Co-crystallising two explosives has allowed researchers to make a compound that is safer but still highly energetic

A team from the University of Michigan,US, has used co-crystallisation to combine two explosives to produce a novel compound that is safer and less likely to explode on impact. 


Crystallising two different compounds allowed researcher to combine explosive power with stability

Team leader Adam Matzger says his group is the first to investigate co-crystallisation as a way to develop new energetic materials - compounds with high, releasable chemical energy. ’Co-crystals are held together in a regular repetitive pattern just like crystalline salts, such as sodium chloride,’ Matzger explains. ’But in a co-crystal, the two components are neutral. They can exist in a pure form, but you can’t have pure Na+ and pure Cl-.’ Co-crystallisation has been used with some success in pharmaceutical applications.


Source: © Angew. Chemie. Int. Ed.

TNT (left) and CL-20 (right)

At the end of last year, Matzger’s team reported using co-crystallisation to make 17 novel solids containing 2,4,6-trinitrotoluene (TNT).1 ’Although this work proved the viability of co-crystallisation, explosive power suffered,’ he says. ’Combining an energetic compound like TNT with a non-energetic compound diluted the explosive power of the final compound so they were not viable as explosives.’  

Now the team have combined two energetic materials in a 1:1 molar ratio: TNT and another explosive called CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane).2 They found this combination ’after lots of trial and error’, says Matzger, who reports that the co-crystal forms readily, despite the lack of strong predictable interactions in the chemical structures of its components.  

’[We have shown] that aliphatic, highly nitrated compounds can co-crystallise,’ Matzger says. ’This co-crystal forms through a series of C-H hydrogen bonds between nitro group oxygens and aliphatic hydrogens, as well as interactions between the electron-deficient ring of TNT and nitro groups of CL-20.’ 

He describes the co-crystal as combining the economy and stability of TNT with the power of CL-20. Although it is not as explosive as pure CL-20, it is more powerful than TNT. Its ability to withstand impact detonation is around doubled relative to CL-20.  

Matzger is now in discussions with interested parties to carry out further testing. Meanwhile, his team is searching for more explosive co-crystals. 

Jon Maienschein, director of the Energetic Materials Center at the Lawrence Livermore National Laboratory, US, says the work is ’a clever idea’ but questions if this is the first co-crystallisation of an energetic material. ’It is a novel approach to use neutral compounds in a 1:1 ratio like this. It’s marvellous work for a university, but they have not been able to do much characterisation, such as real detonation and safety measurements. This is a novel class of material with interesting properties but its usefulness has yet to be proven.’  

Maria Burke

Interesting? Spread the word using the ’tools’ menu on the left.