
A mysterious crystal that melts at two different temperatures - sohkamyung
https://physicstoday.scitation.org/do/10.1063/PT.6.1.20190606a/full/
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nneonneo
The actual publication that resolves this riddle is here:
[https://pubs.acs.org/doi/10.1021/acs.cgd.8b01459](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.

~~~
rkachowski
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?

~~~
nneonneo
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).

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fnordprefect
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.

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duxup
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.

~~~
falcor84
"Once you eliminate the impossible, whatever remains, no matter how
improbable, must be the truth."

~~~
krapp
> "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.

~~~
onei
Seances and faeries strike me as improbable rather than impossible.

~~~
feintruled
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.

~~~
krapp
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.

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im3w1l
> 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.

~~~
dgoldstein0
Maybe presence of acid vapor? But this sounds possibly too sensitive to be
useful.

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

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wrinklz
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.

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mcherm
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.

~~~
kortex
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.

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octocode
>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?

~~~
blawson
I feel like this is basically asking for Melty McMeltface

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

I was so excited too.

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

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logfromblammo
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".

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

