A mysterious crystal that melts at two different temperatures | Hacker News Reader

A mysterious crystal that melts at two different temperatures

physicstoday.scitation.org

206 pointssohkamyung31 comments

The actual publication that resolves this riddle is here: https://pubs.acs.org/doi/10.1021/acs.cgd.8b01459

The gist is that a chemical called acetaldehyde phenylhydrazone (APH) produces crystals in the solid state. Very minute contamination of the crystal can cause the melting point to go from 96 degrees C (uncontaminated) to 56 degrees C (contaminated), even if the contaminating agent (an acid) is virtually undetectable. A contaminated (low melting point crystal) and uncontaminated crystal appear identical under crystal structure and spectroscopy.

The cause turns out to be isomerization. The solid and liquid phases of APH have different stable isomer ratios (solid: 100% “Z” form, liquid: 37% Z, 62% “E” form); the contaminant appears to catalyze the isomeric transformation. With the catalyst, the Z form can transition quickly into the E form, making the liquid phase more accessible and lowering the melting point; without, the solid Z has to melt into the unstable liquid Z which then slowly (spontaneously) changes into the stable Z/E mix.

The paper’s worth a read, if only to see how thoroughly the authors tried to rule out any other possible explanation for this very weird behavior - doing many, many tests of the different solid samples and checking and rechecking equipment.

I have no understanding of chemistry, but you mention that it was contamination which caused the change.

Does this imply that despite the repeated references in the article to the crystals being identical, both when tested in the 19th and 21st century (with "modern structural analysis techniques"), the crystals are in fact different on a minute undetectable level? i.e. "a single molecule in the air" which causes the change?

Crystals are repeating arrangements of atoms bonded in a very particular, repeating way. Modern techniques include looking at the crystal with x-rays, so any serious discrepancies in the patterns would be noticed.

The contamination, they concluded, was not structural (e.g. as a silicon semiconductor might have random atoms replaced with phosphorus or boron) but rather even more minute amounts of acid could act as a catalyst - i.e. an extra component of a reaction that participates, but doesn't actually get consumed - hence can make a difference in tiny amounts. A decent way to think of catalysts is as chemistry's matchmakers - they help reactions happen; sometimes those would happen on their own but take more time, or sometimes they wouldn't happen at all (e.g. if there's not enough energy to react without the shortcut of the catalyst).

Hope that makes some sense.

The paper notes that a 1-in-1000 molar concentration of acid was sufficient to trigger low-temperature melting, indicating that the amount of contamination needed is indeed very, very small. The paper does not discuss exact mechanisms of action, but one of the conjectures is that the acid takes the form of excess hydrogen atoms, which could easily tuck themselves into a crystal structure nearly undetectably.

All it takes is catalyzing a few Z->E transitions when the temperature rises to cause the crystal structure to break down, which would explain the lower melting point despite the small amount of contaminant (catalysts often reduce the energy required to start a reaction, but are not themselves consumed so they can further catalyze other reactions).

So the crystal doesn't melt at two temperatures as per the headline. Instead, it has two stable isomer ratios, which is pretty interesting - generally compounds are stable as one isomer, or the other, correct? Or is that not the case?

Also, aren't E configurations generally more stable, and therefore should have a higher melting point?

It's been years since organic chemistry so perhaps I'm way off here.

You are correct, the E configuration is generally found to be the most stable isomer. It is therefore not suprising to see that in the final liquid state a higher E isomer content is found. The final ratio of interconverting E and Z isomers that you measure in the liquid is determined mainly by the energy difference between the two and the temperature.

However, in the solid state the E isomer does not necessarily give you the more stable crystal structure since the packing of the individual molecules can be very different for each of the two isomers.

I was involved in this research project and can add that interestingly the solid form of the E isomer of this chemical has not been documented so far. It appears that under the conditions that were used, the E isomer of this molecule is found only in the liquid state.

So basically, hydrogen/protons act as a catalyst for APH, a solid phase reactant (!?) that doesn't catalyze a chemical change, but a physical phase change in the reactant (!?) (via catalyzing the isomer change). Do either of those exist anywhere else or are these properties completely novel?

This is a wonderful read. Seriously impressive employment of the scientific method (the way it should be). The take-home message is probably: identify (and, if necessary, test) all of your assumptions. In this case, it was assumed that the different batches of material melted to become the same liquid, since that is pretty much how everything else melts (although that may be open to a new look now). On that assumption, the problem was intractable.

The problem was solved by someone questioning that assumption and finding out that, in fact, different samples melted to become different liquids. Once over that hump, clever application of basic chemistry principles got the rest out. Lovely work.

I can't say I understand all of that but I love a story about investigating something that is happening but seems impossible.

It often happens with my code.

"Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."

> "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."

... authored by a man who believed in seances and faeries, and not an actual scientist.

Seances and faeries strike me as improbable rather than impossible.

Me quantum-tunneling out of my chair to Mars is improbable, but so improbable that it can be considered impossible... an event that will certainly never occur within the entire lifetime of the universe is indistinguishable from one that cannot occur. At that point, differentiating between the improbable and impossible is splitting hairs.

But at least quantum mechanics exists within the framework of what we know to be provable reality. The supernatural doesn't - making it even less probably than me spontaneously teleporting to Mars like John Carter.

The spiritualism that Arthur Conan Doyle believed in and the seances he attended were fraudulent, as were the Cottingley Faerie photos (which he believed were real because the photographs looked convincingly real to him.) I'm certain that he eliminated all other mundane possibilities from his mind to arrive at the supernatural conclusions he did, but he was still wrong.

Mere process of elimination is not sufficient to prove something "however improbable." Once you eliminate the impossible, whatever remains, however improbable, still needs to be proven. Otherwise you wind up in the trap of confirmation bias.

Since the maxim is initially delivered by a detective who is trying to solve human crimes in a realistic universe, I think we can safely assume that by "impossible", he did not mean to descend into solipsism.

Otherwise every locked-room mystery could be solved by "the killer traveled through a folded pocket universe", or "the killer had telekinetic powers and managed to stab the victim in the back from outside the room". And similarly, seances and faeries are improbable enough to be grouped with the impossible for now.

Yes. Conan Doyle's maxim is apt - it's just he misapplied it. In his consideration of the case of Cottingley Fairies, he wrongly considered the girls as too unsophisticated to be able to pull off a deception, and too honest to want to. He eliminated the wrong elements.

That's the problem - the maxim only works in a fictional universe with a protagonist like Holmes who has an uncanny ability to deduce elements of the plot with perfect accuracy from details as minor as the angle of someone's shoulders or the way they tie their tie.

It doesn't work in the real world because it first requires perfect foreknowledge of all of the possible explanations for a phenomenon, as well as perfect confidence that the attempts to disprove all but one of those explanations were correct.

But, as demonstrated with Arthur Conan Doyle and the Cottingley faeries, there may be assumptions one is not willing to challenge (the literal existence of faeries themselves) and possibilities one may not have considered.

Authors and their characters don't believe the same things.

> Although the team’s work breaks genuinely new ground, Meekes cheerfully admits that the circumstances under which the melting-point suppression occurs are so specific that the research is unlikely to have useful applications.

Whenever I hear of something extremely sensitive, it makes me want to turn it into a measurement device.

Not clear what you'd use this to measure though.

Maybe presence of acid vapor? But this sounds possibly too sensitive to be useful.

I was thinking in space... measuring the corona of a comet or something. Anywhere you have a very faint hint of something

The higher melting point is not the true thermodynamic melting point, which must be reversible. It's more akin to supersaturation, where a solvent temporarily holds more solid in the absence of a "seed" to induce crystallization. Really interesting though, and the historical connection even more so.

A fascinating piece of chemistry. I like everything about the article except the claim, near the beginning, that it melts into two different liquid forms. I would have preferred to express it as two different melting processes (one catalyzed by tiny amounts of impurities, the other not catalyzed).

Normally, it is chemical reactions that get catalyzed, not state changes, but the behavior of this chemical is an exception.

No, it is truly a different chemical form, as evidenced by the shift in NMR spectrum.

Isomers often have an energy barrier between the two forms which can be overcome under certain conditions. Infamously, thalidomide easily converted between isomers, even if it was prepared chirally, it would convert in-vivo to the teratogenic form, due to a loose alpha hydrogen.

Usually, it's something like a chemical rearrangement, like the alpha hydrogen popping off then another attaching, inverting chirality.

At some point, this barrier can be low enough that it's very sensitive to these rearrangements, even a tiny fraction of acid or base can swing it.

>The team hasn’t even coined a name for the physical process by which identical solids can melt into distinct liquids. “If someone else wants to name it, then they can,” Threlfall says.

I think this is a challenge. Is HN up to the task?

I feel like this is basically asking for Melty McMeltface

Sounds like a desire for it to be 'Threlfallic melting' to me...

bi-solid, bi-liquid, bi-melting point. Unless it already means something else.

Corrected headline would be "two different substances melt at different temperatures". :(

I was so excited too.

No, this is the same substance that melts into two different liquids as the article states

I sense a demonstration wherein a large APH crystal prepared in an alkaline environment is heated to 60 degrees, and then a single drop of acid is dripped onto it, and the whole thing melts away.

My vote for nomenclature is "catalyzed isomerization melting".

makes me wonder how many ideas die(d) like this because they can't/couldn't reach the adequate attention from the public.

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