The Gnome is a nerd. Yep. I don’t research because I have to, I have to research.
Someone on Ravelry asked a question about dyes (not an uncommon occurrence in a dyeing group). Specifically, it was a question about why acid dyeing yellow is a pain in the butt and falls out of solution.
Now, most people would satisfy themselves with something to the effect of, “It’s a yellow thing” or even a “It’s a chemistry thing.” But not me, oh no, none of that simple answer for THIS gnome!
You see, I am one of those freaks you might have known in college. Remember how all the bio and chem majors would always moan about this horrible class they had? A class who’s horror reached almost mythic proportions, a monolith almost insurmountable? That class was called, “Orgo” short for “Organic Chemistry.” And I was that kid, yeah, That Kid. The one who loved it. Even when I was failing it, I found it fascinating.
So when someone asked, “Why does my yellow dye always fall out of solution as soon as its cool?” I started researching (in my spare moments when I wasn’t dunking my yeast in formaldehyde). And you know what? I came up with some answers! Huzzah!
So, you probably want to know what the answer is, don’t you? Well, I’ll tell you. I’m going to try to write this such that it goes from the simplest explanation to the most complete explanation, so you can read as far as you want and not be overwhelmed. Because the truth is, most people don’t care about the in depth chemistry. But for those who do, I’ll try to explain things in non-jargoned terms (or explain the jargon at least).
Question: Why does my yellow dye fall out of solution when cool?
Corollary Question: Why do some of my red dyes form “jello” when cooler?
Most basic answer: Because your yellow dye hates water
Most basic corollary answer: Because your red dye likes itself
More in-depth answer: Chemistry.
One of the most basic divisions of chemicals is between those that love water, called “hydrophillic” (meaning, literally, “loves water”), and those that hate water, called “hydrophobic” (meaning “fears water”).
Example: Oil is hydrophobic. That’s why it makes little bubbles when you mix it with water.
Your yellow dyes are “hydrophobic,” thus the most basic answer.
Hydrophobicity, the “amount” a molecule hates water, is controlled by how “balanced” a molecule is. Think of an unbalanced, water loving molecule like a magnet. If a molecule is not balanced, its exactly like a magnet, with two ends or “poles.” Those poles can attract other unbalanced magnets. Strong magnets are all about getting together with other strong magnets. They have fun molecule parties. If the magnets are strong enough… well you can’t pull those magnets back apart again!
So the trick is that the balanced molecules don’t get along with the unbalanced ones. All those magnet molecules want to cluster together, so they can line up South poles near North poles and balance each other out, right? But the balanced molecules don’t have any poles, so any magnet nearby doesn’t have anything to balance it out.
This makes the magnets cranky.
So the magnet molecules all get together and line up end to end around the non-magnet molecules and make itty bitty bubbles around them, ostracizing them from the party. Bam, you’ve got bubbles of oil! (or yellow dye). If all those water molecules get really lined up (ordered) then you get… ice! Ice is highly ordered water.
I can hear you now, “So wait, if my dye hates water, why does it ever go into solution? And why doesn’t my blue dye do that too?”
Quick answer: ‘Cause your dye isn’t as hydrophobic (water hating) as oil, and not all dyes were created equal.
When you heat up your water, all those magnets get hyper (think steam!). When they’re bouncing around, they don’t line up very well (the ice melts) and so they can’t ostracize those non-magnet dye particles as well, and the little bubbles burst and your dye dissolves into solution, until it cools down again and falls out.
But why doesn’t it happen to your blue dye? Why is yellow the worst?
Here comes the chemistry. Ooh scary. Nope, not really. Just gonna show you some pretty pictures and point at things.
Here is the core of a yellow acid dye. This is called a “trityl” group.
See those hexagons with the little double lines? Those are very hydrophobic. They really don’t get along with water well, because they’re very very balanced. There’s some little bits hanging off that aren’t shown here, which make it kinda go into water if you heat it, but not enough to make it stay there.
Here’s another yellow acid dye
Again, see those rings? Hydrophobic. The little bits hanging off the two ends are what make it yellow (by changing the way it interacts with light). Like before, they make it able to go into water, but not well.
“But wait, if this is what my dye looks like, why doesn’t the green do this? Or the blue? If green is blue and yellow, why doesn’t the yellow part fall out?”
Ah, right. Green. The answer is, depends on the green. Some greens are mixed blue and yellow, but some are “pure” greens, that is the dye itself is green. Pure greens are similar to yellow, but with bigger hydrophillic water friendly groups.
See those big hanging off bits (moieties)? Those make this pure green dye a lot more water friendly than it’s yellow cousin. You can also see how the big rings aren’t as evenly spaced as in the yellow example. This lets water into that center space, again making it more soluble.
“Ok, but what about my corollary question? Why the heck is my red making jello?”
Right, the jello. Ok, we’re going to go back to that second yellow dye
Remember? Now, reds are made from a similar base, but the big water-hating rings are a little further apart, which lets water in between (like that twist in the green dye), but also the way they make them red, and not yellow, is by adding big doo-dads on the two ends (where the HO- is and on the far end).
These moieties (doo-dads) make the whole molecule unbalanced enough to interact with the water. But in some cases, if the end magnets are strong and the middle part water hating, your red can line up in big long strings, end to end, making balanced ropes with water hating middles! This lining up is called “polymerization” and is exactly what happens in jello, and is similar to the ordering of ice. Warming up the solution makes the magnets hyper again and things move around and your red goes back into solution.
So, that’s why your yellow falls out of solution and your red sometimes makes jello.