Liquid Explosives: TATP

Note: TATP is the explosive linked to the recent Airline Terrorist Plot

An explosive favored by suicide bombers – triacetone triperoxide(TATP) -- is extremely unstable and prone to explode unexpectedly, a disadvantage that has made it useless for other applications. But it is hard to detect and extremely easy to make from two widely available chemicals: acetone and hydrogen peroxide. Now, an international team of scientists led by Ehud Keinan of the Institute of Catalysis Science and Technology of the Technion-Israel Institute of Technology has discovered the surprising reason why TATP is so easy to make--it's a different kind of explosive than practically any in use.

"Most explosives are energetic materials," explains Keinan. "They have a lot of energy chemically stored in them. In an explosion, that energy is released suddenly, generating a huge amount of heat. The heat in turn creates the explosive expansion." To get the energy into the explosive in the first place, it has to be supplied from something, generally in the form of heat. TNT, for example, has to cooked at high temperature for its high-energy chemical bonds to form. Since nitrogen compounds are good at storing energy, most conventional explosives contain nitrogen, a property that makes them relatively easy to detect.

But TATP is different. It is formed at room temperature and does not require any input of heat. Nor does it contain nitrogen compounds. It is in fact a carbohydrate-type compound somewhat related to sugar. So the question is--how can it explode if the energy is not pumped into it in the first place?

The research team demonstrated that TATP exploded not by releasing thermal energy, but by suddenly breaking each molecule of TATP in the solid state into four molecules of gas. Although the gas is at room temperature, it has the same density as the solid, and four times as many molecules, so it has 200 times the pressure of the surrounding air. This enormous pressure – one-a-half tons per square inch – then pushes outward, creating an explosive force 80% greater than that of TNT.

"There is no increase in energy when the molecules break apart," says Keinan, "but there is a sudden increase in entropy." Entropy is a measure of the degree of disorder in a system, and the randomly moving gas molecules have far more entropy than the orderly TATP crystal from which they are produced. When entropy increases in a system, energy can be derived from it, such as the kinetic energy of an explosion. In a TATP explosion, the gas molecules give up their energy of motion to the surroundings, in the process creating the shock wave that does the damage.