Solubility Studies 2: The Thermodynamics of DMT Crystallization - Freebase Solubility Curve Options
#1 Posted : 11/19/2019 8:03:24 AM

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For my 2nd solubility study, I examined the solubility of DMT freebase in n-Heptane over a range of temperatures (20C - 105C).

Only the preliminary results are in, but I wanted to share anyway because I find the data too exciting. I've wanted to know this for years !

The Word document contains method procedures, graphs for the solubility curve in terms of mL solution as well as mL solvent, and some of the initial raw data.

Finally, the crystallization process can start to be controlled Big grin

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#2 Posted : 11/19/2019 6:59:31 PM

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#3 Posted : 11/19/2019 10:21:11 PM

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A safety disclaimer might also be welcome considering the heating of petrols.
#4 Posted : 11/22/2019 7:02:37 AM

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More results in. Attached are the solubility curves for the 2nd and 3rd pulls I did of the same starting material. These pulls were done with the hot plate at 90C and 100C, respectively, which translates to a solution temperature of roughly, idk 65C & ~75C. (Pull 1: set temp = 80C, real temp = ~57-60C).

Couple things which are interesting:

Freebase from Pull 2 demonstrated higher solubility than Pull 1. Interestingly, the slopes of the two linear trendlines were nearly identical: y= 0.4313x - 13.075 for Pull 1, and for Pull 2 y= 0.4425 - 5.769, so the solubility of DMT from pull 2 was consistently ~7 mg/mL higher
I think this has to do with the fact that the freebase from Pull 1 was much more crystalline than 2, and even though I swirled them all as they were heating it could have been that if I had waited longer to get the crystals into solution and reach equilibration the solubility values would be significantly higher.

Pulls 3 & 4 (4 @ 110C hot plate setting) were far more oily and exhibited no crystallinity at all. I didn't get a full curve for Pull 3, but it seems like its solubility begins to skyrocket around 70C which would make sense -- oiling out is usually from too high a supersaturation, and if the extraction temp was 75C then that would have definitely affected the crystallization

At the end is a graph of all 3 combined.
Samvidbuho attached the following image(s):
Freebase Solubility Curve for n-Heptane (mg-mL solvent), Pulls 2 & 3.PNG (36kb) downloaded 92 time(s).
Freebase Solubility Curve for n-Heptane (mmol-mol), Pulls 2 & 3.PNG (35kb) downloaded 93 time(s).
Freebase Solubility Curve for n-Heptane (mg-mL solvent), Pulls 1, 2 & 3.PNG (21kb) downloaded 92 time(s).
Freebase Solubility Curve for n-Heptane (mmol-mol), Pulls 1, 2 & 3.PNG (16kb) downloaded 91 time(s).
#5 Posted : 11/22/2019 7:48:05 AM

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I'd like to share something I just learned, and run by what I'm trying to figure out.

Supersaturation is the driving force of all crystallization processes. That's why the solubility curves are so crucial to know, because then you can calculate your supersaturation ratio and try and design a crystallization scheme which maintains that ratio within the metastable zone width (MSZW).

There is a widely expressed way of correlating supersaturation with nucleation rate and crystal growth, and it is probably the only graph one is likely to find when researching the relationship. However, there is a more accurate way of describing this relationship that I found in Mullin's "Crystallization, 4th ed." (THE Source) which shows that the conventional ways of expressing this relationship do not apply for DMT crystallization.

The 1st picture are the two methods of describing this correlation with supersaturation and nucleation rate / crystal growth. The graph on the left is Mullin's (source material reproduced as well below), the right something you will find anywhere on the internet, here from a white paper from Mettler Toledo.

Conventional theory will tell you that nucleation rate increases exponentially with increasing supersaturation. It would mean: the higher you supersaturate your solution, the more your end product will be a bunch of tiny crystals rather than. Lower supersaturation ratios will favor crystal growth and lead to larger crystals.
Yet, looking at the graph Mullin has, we see that this is just the theoretical vision, and not what happens in reality. In practice, there is a maximum and then a sharp decrease. To quote:
"Melts frequently demonstrate abnormal nucleation characteristics [...]. The rate of nucleation usually follows an exponential curve (solid line in Figure 5.2) as the supercooling is increased, but reaches a maximum and subsequently decreases (broken curve in Figure 5.2). Tamman (1925) suggested that this behavior was caused by the sharp increase in viscosity with supercooling which restricted molecular movement and inhibited the formation of ordered crystal structures."

I attached pictures of the Pull 1 sample extracted at 97.5C (give or take) so the supersaturation ratio at RT was huge. Nucleation should have been exponential, but the crystals were actually fairly well developed, and several but not by any large amount. The next picture is the pull at 85C, which also should have yielded a high number of little crystals but actually formed just into a single crystal.

The most interesting formations occurred with the ones at 75C, 65C, and 40C, and they are shown at the end. Mullin points out that this reversal of the nucleation rate has been confined to melts, but also has been noted in extremely viscous solutions.
We are always working with melts with DMT (unless you stick to below 46C), so this is directly applicable.
Two of the samples demonstrating this strange crystal formation came from melted samples, although not the third.

The first page of the source material discusses three main variables which govern the rate of nucleation: temperature, degree of supersaturation, and interfacial tension. I think the latter played a huge part in the formation of this crystal, as well as the lowered degree of supersaturation.

It's, well, crystallization is complicated, I'm not quite sure exactly yet how to describe these processes completely, but I do think it's good to bear in mind this other relationship between some of these values which you probably won't find elsewhere.
Samvidbuho attached the following image(s):
Two Differing Models on the Effects of Supersaturation on the Rate of Nucleation.PNG (102kb) downloaded 87 time(s).
Pull 1, 97.5C, Extract.jpg (1,305kb) downloaded 87 time(s).
Pull 1, 85.0C, Extract.jpg (628kb) downloaded 85 time(s).
Pull 1, 75.0C, Extract_time 1.JPG (217kb) downloaded 84 time(s).
Pull 1, 75.0C, Extract_time 2(b).jpg (1,291kb) downloaded 85 time(s).
Pull 1, 40.0C (lbottom) & 65.0C (top), Extracts.JPG (306kb) downloaded 85 time(s).
Pull 1, 65.0C, Extract (1).JPG (160kb) downloaded 84 time(s).
Chapter 5 - Nucleation, pp. 184-185 ('Crystallization', 4th ed., Mullin).PNG (229kb) downloaded 84 time(s).
Chapter 5 - Nucleation, pp. 186-187 ('Crystallization', 4th ed., Mullin).PNG (285kb) downloaded 84 time(s).
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