Growing Fat Rocks of Stuff

loki · guinea pig

I came up with a possibly stupid idea this evening, but watching a feeble attempt at testing the idea, it did look quite convincing, thus I feel I should put this idea out there.

The idea goes like this: A crudely crystallised substance is added to a small amount of a solvent which only sparingly dissolves the substance is put on reflux (or just covered in a polyethylene plastic wrap pushed inwards somewhat so that it doesn't tend to drip anything over the lip of the refluxing vessel), for some number of hours, determined by observation of a level of crystal growth that one desires.

The preliminary attempt at this process did indeed appear to result in the growth of what definitely seemed to be completely pure substances. In a solution known to be holding some amount of three different crystalline materials, three different crystal forms were observed. In a period of about 3 hours what looked like completely pure crystals of about 9/10ths of a millimetre had formed. One was rhombic, another looked like it might be octahedral and another looked vaguely hexagonal.

The crystals did grow with time and heat and agitation, in what was very clearly a very pure form. Probably would work better with a more minimal amount of the solvent (just enough so it can swirl around in the liquid from convection) and with stirring, shaking or something like that.

Obviously this idea was based on accepted theory about crystallisation and purity, but run in a different way to the usual slow cooling. The theory was that with a small amount of the crystalline substance dissolving, in time, and accelerated by heat (obviously heat must be kept below the mp of a freebase if one were working a low boiling freebase which does form a solid at room temperature), the smaller crystals in the solution dissolve faster than the big ones, which then aggregate the substance lost from the smaller ones more effectively due to surface area.

A concrete example of how this might be performed in practise was reaction product from a PIE reaction. This product is sparingly soluble in wet (as it comes from the hardware store) acetone. The crystalline material is added to a small amount of the wet acetone and refluxed. This indeed appears to cause the growth of crystals characteristic of the substances expected (grey translucent rhombic crystals and shard-like pointy ones... is there a proper name for that crystal form, or is it just pyramidal)

It has not been established what the optimal arrangement for this crystallisation method is, whether it is better to have ample fluid or just a small amount, but having watched a test of this being done with ample, it seemed to me that the volume of solvent would have slowed down the solvation and precipitations due to the much greater distances between the crystals.

Swim believes that this method, if let to run for say 24 hours, or whatever, may depend on quantity and technique etc, should theoretically eventually result in singular, pure crystals of whatever salts were in the mixture.

As the process proceeded, it could be seen that the accretion of mass to the larger crystals and loss from the smaller ones did indeed occur. The initial material put into this process was from a rapid freezer cooled recrystallisation in dry acetone and IPA, which turned out very fluffy, not suprisingly. The fluffiness gradually disappeared as the heating proceeded, and gradually all the crystals in the solution adopted their pure, simple forms, free of any attachment to the edges of the glass. And with more time the crystals did indeed appear to increase in size and diminish in number.

More tests should be done. Swim may do further tests on the idea, plenty of crystalline substances could be tested with this idea, the desired end goal being the growth of a singular, completely pure crystal. Tests with two different salts will probably be done to see if this method might possibly serve to effectively isolate two pure substances, given enough time to form a single pure crystal. Swim thought that the method may well be applicable to certain types of extractions too, or as an effective method of isolating a product from side products and unreacted precursors, for example.

loki · guinea pig

after thinking about it some more, from watching the process, a few additions to the theory of how to do it properly come to mind.

one thing is the solvent used must be either pure or the mixtures use must be azeotropic. changes in solubilities due to, for example the higher volatility of acetone compared to water cause the crystals to be redissolved as the gas above the liquid becomes saturated with acetone and the water from the air and the air itself becomes more dissolved in the liquid. the solubility of the solvent being as low as possible is desirable if the process is done under reflux, dry acetone, for example, should cause desoxyephedrine to grow crystals larger under reflux, or in sealed conditions with temperatures above acetone's boiling point. supercritical gases could be used this way too, although finding a preferably transparent container which can hold the gas in the supercritical liquid state might be tricky

lurker4711 · Sat Jun 18, 2005 6:22 am

I would suggest to adapt the methods used for producing monocrystalline silicium for wafers: A slowly rotating needle (vibration free) is dipped into the melt (high purity) of the compound you want to produce a (mono)crystal from and then very slowly removed from the melt. This has of course to be done with a protecting inert gas - argon for example. The technical setup may be not trivial, but it should produce a solid monocrystall from the substrate (in form of a rod).

A monocrystalline rod of whatever! Besides that this was never seen before and may have some interesting physical properties it is unknown to the forces of evil and good to store the compound in an ultracompact way for a long time unaffected because of the low surface area prone to attack.

A project worth considering if I ever should start to abuse massive amounts of certain amines again, for this would provide me with the motivation and the substrate.

Besides this, crystallization over a longer time - a week or more - from plain water produces big ones. A certain, not big but regualary temp. change and water as solvent are known to produce excellent results regarding size and purity.
There was a very good post from Xtaldoc at the-hive dealing with this.

A monocrystall from the melt will be superior of course. It must be!