Those that work in the pharmaceutical industry will be very aware of the issues associated with final physical form, such as salt selection and polymorphism and its control.
The issue of co-crystals is one that crops up occasionally in the quest for suitable forms of APIs so I was interested to note this example as a reminder that, of course, the solid state is important in other fields of chemistry. A J Matzger et al (Angew Chem Int Ed, 2011, 50, 8960) describe a co-crystal between the relatively new energetic material CL-20, which is shock sensitive, and the well known explosive TNT. One might be forgiven for thinking this is a recipe for disaster – a co-crystal of one explosive material with another?
In fact, the paper introduces the subject very nicely for those of us not accustomed to design of such materials, indicating that past attempts to stabilise explosives as co-crystals with non-energetic partners can work, but inevitably results in dilution of the energetic component and hence reduction in explosive power. This co-crystal between two energetic materials realises benefits in terms of enhanced stability/reduced shock sensitivity but of course maintains the explosive power. A further specific feature is that on heating the co-crystal, at a certain point the TNT liquefies and on cooling the input co-crsytal is not regenerated, but instead the mixture is again shock sensitive, thereby presenting an opportunity to use this strategy to design materials with improved handling characteristics for transport which can be altered at the point of use.
For interest, the structure of CL-20 is illustrated above.