So, this is the subsequent post to Part 2 if it wasn’t obvious by now. This post is going to be about acid dyes, how it interacts with wool, why you soak yarn in water, and what mordants are for! (Warning, slightly long post).
Acid dyes has acid functional groups like —SO3H (Sulfonic acid) or —COOH (Carboxylic acid). And example of one of the dyes is Direct Red 81. The charged groups under normal conditions would be -SO3H.
They become charged groups when in water. The reaction in the water would be:
dye — (SO3H)n + H2O ——> dye — (SO3 -)n + H3O+
But guess what? It also happens to the keratin in the wool. From the wool you’ll have spots where a fragment of the keratin ionized. The amine groups and the carboxylic groups ionize. The two groups are NH3+ and COO-. The ionized amine group and the ionized sulfonic acid create an ionic bond. The more acidic the bath is the better, however too high of a pH level will disrupt some of the other bonds. I’m actually going to do an experiment on how pH levels affect the color of yarn using natural dyes.
When the pH levels get too low (acidic), all the salt bridges, hydrogen bonds, and disulfide bonds get disrupted. This is called denaturation. The fiber loses its shape. Also, too much acid could reverse the reaction that’s above, changing how much dye will be willing to react.
This is where this thing called mordants comes in! Mordants are cationic (positively charged) metal ions that you usually soak your yarn in to dye. Some examples of mordants are alum, iron, chrome, copper, etc (Source). These metal ions have to be able to create coordinate covalent bonds. The mordants attach to the space in the middle of hydrogen bond sites and create a bridge between the keratin and the dye molecules. The mordants usually attach to the fiber through oxygen from —OH or —SO3 -. The coordinate covalent bonds create a colorfast effect, which is what you would want if you were dyeing fiber.