it must be the Fifties, or maybe even the late Forties
It must be quite early, because he wrote it to practice the "turgid" style that he felt necessary for his PhD thesis. It was published before his interview and he was quite nervous, because he was afraid that the interviewer would have read it and consider him to be too frivolous to be awarded such an august honour. They reached the end with no mention of it, much to Asimov's relief, but just as he thought it was all over, the interviewer said: "One final question: what can you tell me about resublimated thiotimoline?".
The full tale and the actual story are in, I think, "Nightfall and other stories". It's been a long time since I read it, so my memory might be hazy there.
both of them vanish in mutual annihilation
I thought it was just that it dissolved before it hit the water, rather than any violent reaction. It was because thiotimoline supposedly possessed an inordinate number of hydrophilic bits (hydroxyl groups and so on). The reaction was probably highly exothermic (as well as endochronic), but I don't think annihilation (a la matter-antimatter) was involved.
