Water goes with a bang

US researchers have found that water appears to play a key role as a catalyst in complex explosions. The discovery could give insight into designing safer, less sensitive explosives - or provide new clues to how planets and stars were formed. 

Explosions are often extremely fast - so it can be difficult to probe what is happening at a molecular level. Now, researchers at Lawrence Livermore National Laboratory in the US have taken a detailed look at the process by modelling explosions of pentaerythritol tetranitrate (PETN) using BlueGene/L, one of the most powerful supercomputers in the  world. 

’Water is usually considered to be one of the final products of an explosive reaction,’ says Christine Wu, the lead researcher on the project, ’but in fact, we found that water has to form much earlier.’

One reason for this is the close proximity of oxygen and hydrogen molecules on explosive molecules, allowing water molecules to assemble easily. ’And once the water has formed, it does not just sit there,’ Wu adds. ’Under such high temperature and pressure, the physical properties of water change dramatically. In this case, the molecules are constantly breaking apart - becoming a continuous source of oxidising and reducing agents.’

In order to proceed to the final gas products of CO₂ and N₂, oxygen atoms need to be transferred from nitro groups to carbons, and the team found that water assists this process by stripping off oxygen atoms and shuttling them to other molecules. 

’Water acts like a baton, passing oxygen atoms to where they are needed,’ Wu explains, adding that this drives forward the chain reaction and contributes to the violence of explosions. 

Another indication from the simulations is that water formation could be related to the sensitivity of the explosive. Since the same mechanisms are likely to be at work in other explosives, such as TNT or RDX, understanding how the explosion occurs in detail could allow better explosives that are both powerful and safe to handle. 

The research could also have implications for geochemists, says Jung-Fu Lin at the University of Texas at Austin, US. ’This study reports a new dimension of behaviour of water under high pressure-temperature conditions that are relevant to a number of forefront questions in planetary science,’ he told Chemistry World.

’Since the constituent elements in the explosive PETN are likely to be present in the interiors of icy planets such as Uranus and Neptune, this study means that the chemistry, composition, and dynamics of these planets may not be as simple as we thought,’ Lin adds.

Lewis Brindley