For manufacturers of drugs and chocolate bars, an understanding of how they crystallise can mean the difference between a best-selling product and a flop. X-ray diffraction could help them get a clearer picture at the atomic level.
The taste and feel of chocolate in the mouth depends a lot on the crystal form of the cocoa solids, while some medicines work more effectively in one polymorphic form than another. Until now a crystal clear understanding at the atomic level of how different polymorphs form in everything from chocolate to medicine has been little more than trial and error except in the laboratory setting of the vacuum. Now, Elias Vlieg of the Department of Solid State Chemistry, at the University of Nijmegen, describes how X-ray diffraction (XRD) techniques can be used to study crystals as they form and so provide clues as to how their growth can be better controlled. The chance of tastier chocolate and more efficacious drugs is on the horizon.
If the growth of crystals were clear-cut, there would be no need to study crystal growth, but many compounds can crystallise in different – polymorphic – forms. Even a material as seemingly simple as carbon has several polymorphs – graphite, diamond and fullerite. The differences between polymorphs of the same compound can be tiny, an atom shifted slightly to the left, or a tighter angle between two bonds. But, they can also be quite large differences that impact on the overall properties of the solid. For a drug in solid form this can have a real impact on how well it is absorbed by the body. One polymorph may take longer to be dissolved and absorbed while another might be faster acting. The result can also alter the drug’s side-effects. A slowly absorbed drug might sit in the stomach too long and cause irritation of the lining of the stomach for instance.
On the lighter side, the minute crystals of cocoa solids in a chocolate bar affect how the bar melts in the mouth. One crystal form may have a more pleasing texture on the tongue than another. According to Vlieg, XRD has been wholly successful in observing crystal growth in a vacuum. But for crystal growth from the more industrially realistic setting of a solution, melt or solid, it has until recently been little more than a dream tool.
Now, XRD is beginning to offer information on the structure of both sides of a growing interface. This, explains Vlieg, means that structural details like relaxation and reconstruction on the crystal surface and ordering in the solution can be included in the theoretical description of crystal growth.
Understanding crystal growth in vacuum and beyond, Surface Science, in press.