Wednesday, June 22, 2011

The 4-benzyltoluene melting point twist

Evan Curtin and I were in the lab this morning to follow up on our effort to curate the melting point of 4-benzyltoluene. I identified the next step to confirm an upper limit of -15 C:
With the information available thus far from our experiments (UC-EXP266), we think it is unlikely that the +4.6 C value can be correct because we observed no solidification after 2 days at -15 C. The patent reports that solidification of some viscous mixtures took up to a full week but we did not observe an appreciable increase in viscosity for 4-benzyltoluene at -15 C. But in order to be sure we will first freeze the sample again below -40 C and let it warm up to -15 C in the freezer and confirm that it melts completely.
But when we took the sample out of the freezer after 16 days it was completely frozen!


This now effectively ruled out the -30 C value and re-opened the possibility that the +4.6 C value could be the best estimate. Learning from our previous failed attempt to observe a temperature plateau when heating the sample, this time we let it warm as slowly as possible by leaving it in an ice water bath inside of a Styrofoam container. This worked much better as the sample warmed a few degrees over several hours. This time Evan observed a clear transition from the solid to the liquid phase in the 4-6 C range.(UC-EXP266)

The curation record for the melting point of 4-benzyltoluene now looks like this:

When I introduce the concept of Open Notebook Science in my talks I usually make the point that there are no facts - just measurements embedded within assumptions.

The 4-benzyltoluene melting point story is a really good example of this principle. When I stated that I thought that "it is unlikely that the +4.6 C value can be correct because we observed no solidification after 2 days at -15 C", it was not the measurement that was in error - it was the interpretation. And when new information came to light, an experiment was proposed to either challenge or further support that interpretation. There were never any "facts" in this story (nor is the +4.6 C value a "fact" from these results).

I think that this is how science functions best and most efficiently. Unfortunately we don't usually have access to all pertinent raw measurements, assumptions and interpretations. I would be extremely interested in seeing how the -30 C value was determined. This is actually the value provided by the company that sold us this batch of material (as well as the PhysProp entry in the image above). Because of slow crystallization, I can see how this could happen if the temperature was dropped until solidification was observed. In our observations, the -30 C to -35 C range is roughly where we observed rapid solidification upon cooling. (UC-EXP266)

2 Comments:

At 7:58 AM, Blogger Poliin said...

The reason for the observation of low temperature as a point where solidification starts is that crystallization cannot start without nuclei - crystal seeds within the liquid, or any other particles which would enable crystallization.
Possibly useful way to find the real melting point without problems with nucleation would be to use sonication along with freezing.

General explanations related to this topic can be found here:
http://mnyukh.com/

 
At 11:48 AM, Blogger Jean-Claude Bradley said...

Thanks for the comment Poliin - indeed great care needs to be taken when interpreting melting point implications from freezing experiments. Only by slowly heating the frozen solid were we able to get more reliable information.

 

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