|Each line segment represents one bond between two carbon atoms. Double bonds are two lines.|
I think the pens use ink with long carbon chains or rings that have this property mixed with another substance that reacts with the ink when heated. I don’t have direct evidence for this, because I haven’t done a chemical analysis of the ink, but as soon as I realized that heat was what “erased” the ink, I came up with a working hypothesis. Now I should clarify that this could be completely inorganic for all I know. I'm not a dye chemist, so I don't even know what's likely, but there are inorganic ways of getting color as well as ways of combining organic and inorganic substances. I'm focused on conjugation here because it's a cool concept that's easy to explain.
Breaking double bonds with heat and turning them into single bonds is “turning off” the conjugation. The ink is still there, but when you heat it, it becomes clear. These sorts of reactions, where double bonds are reduced to single bonds tend to be reversible, and you reverse them with-wait for it... temperature. So I “erased” a message and stuck the piece of paper in the freezer. When I pulled it out, it was restored.
|This is an example of a reversible conversion where the amount of conjugation changes. This would require the presence of another substance to convert, since there is a different number of atoms in each.|
I realized then that there’s another, neater way to do this. Normal visible light usually needs a fair amount of conjugation (again, that’s long chains of carbon with alternating double and single bonds) for color to be affected. When you have long chains like this, you usually can’t break down all the double bonds at once, instead you just break some of them, and create smaller regions of the molecule that have conjugation. Visible light is too low energy to be affected by this, but a higher energy light source will react very differently. Ultraviolet light will show conjugation that visible light will not. It’s probably more complicated than this. So I tried my blacklight on it.
Boom. You now have a recipe for invisible ink that you can buy at the store. You don’t need a black light, but it helps. Of course, the biggest problem is that the pens still leaves indentations that are visible. There are some simple solutions: Write in between the lines of a real message. People won’t look too closely at it.
Another solution is to write on paper that isn’t so white. Just don’t use construction paper, the paper tends to be of such low quality that when you erase the message, you actually end up rubbing it away. The best paper choice is a neon color. They turn up bright under a blacklight and give good contrast.
Have fun with it!