Sugar, table salt, even chocolate- crystallisation is how we get them as we do. It’s always been a technique of purification, perhaps a poor man’s technique in this age of chromatographic separations, but an effective way nonetheless of getting one pure substance out of a cesspit of gawd-awful gunge. That’s how it was; until 1844, when a man called Wohler separated something called quinhydrone. Quinhydrone has two different chemical compounds in its crystal structure; which means, in simple words, the constituent compounds like each other way too much to let each other go.
My research is in a discipline with a fancy name- crystal engineering. I am a matchmaker. I search for compound-pairs that would be likely to love each other until death do them part (Death, in this case, involving a load of nerdy stuff like electron beams in high vacuum or evil-smelling elixirs. I need to get a life. I know.). Believe it or not, crystal engineers all over the world have voted to form an entire database of compounds for finding these compound-pairs. Yup, we take our matchmaking seriously. We do. Methodologies have been advanced quite a bit in the last two decades or so, to the extent that we can narrow the search down to only a few potential partners just by looking at a few arcane diagrams. Partners, mind you. Only a pair.
The big question we in the GRD group have been asking is, “What about a family, then?”. How many of these little buggers can be forced to coexist in one single box without burning it down? Two? Sure. Three? Well… I myself have worked on ternary systems; and constructing them is devilishly difficult. Four? Unimaginable. At least until 2016, when a senior of mine painstakingly constructed a series of them. Five? Six? This paper, of which I happen to be a coauthor, says YES!