Puzzling crystals in the drizzle

 

The life of an academic involves a certain amount of travel, in my case to Manchester in January. This as glamorous as it sounds, the drizzle has been unrelenting. Although on the bright side I was able to finish my talk for tomorrow in the Lass o Gowrie, which I can recommend; friendly barstaff, and the Citrus IPA was good. Tomorrow, I am going to give a talk about growing crystals, in particular growing crystals of a small molecule called glycine. We* studied the glycine molecule because when crystallised from water, it forms not one but two types of crystals.

These two types have different crystal lattices. Crystals are, by definition, solids where the molecules are arranged in a regular repeating lattice, and glycine commonly forms two type of crystals, with different repeating lattices. One is called the α polymorph, and the other is called the γ polymorph.

Above are two pictures of two crystals of glycine — apologies for the poor resolution, the crystals are only a few millimetres across and the images are cropped-out parts of much bigger images. Also, in both cases there are prominent circles, these are the sides of the circular wells the crystals are in, you should ignore those. Even at this resolution, the left-hand image clearly shows a long thin crystal — it goes from bottom-left to top-right. While the crystal at the bottom in the right-hand image is blocky not needle-shaped.

You might think: OK we have two polymorphs, α and γ, and two crystal shapes, blocky and needle-shaped, so either blocky are α and the needle-like ones are γ, or vice versa. But no, we used a technique called Raman spectroscopy, to show that both crystals above are α. As I will tell the assembled University of Manchester chemical engineers tomorrow, we have little idea of why glycine molecules in exactly the same crystal lattice can form crystals of very different shapes.

The needles grow in a characteristic way, the crystal grows very rapidly along the long axis of the needles, at least ten time faster than a blocky crystal does in any direction. It looks like early on in the formation of the crystal, in some (needles) but not others (blocky) something is triggered that turbocharges growth along one axis (only). Although we don’t know what it is, we do have some control over it, and this may be useful. When chemical engineers are making crystals in factories, by the kilo or by the ton, they often need to avoid needle-like crystals as these crystals can clog machinery instead of flowing smoothly. So tomorrow, I’ll be talking to the right people to see if our results are useful or not, I am looking forward to it.

* “We” here is the PhD supervisor’s we, my then PhD student Laurie Little did all the hard work, and discovered this odd behaviour of glycine crystals. We published this work last year, and it was the heart of Laurie’s PhD thesis.

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