyl groups on the cotton t-shirt, they have electrons that "attack" this portion of the molecule. That reaction works something like this:
The dots and curved arrows just stand for electrons moving around (when electrons move around it sounds like this: "flup, saak" j/k). You are probably familiar with the by-product of this reaction, HCl, or hydrochloric acid. That molecule helps explain why people often pre-soak the fabrics they are going to tie-dye in an aqueous solution of soda ash, aka sodium carbonate (Na2CO3). Sodium carbonate is what is known as a "base". Bases love to bond with hydrogen atoms, but they make the hydrogen atoms leave their electrons behind. A hydrogen atom without any electrons is called a proton. Mixing soda ash and hydrochloric acid looks something like this:
The soda ash works in two ways. First it soaks up the protons from the hydrochloric acid, which helps prevent the acid from doing damage to the shirt molecules and dye molecules. Second, it helps speed the whole process up by sucking up protons from some of those structures above. If you use a weaker base such as sodium bicarbonate (NaHCO3), I hear you have to heat the shirt to speed the reaction up. When you use soda ash, no heat is necessary.
THE REALLY COOL PART: The dye molecule has become a part of the t-shirt cellulose molecule, through a so-called "covalent bond". Together they make a new molecule, and there is nooooo waaaaay that soap will wash that away. It takes a lot of energy to break covalent bonds. My guess about the Rit dye is that it basically just sticks to the cellulose through "non-covalent interactions", which tend to be a lot weaker and easier to wash away and bleed all over. The moral: molecules are strong.
Now, check out this sweet time-lapse-instructive-tie-dye-video: